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| a FEATURES Complete Multistage Logarithmic Amplifier 92 dB Dynamic Range: -75 dBm to +17 dBm to -90 dBm Using Matching Network Single Supply of 2.7 V Min at 7.5 mA Typical DC-500 MHz Operation, 1 dB Linearity Slope of 25 mV/dB, Intercept of -84 dBm Highly Stable Scaling Over Temperature Fully Differential DC-Coupled Signal Path 100 ns Power-Up Time, 150 A Sleep Current APPLICATIONS Conversion of Signal Level to Decibel Form Transmitter Antenna Power Measurement Receiver Signal Strength Indication (RSSI) Low Cost Radar and Sonar Signal Processing Network and Spectrum Analyzers (to 120 dB) Signal Level Determination Down to 20 Hz True Decibel AC Mode for Multimeters Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier AD8307 FUNCTIONAL BLOCK DIAGRAM AD8307 VPS SUPPLY 7.5mA BANDGAP REFERENCE AND BIASING SIX 14.3dB 900MHz AMPLIFIER STAGES MIRROR 3 NINE DETECTOR CELLS SPACED 14.3dB COMMON COM INPUT-OFFSET COMPENSATION LOOP 2 12.5k COM OFS OFS. ADJ. 2A /dB OUT OUTPUT ENB ENABLE INT +INPUT -INPUT INP 1.15k INM INT. ADJ PRODUCT DESCRIPTION The AD8307 is the first logarithmic amplifier in an 8-lead (SO-8) package. It is a complete 500 MHz monolithic demodulating logarithmic amplifier based on the progressive compression (successive detection) technique, providing a dynamic range of 92 dB to 3 dB law-conformance and 88 dB to a tight 1 dB error bound at all frequencies up to 100 MHz. It is extremely stable and easy to use, requiring no significant external components. A single supply voltage of 2.7 V to 5.5 V at 7.5 mA is needed, corresponding to an unprecedented power consumption of only 22.5 mW at 3 V. A fast-acting CMOS-compatible control pin can disable the AD8307 to a standby current of under 150 A. Each of the cascaded amplifier/limiter cells has a small-signal gain of 14.3 dB, with a -3 dB bandwidth of 900 MHz. The input is fully differential and at a moderately high impedance (1.1 k in parallel with about 1.4 pF). The AD8307 provides a basic dynamic range extending from approximately -75 dBm (where dBm refers to a 50 source, that is, a sine amplitude of about 56 V) up to +17 dBm (a sine amplitude of 2.2 V). A simple input-matching network can lower this range to -88 dBm to +3 dBm. The logarithmic linearity is typically within 0.3 dB up to 100 MHz over the central portion of this range, and is degraded only slightly at 500 MHz. There is no minimum frequency limit; the AD8307 may be used at audio frequencies (20 Hz) or even lower. The output is a voltage scaled 25 mV/dB, generated by a current of nominally 2 A/dB through an internal 12.5 k resistor. This voltage varies from 0.25 V at an input of -74 dBm (that is, the ac intercept is at -84 dBm, a 20 V rms sine input), up to 2.5 V for an input of +16 dBm. This slope and intercept can be trimmed using external adjustments. Using a 2.7 V supply, the output scaling may be lowered, for example to 15 mV/dB, to permit utilization of the full dynamic range. The AD8307 exhibits excellent supply insensitivity and temperature stability of the scaling parameters. The unique combination of low cost, small size, low power consumption, high accuracy and stability, very high dynamic range, and a frequency range encompassing audio through IF to UHF, make this product useful in numerous applications requiring the reduction of a signal to its decibel equivalent. The AD8307 is available in the industrial temperature range of -40C to +85C, and in 8-lead SOIC and PDIP packages. REV. A Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 1999 AD8307-SPECIFICATIONS (V = +5 V, T = 25 C, R 1 M S A L , unless otherwise noted) Min -72 Typ Max 16 1 27 27 -77 -76 Units dBm dB dB mV/dB mV/dB V dBm dBm nV/Hz dBm k pF ns ns MHz Hz MHz dB V V V V/C k pF A V mA A Parameter GENERAL CHARACTERISTICS Input Range ( 1 dB Error) Logarithmic Conformance Logarithmic Slope vs. Temperature Logarithmic Intercept vs. Temperature Input Noise Spectral Density Operating Noise Floor Output Resistance Internal Load Capacitance Response Time Conditions Expressed in dBm re 50 f 100 MHz, Central 80 dB f = 500 MHz, Central 75 dB Unadjusted1 Sine Amplitude; Unadjusted2 Equivalent Sine Power in 50 Inputs Shorted RSOURCE = 50 /2 Pin 4 to Ground Small Signal, 10%-90%, 0 mV-100 mV, CL = 2 pF Large Signal, 10%-90%, 0 V-2.4 V, CL = 2 pF Input AC-Coupled -3 dB 23 23 -87 -88 0.3 0.5 25 20 -84 1.5 -78 12.5 3.5 400 500 500 10 900 14.3 3.2 1.6 50 0.8 1.1 1.4 10 10 15 Upper Usable Frequency3 Lower Usable Frequency AMPLIFIER CELL CHARACTERISTICS Cell Bandwidth Cell Gain INPUT CHARACTERISTICS DC Common-Mode Voltage Common-Mode Range DC Input Offset Voltage4 Incremental Input Resistance Input Capacitance Bias Current POWER INTERFACES Supply Voltage Supply Current Disabled Inputs AC-Coupled Either Input (Small Signal) RSOURCE 50 Drift Differential Either Pin to Ground Either Input -0.3 VS - 1 500 25 5.5 10 750 VENB 2 V VENB 1 V 2.7 8 150 NOTES 1 This may be adjusted downward by adding a shunt resistor from the Output to Ground. A 50 k resistor will reduce the nominal slope to 20 mV/dB. 2 This may be adjusted in either direction by a voltage applied to Pin 5, with a scale factor of 8 dB/V. 3 See Application on 900 MHz operation. 4 Normally nulled automatically by internal offset correction loop. May be manually nulled by a voltage applied between Pin 3 and Ground; see APPLICATIONS. Specifications subject to change without notice. -2- REV. A AD8307 ABSOLUTE MAXIMUM RATINGS* Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +7.5 V Input Voltage (Pins 1, 8) . . . . . . . . . . . . . . . . . . . . . . VSUPPLY Storage Temperature Range, N, R . . . . . . . . -65C to +125C Ambient Temperature Range, Rated Performance Industrial, AD8307AN, AD8307AR . . . . . . . . . . . . . -40C to +85C Lead Temperature Range (Soldering 10 sec) . . . . . . . +300C *Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may effect device reliability. ORDERING GUIDE Model AD8307AR AD8307AN AD8307AR-REEL AD8307AR-REEL7 AD8307-EB Temperature Range -40C to +85C -40C to +85C Package Descriptions SOIC Plastic DIP 13" REEL 7" REEL Evaluation Board Package Options SO-8 N-8 CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD8307 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. WARNING! ESD SENSITIVE DEVICE PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS Pin INM 1 COM 2 8 Name INM COM OFS OUT INT ENB VPS INP Function Signal Input, Minus Polarity; Normally at VPOS/2. Common Pin (Usually Grounded). Offset Adjustment; External Capacitor Connection. Logarithmic (RSSI) Output Voltage; ROUT = 12.5 k. Intercept Adjustment; 6 dB (See Text). CMOS-compatible Chip Enable; Active when "HI." Positive Supply, 2.7 V-5.5 V. Signal Input, Plus Polarity; Normally at VPOS/ 2. Note: Due to the symmetrical nature of the response, there is no special significance to the sign of the two input pins. DC resistance from INP to INM = 1.1 k. INP 7 VPS AD8307 TOP VIEW OFS 3 (Not to Scale) 6 ENB 5 1 2 3 4 5 6 7 8 OUT 4 INT REV. A -3- AD8307-Typical Performance Characteristics 8 7 3 2 SUPPLY CURRENT - mA 6 1 ERROR - dB 5 4 3 2 TEMPERATURE ERROR @ +85 C 0 -1 TEMPERATURE ERROR @ +25 C TEMPERATURE ERROR @ -40 C -2 1 0 1.0 -3 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 -80 -60 -40 -20 0 20 VENB - Volts INPUT LEVEL - dBm Figure 1. Supply Current vs. VENB Voltage (5 V) Figure 4. Log Conformance vs. Input Level (dBm) at 25C, 85C, -40C 8 7 3 INPUT FREQUENCY 10MHz SUPPLY CURRENT - mA 6 VOUT - Volts 5 4 3 2 1 0 1.0 2 INPUT FREQUENCY 100MHz 1 INPUT FREQUENCY 300MHz INPUT FREQUENCY 500MHz 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 0 -80 -60 -40 -20 0 20 VENB - Volts INPUT LEVEL - dBm Figure 2. Supply Current vs. VENB Voltage (3 V) Figure 5. VOUT vs. Input Level (dBm) at Various Frequencies 3 1.5 2 1.0 1 FREQUENCY INPUT = 300MHz 0.5 ERROR - dB ERROR - dB CFO VALUE = 0.01 F 0 CFO VALUE = 1 F CFO VALUE = 0.1 F -1.0 0 -1 FREQUENCY INPUT = 100MHz -2 -0.5 -3 -80 -60 -40 -20 INPUT LEVEL - dBm 0 20 - 1.5 - 80 - 60 - 40 - 20 INPUT LEVEL - dBm 0 20 Figure 3. Log Conformance vs. Input Level (dBm) @ 100 MHz, 300 MHz Figure 6. Log Conformance vs. CFO Values at 1 kHz Input Frequency -4- REV. A AD8307 3.0 3 2.5 INT PIN = 3.0V 10MHz, INT = -96.52dBm 2 100MHz 1 2.0 VOUT - Volts ERROR - dB INT PIN = 4.0V 10MHz, INT = -87.71dBm 1.5 + INPUT 0 1.0 NO CONNECT ON INT 10MHz, INT = -82.90dBm -1 - INPUT 0.5 -2 0 -80 -70 -60 -50 -40 -30 -20 -10 INPUT LEVEL - dBm 0 10 20 -3 -80 -60 -40 -20 INPUT LEVEL - dBm 0 20 Figure 7. VOUT vs. Input Level at 5 V Supply; Showing Intercept Adjustment Figure 10. Log Conformance vs. Input Level at 100 MHz; Showing Response to Alternative Inputs 3.0 INT VOLTAGE INT = 1.0V, INT = -86dBm 3 2.5 2 500MHz 2.0 VOUT - Volts 1.5 ERROR - dB INT VOLTAGE INT NO CONNECT, INT = -71dBm 1 0 1.0 -1 100MHz 0.5 INT VOLTAGE INT = 2.0V, INT = -78dBm -2 0 -80 -70 -60 -50 -40 -30 -20 -10 0 10 -3 -90 -70 INPUT LEVEL - dBm -50 -30 -10 INPUT LEVEL - dBm 10 Figure 8. VOUT vs. Input Level at 3 V Supply Using AD820 as Buffer, Gain = +2; Showing Intercept Adjustment Figure 11. Log Conformance vs. Input at 100 MHz, 500 MHz; Input Driven Differentially Using Transformer 2.5 3 2.0 2 500MHz 1 ERROR - dB VOUT - Volts 1.5 100MHz @ -40 C 0 100MHz -1 10MHz 1.0 100MHz @ +25 C 0.5 100MHz @ +85 C 0 -80 -2 -60 -40 -20 INPUT LEVEL - dBm 0 20 -3 -70 -60 -50 -40 -30 -20 -10 0 10 20 INPUT LEVEL - dBm Figure 9. VOUT vs. Input Level at Three Temperatures (-40C, +25C, +85C) Figure 12. Log Conformance vs. Input Level at 3 V Supply Using AD820 as Buffer, Gain = +2 REV. A -5- AD8307 2V Ch1 200mV VOUT CH 1 Ch1 500mV VOUT CH 1 CH 1 GND VENB CH 2 INPUT SIGNAL CH 2 CH 2 GND GND Ch2 2.00V 500ns Ch2 1.00V 200ns Figure 13. Power-Up Response Time Figure 16. VOUT Rise Time VOUT CH 1 Ch1 200mV Ch1 500mV 2.5V VENB CH 2 INPUT SIGNAL CH 2 CH 2 GND VOUT CH 1 CH 1 GND GND Ch2 2.00V 500ns Ch2 1.00V 200ns Figure 14. Power-Down Response Time Figure 17. Large Signal Response Time VPS = +5.0V HP8648B SIGNAL GENERATOR 1nF RF OUT NC INP VPS ENB INT 52.3 0.1 F HP8112A PULSE GENERATOR OUT SYNCH OUT HP8648B 10MHz REF CLK SIGNAL GENERATOR PULSE MODE IN PULSE MODULATION VPS = +5.0V MODE RF OUT 1nF 0.1 F EXT TRIG OUT HP8112A PULSE GENERATOR TRIG OUT NC AD8307 INM COM OFS OUT NC 1nF NC = NO CONNECT TEK P6139A 10x PROBE TEK744A SCOPE 52.3 INP VSP ENB INT AD8307 INM COM OFS OUT TRIG 1nF NC TEK P6204 FET PROBE TEK744A SCOPE TRIG NC = NO CONNECT Figure 15. Test Setup For Power-Up/Power-Down Response Time Figure 18. Test Setup For VOUT Pulse Response -6- REV. A AD8307 LOG AMP THEORY Logarithmic amplifiers perform a more complex operation than that of classical linear amplifiers, and their circuitry is significantly different. A good grasp of what log amps do, and how they do it, will avoid many pitfalls in their application. The essential purpose of a log amp is not to amplify, though amplification is utilized to achieve the function. Rather, it is to compress a signal of wide dynamic range to its decibel equivalent. It is thus a measurement device. A better term might be logarithmic converter, since its basic function is the conversion of a signal from one domain of representation to another, via a precise nonlinear transformation. Logarithmic compression leads to situations that may be confusing or paradoxical. For example, a voltage offset added to the output of a log amp is equivalent to a gain increase ahead of its input. In the usual case where all the variables are voltages, and regardless of the particular structure, the relationship between the variables can be expressed as: VOUT = VY log (VIN /VX) where: VOUT is the output voltage, VY is called the slope voltage; the logarithm is usually taken to base-ten (in which case VY is also the volts-per-decade), VIN is the input voltage, and VX is called the intercept voltage. All log amps implicitly require two references, here VX and VY, which determine the scaling of the circuit. The absolute accuracy of a log amp cannot be any better than the accuracy of its scaling references. Equation 1 is mathematically incomplete in representing the behavior of a demodulating log amp such as the AD8307, where VIN has an alternating sign. However, the basic principles are unaffected, and we can safely use this as our starting point in the analyses of log amp scaling which follow. VOUT 5VY 4VY VSHIFT 3VY 2VY VY LOG VIN VOUT = 0 VIN = 10-2VX -40dBc VIN = VX 0dBc VIN = 102VX +40dBc VIN = 104VX +80dBc LOWER INTERCEPT continue indefinitely in both directions. The dotted line shows that the effect of adding an offset voltage VSHIFT to the output is to lower the effective intercept voltage VX. Exactly the same alteration could be achieved raising the gain (or signal level) ahead of the log amp by the factor VSHIFT/VY. For example, if VY is 500 mV per decade (that is, 25 mV/dB, as for the AD8307), an offset of +150 mV added to the output will appear to lower the intercept by two tenths of a decade, or 6 dB. Adding an offset to the output is thus indistinguishable from applying an input level that is 6 dB higher. The log amp function described by Equation 1 differs from that of a linear amplifier in that the incremental gain VOUT/VIN is a very strong function of the instantaneous value of VIN, as is apparent by calculating the derivative. For the case where the logarithmic base is e, we have: VOUT VIN = VY VIN (2) (1) That is, the incremental gain is inversely proportional to the instantaneous value of the input voltage. This remains true for any logarithmic base, which is chosen as 10 for all decibelrelated purposes. It follows that a perfect log amp would be required to have infinite gain under classical small-signal (zeroamplitude) conditions. Less ideally, this result indicates that, whatever means are used to implement a log amp, accurate response under small-signal conditions (that is, at the lower end of the dynamic range) demands the provision of a very high gain-bandwidth product. A further consequence of this high gain is that, in the absence of an input signal, even very small amounts of thermal noise at the input of a log amp will cause a finite output for zero input, resulting in the response line curving away from the ideal shown in Figure 19 toward a finite baseline, which can be either above or below the intercept. Note that the value given for this intercept may be an extrapolated value, in which case the output may not cross zero, or even reach it, as is the case for the AD8307. While Equation 1 is fundamentally correct, a simpler formula is appropriate for specifying the calibration attributes of a log amp like the AD8307, which demodulates a sine wave input: VOUT = VSLOPE (PIN - P0) where: VOUT is the demodulated and filtered baseband (video or RSSI) output, VSLOPE is the logarithmic slope, now expressed in volts/dB (typically between 15 and 30 mV/dB), PIN is the input power, expressed in decibels relative to some reference power level, and P0 is the logarithmic intercept, expressed in decibels relative to the same reference level. The most widely used reference in RF systems is decibels above 1 mW in 50 , written dBm. Note that the quantity (PIN - P0) is just dB. The logarithmic function disappears from the formula because the conversion has already been implicitly performed in stating the input in decibels. This is strictly a concession to popular convention: log amps manifestly do not respond to power (tacitly, power absorbed at the input), but, rather, to input -7- (3) -2VY Figure 19. Ideal Log Amp Function Figure 19 shows the input/output relationship of an ideal log amp, conforming to Equation 1. The horizontal scale is logarithmic and spans a wide dynamic range, shown here as over 120 dB, or six decades. The output passes through zero (the log-intercept) at the unique value VIN = VX and would ideally become negative for inputs below the intercept. In the ideal case, the straight line describing VOUT for all values of VIN would REV. A AD8307 voltage. The use of dBV (decibels with respect to 1 V rms) would be more precise, though still incomplete, since waveform is involved, too. Since most users think about and specify RF signals in terms of power--even more specifically, in dBm re 50 --we will use this convention in specifying the performance of the AD8307. Progressive Compression in the case of the AD8307, VY is traceable to an on-chip bandgap reference, while VX is derived from the thermal voltage kT/q and later temperature-corrected. Let the input of an N-cell cascade be VIN, and the final output VOUT. For small signals, the overall gain is simply AN. A sixstage system in which A = 5 (14 dB) has an overall gain of 15,625 (84 dB). The importance of a very high small-signal gain in implementing the logarithmic function has been noted; however, this parameter is of only incidental interest in the design of log amps. From here onward, rather than considering gain, we will analyze the overall nonlinear behavior of the cascade in response to a simple dc input, corresponding to the VIN of Equation 1. For very small inputs, the output from the first cell is V1 = AVIN; from the second, V2 = A2 VIN, and so on, up to VN = AN VIN. At a certain value of VIN, the input to the Nth cell, VN-1, is exactly equal to the knee voltage EK. Thus, VOUT = AEK and since there are N-1 cells of gain A ahead of this node, we can calculate that VIN = EK /AN-1. This unique situation corresponds to the lin-log transition, labeled 1 on Figure 22. Below this input, the cascade of gain cells is acting as a simple linear amplifier, while for higher values of VIN, it enters into a series of segments which lie on a logarithmic approximation (dotted line). VOUT Most high speed high dynamic range log amps use a cascade of nonlinear amplifier cells (Figure 20) to generate the logarithmic function from a series of contiguous segments, a type of piecewise-linear technique. This basic topology immediately opens up the possibility of enormous gain-bandwidth products. For example, the AD8307 employs six cells in its main signal path, each having a small-signal gain of 14.3 dB (x5.2) and a -3 dB bandwidth of about 900 MHz; the overall gain is about 20,000 (86 dB) and the overall bandwidth of the chain is some 500 MHz, resulting in the incredible gain-bandwidth product (GBW) of 10,000 GHz, about a million times that of a typical op amp. This very high GBW is an essential prerequisite to accurate operation under small-signal conditions and at high frequencies. Equation 2 reminds us, however, that the incremental gain will decrease rapidly as VIN increases. The AD8307 continues to exhibit an essentially logarithmic response down to inputs as small as 50 V at 500 MHz. STAGE 1 STAGE 2 STAGE N -1 STAGE N VX A A A A VW (4A-3) EK (3A-2) EK (A-1) EK (2A-1) EK AEK 0 EK/AN-1 EK/AN-2 EK/AN-3 EK/AN-4 Figure 20. Cascade of Nonlinear Gain Cells To develop the theory, we will first consider a slightly different scheme to that employed in the AD8307, but which is simpler to explain and mathematically more straightforward to analyze. This approach is based on a nonlinear amplifier unit, which we may call an A/1 cell, having the transfer characteristic shown in Figure 21. The local small-signal gain VOUT/VIN is A, maintained for all inputs up to the knee voltage EK, above which the incremental gain drops to unity. The function is symmetrical: the same drop in gain occurs for instantaneous values of VIN less than -EK. The large-signal gain has a value of A for inputs in the range -EK VIN +EK, but falls asymptotically toward unity for very large inputs. In logarithmic amplifiers based on this amplifier function, both the slope voltage and the intercept voltage must be traceable to the one reference voltage, EK. Therefore, in this fundamental analysis, the calibration accuracy of the log amp is dependent solely on this voltage. In practice, it is possible to separate the basic references used to determine VY and VX and RATIO OF A LOG VIN Figure 22. The First Three Transitions OUTPUT AEK A/1 SLOPE = 1 SLOPE = A 0 Continuing this analysis, we find that the next transition occurs when the input to the (N-1) stage just reaches EK; that is, when VIN = EK /AN-2. The output of this stage is then exactly AEK, and it is easily demonstrated (from the function shown in Figure 21) that the output of the final stage is (2A-1) EK (labeled on Figure 22). Thus, the output has changed by an amount (A-1)EK for a change in VIN from EK /AN-1 to EK /AN-2, that is, a ratio change of A. At the next critical point, labeled , we find the input is again A times larger and VOUT has increased to (3A-2)EK, that is, by another linear increment of (A-1)EK. Further analysis shows that right up to the point where the input to the first cell is above the knee voltage, VOUT changes by (A-1)EK for a ratio change of A in VIN. This can be expressed as a certain fraction of a decade, which is simply log10(A). For example, when A = 5 a transition in the piecewise linear output function occurs at regular intervals of 0.7 decade (that is, log10(A), or 14 dB divided by 20 dB). This insight allows us to immediately write the Volts per Decade scaling parameter, which is also the Scaling Voltage VY, when using base-10 logarithms, as: EK INPUT VY = Linear Change in VOUT Decades Change in VIN = Figure 21. The A/1 Amplifier Function ( A - 1)E log ( A) 10 K (4) -8- REV. A AD8307 Note that only two design parameters are involved in determining VY, namely, the cell gain A and the knee voltage EK, while N, the number of stages, is unimportant in setting the slope of the overall function. For A = 5 and EK = 100 mV, the slope would be a rather awkward 572.3 mV per decade (28.6 mV/dB). A well designed log amp will have rational scaling parameters. The intercept voltage can be determined by using two pairs of transition points on the output function (consider Figure 22). The result is: VX = N +1/ ( A-1) ) A( VY = A EK log 10 A () (6) EK (5) For the case under consideration, using N = 6, we calculate VZ = 4.28 V. However, we need to be careful about the interpretation of this parameter, since it was earlier defined as the input voltage at which the output passes through zero (see Figure 19). But clearly, in the absence of noise and offsets, the output of the amplifier chain shown in Figure 21 can be zero when, and only when, VIN = 0. This anomaly is due to the finite gain of the cascaded amplifier, which results in a failure to maintain the logarithmic approximation below the lin-log transition (point in Figure 22). Closer analysis shows that the voltage given by Equation 5 represents the extrapolated, rather than actual, intercept. Demodulating Log Amps Preference for the A/0 style of log amp, over one using A/1 cells, stems from several considerations. The first is that an A/0 cell can be very simple. In the AD8307 it is based on a bipolartransistor differential pair, having resistive loads RL and an emitter current source, IE. This will exhibit an equivalent kneevoltage of EK = 2 kT/q and a small signal gain of A = IERL /EK. The large signal transfer function is the hyperbolic tangent (see dotted line in Figure 23). This function is very precise, and the deviation from an ideal A/0 form is not detrimental. In fact, the rounded shoulders of the tanh function beneficially result in a lower ripple in the logarithmic conformance than that obtained using an ideal A/0 function. An amplifier built of these cells is entirely differential in structure and can thus be rendered very insensitive to disturbances on the supply lines and, with careful design, to temperature variations. The output of each gain cell has an associated transconductance (gm) cell, which converts the differential output voltage of the cell to a pair of differential currents, which are summed simply by connecting the outputs of all the gm (detector) stages in parallel. The total current is then converted back to a voltage by a transresistance stage, to generate the logarithmic output. This scheme is depicted, in single-sided form, in Figure 24. AVIN VIN gm A/0 A/0 A2 VIN A/0 A3 VIN A/0 A4 VIN VLIM gm IOUT Log amps based on a cascade of A/1 cells are useful in baseband applications, because they do not demodulate their input signal. However, baseband and demodulating log amps alike can be made using a different type of amplifier stage, which we will call an A/0 cell. Its function differs from that of the A/1 cell in that the gain above the knee voltage EK falls to zero, as shown by the solid line in Figure 23. This is also known as the limiter function, and a chain of N such cells is often used to generate a hard-limited output, in recovering the signal in FM and PM modes. gm gm gm Figure 24. Log Amp Using A/0 Stages and Auxiliary Summing Cells SLOPE = 0 AEK OUTPUT A/0 TANH SLOPE = A 0 EK INPUT Figure 23. A/0 Amplifier Functions (Ideal and Tanh) The chief advantage of this approach is that the slope voltage may now be decoupled from the knee-voltage EK = 2 kT/q, which is inherently PTAT. By contrast, the simple summation of the cell outputs would result in a very high temperature coefficient of the slope voltage given by Equation 6. To do this, the detector stages are biased with currents (not shown in the Figure) which are rendered stable with temperature. These are derived either from the supply voltage (as in the AD606 and AD608) or from an internal bandgap reference (as in the AD640 and AD8307). This topology affords complete control over the magnitude and temperature behavior of the logarithmic slope, decoupling it completely from EK. A further step is yet needed to achieve the demodulation response, required when the log amp is to convert an alternating input into a quasi-dc baseband output. This is achieved by altering the gm cells used for summation purposes to also implement the rectification function. Early discrete log amps based on the progressive compression technique used half-wave rectifiers. This made post-detection filtering difficult. The AD640 was the first commercial monolithic log amp to use a full-wave rectifier, a practice followed in all subsequent Analog Devices types. The AD640, AD606, AD608, AD8307 and various other Analog Devices communications products incorporating a logarithmic IF amplifier all use this technique. It will be apparent that the output of the last stage can no longer provide the logarithmic output, since this remains unchanged for all inputs above the limiting threshold, which occurs at VIN = EK /AN-1. Instead, the logarithmic output is now generated by summing the outputs of all the stages. The full analysis for this type of log amp is only slightly more complicated than that of the previous case. It is readily shown that, for practical purpose, the intercept voltage VX is identical to that given in Equation 5, while the slope voltage is: REV. A -9- AD8307 We can model these detectors as being essentially linear gm cells, but producing an output current independent of the sign of the voltage applied to the input of each cell. That is, they implement the absolute-value function. Since the output from the later A/0 stages closely approximates an amplitude-symmetric square wave for even moderate input levels (most stages of the amplifier chain operate in a limiting mode), the current output from each detector is almost constant over each period of the input. Somewhat earlier detectors stages produce a waveform having only very brief dropouts, while the detectors nearest the input produce a low level almost-sinusoidal waveform at twice the input frequency. These aspects of the detector system result in a signal that is easily filtered, resulting in low residual ripple on the output. Intercept Calibration Offset Control In a monolithic log amp, direct-coupling between the stages is used for several reasons. First, this avoids the use of coupling capacitors, which may typically have a chip area equal to that of a basic gain cell, thus considerably increasing die size. Second, the capacitor values predetermine the lowest frequency at which the log amp can operate; for moderate values, this may be as high as 30 MHz, limiting the application range. Third, the parasitic (back-plate) capacitance lowers the bandwidth of the cell, further limiting the applications. But the very high dc gain of a direct-coupled amplifier raises a practical issue. An offset voltage in the early stages of the chain is indistinguishable from a `real' signal. If it were as high as, say, 400 V, it would be 18 dB larger than the smallest ac signal (50 V), potentially reducing the dynamic range by this amount. This problem is averted by using a global feedback path from the last stage to the first, which corrects this offset in a similar fashion to the dc negative feedback applied around an op amp. The high frequency components of the signal must, of course, be removed, to prevent a reduction of the HF gain in the forward path. In the AD8307, this is achieved by an on-chip filter, providing sufficient suppression of HF feedback to allow operation above 1 MHz. To extend the range below this frequency, an external capacitor may be added. This permits the high pass corner to be lowered to audio frequencies using a capacitor of modest value. Note that this capacitor has no effect on the minimum signal frequency for input levels above the offset voltage: this extends down to dc (for a signal applied directly to the input pins). The offset voltage will vary from part to part; some will exhibit essentially stable offsets of under 100 V, without the benefit of an offset adjustment. Extension of Range All monolithic log amps from Analog Devices include accurate means to position the intercept voltage VX (or equivalent power for a demodulating log amp). Using the scheme shown in Figure 24, the basic value of the intercept level departs considerably from that predicted by the simpler analyses given earlier. However, the intrinsic intercept voltage is still proportional to EK, which is PTAT (Equation 5). Recalling that the addition of an offset to the output produces an effect which is indistinguishable from a change in the position of the intercept, we can cancel the left-right motion of VX resulting from the temperature variation of EK by adding an offset having the required temperature behavior. The precise temperature-shaping of the intercept-positioning offset results in a log amp having stable scaling parameters, making it a true measurement device, for example, as a calibrated Received Signal Strength Indicator (RSSI). In this application, one is more interested in the value of the output for an input waveform which is invariably sinusoidal. The input level may alternatively be stated as an equivalent power, in dBm, but here we must step carefully. It is essential to know the load impedance in which this power is presumed to be measured. In RF practice, it is generally safe to assume a reference impedance of 50 , in which 0 dBm (1 mW) corresponds to a sinusoidal amplitude of 316.2 mV (223.6 mV rms). The intercept may likewise be specified in dBm. For the AD8307, it is positioned at -84 dBm, corresponding to a sine amplitude of 20 V. It is important to bear in mind that log amps do not respond to power, but to the voltage applied to their input. The AD8307 presents a nominal input impedance much higher than 50 (typically 1.1 k at low frequencies). A simple input matching network can considerably improve the sensitivity of this type of log amp. This will increase the voltage applied to the input and thus alter the intercept. For a 50 match, the voltage gain is 4.8 and the whole dynamic range moves down by 13.6 dB (see Figure 33). Note that the effective intercept is a function of waveform. For example, a square-wave input will read 6 dB higher than a sine wave of the same amplitude, and a Gaussian noise input 0.5 dB higher than a sine wave of the same rms value. The theoretical dynamic range for the basic log amp shown in Figure 24 is AN. For A = 5.2 (14.3 dB) and N = 6, it is 20,000 or 86 dB. The actual lower end of the dynamic range is largely determined by the thermal noise floor, measured at the input of the chain of amplifiers. The upper end of the range is extended upward by the addition of top-end detectors. The input signal is applied to a tapped attenuator, and progressively smaller signals are applied to three passive rectifying gm cells whose outputs are summed with those of the main detectors. With care in design, the extension to the dynamic range can be seamless over the full frequency range. For the AD8307 it amounts to a further 27 dB. The total dynamic range is thus theoretically 113 dB. The specified range of 90 dB (-74 dBm to +16 dBm) is that for high accuracy, calibrated operation, and includes the low end degradation due to thermal noise, and the top end reduction due to voltage limitations. The additional stages are not, however, redundant, but are needed to maintain accurate logarithmic conformance over the central region of the dynamic range, and in extending the usable range considerably beyond the specified range. In applications where log-conformance is less demanding, the AD8307 can provide over 95 dB of range. -10- REV. A AD8307 PRODUCT OVERVIEW The AD8307 comprises six main amplifier/limiter stages, each having a gain of 14.3 dB and small signal bandwidth of 900 MHz; the overall gain is 86 dB with a -3 dB bandwidth of 500 MHz. These six cells, and their associated gm-styled full-wave detectors, handle the lower two-thirds of the dynamic range. Three top-end detectors, placed at 14.3 dB taps on a passive attenuator, handle the upper third of the 90 dB range. Biasing for these cells is provided by two references: one determines their gain; the other is a bandgap circuit that determines the logarithmic slope and stabilizes it against supply- and temperature-variations. The AD8307 may be enabled/disabled by a CMOS-compatible level at ENB (Pin 6). The first amplifier stage provides a low voltage noise spectral density (1.5 nV/Hz). The differential current-mode outputs of the nine detectors are summed and then converted to single-sided form in the output stage, nominally scaled 2 A/dB. The logarithmic output voltage is developed by applying this current to an on-chip 12.5 k resistor, resulting in a logarithmic slope of 25 mV/dB (i.e., 500 mV/decade) at OUT. This voltage is not buffered, allowing the use of a variety of special output interfaces, including the addition of post-demodulation filtering. The last detector stage includes a modification to temperature-stabilize the log intercept, which is accurately positioned to make optimal use of the full output voltage range available. The intercept may be adjusted using the pin INT, which adds or subtracts a small current to the signal current. AD8307 VPS SUPPLY 7.5mA BAND GAP REFERENCE AND BIASING SIX 14.3dB 900MHz AMPLIFIER STAGES MIRROR 3 NINE DETECTOR CELLS SPACED 14.3dB COMMON COM INPUT- OFFSET COMPENSATION LOOP 2 2A /dB OUT OUTPUT 12.5k COM OFS COM OFS. ADJ. ENB junction capacitances associated with them, due to active devices or ESD protection; these may be neither accurate nor stable. Component numbering in each of these interface diagrams is local. Enable Interface The chip-enable interface is shown in Figure 26. The currents in the diode-connected transistors control the turn-on and turnoff states of the bandgap reference and the bias generator, and are a maximum of 100 A when Pin 6 is taken to 5 V, under worst case conditions. Left unconnected, or at a voltage below 1 V, the AD8307 will be disabled and consume a sleep current of under 50 A; tied to the supply, or a voltage above 2 V, it will be fully enabled. The internal bias circuitry is very fast (typically <100 ns for either OFF or ON), and in practice the latency period before the log amp exhibits its full dynamic range is more likely to be limited by factors relating to the use of ac coupling at the input or the settling of the offset-control loop (see following sections). AD8307 TO BIAS STAGES 40k ENB COM Figure 26. Enable Interface VPS ENABLE S INT 125 INT. ADJ +INPUT -INPUT INP 1.1k INM 6k COM CP INP CD INM CM COM TYP +2.2V FOR +3V SUPPLY, +3.2V AT +5V S 4k TOP-END DETECTORS ~3k Q2 2k 6k Q1 IE 2.4mA Figure 25. Main Features of the AD8307 The last gain stage also includes an offset-sensing cell. This generates a bipolarity output current when the main signal path has an imbalance due to accumulated dc offsets. This current is integrated by an on-chip capacitor (which may be increased in value by an off-chip component, at OFS). The resulting voltage is used to null the offset at the output of the first stage. Since it does not involve the signal input connections, whose ac coupling capacitors otherwise introduce a second pole in the feedback path, the stability of the offset correction loop is assured. The AD8307 is built on an advanced dielectrically-isolated complementary bipolar process. Most resistors are thin-film types having a low temperature coefficient of resistance (TCR) and high linearity under large signal conditions. Their absolute tolerance will typically be within 20%. Similarly, the capacitors have a typical tolerance of 15% and essentially zero temperature or voltage sensitivity. Most interfaces have additional small COM Figure 27. Signal Input Interface Input Interface Figure 27 shows the essentials of the signal input interface. CP and CM are the parasitic capacitances to ground; CD is the differential input capacitance, mostly due to Q1 and Q2. In most applications both input pins are ac-coupled. The switches S close when Enable is asserted. When disabled, the inputs float, bias current IE is shut off, and the coupling capacitors remain charged. If the log amp is disabled for long periods, small leakage currents will discharge these capacitors. If they are poorly matched, charging currents at power-up can generate a transient input voltage which may block the lower reaches of the dynamic range until it has become much less than the signal. REV. A -11- AD8307 In most applications, the signal will be single-sided, and may be applied to either Pin 1 or Pin 8, with the other pin ac-coupled to ground. Under these conditions, the largest input signal that can be handled by the AD8307 is +10 dBm (sine amplitude of 1 V) when operating from a 3 V supply; a +16 dBm may be handled using a 5 V supply. The full 16 dBm may be achieved for supplies down to 2.7 V, using a fully balanced drive. For frequencies above about 10 MHz, this is most easily achieved using a matching network (see below). Using such a network, having an inductor at the input, the input transient noted above is eliminated. Occasionally, it may be desirable to use the dc-coupled potential of the AD8307. The main challenge here is to present signals to the log amp at the elevated common-mode input level, requiring the use of low noise, low offset buffer amplifiers. Using dual supplies of 3 V, the input pins may operate at ground potential. Offset Interface The offset feedback is limited to a range 400 V; signals larger than this override the offset control loop, which only impacts performance for very small inputs. An external capacitor reduces the high pass corner to arbitrarily low frequencies; using 1 F this corner is below 10 Hz. All ADI log amps use an offsetnulling loop; the AD8307 differs in using this single-sided form. Output Interface The input-referred dc offsets in the signal path are nulled via the interface associated with Pin 3, shown in Figure 28. Q1 and Q2 are the first stage input transistors, with their corresponding load resistors (125 ). Q3 and Q4 generate small currents, which can introduce a dc offset into the signal path. When the voltage on OFS is at about 1.5 V, these currents are equal, and nominally 16 A. When OFS is taken to ground, Q4 is off and the effect of the current in Q3 is to generate an offset voltage of 16 A x 125 = 2 mV. Since the first stage gain is x5, this is equivalent to a input offset (INP to INM) of 400 V. When OFS is taken to its most positive value, the input-referred offset is reversed, to -400 V. If true dc-coupling is needed, down to very small inputs, this automatic loop must be disabled, and the residual offset eliminated using a manual adjustment, as explained in the next section. In normal operation, however, using an ac-coupled input signal, the OFS pin should be left open. Any residual input-offset voltage is then automatically nulled by the action of the feedback loop. The gm cell, which is gated off when the chip is disabled, converts any output offset (sensed at a point near the end of the cascade of amplifiers) to a current. This is integrated by the onchip capacitor CHP, plus any added external capacitance COFS, so as to generate an error voltage, which is applied back to the input stage in the polarity needed to null the output offset. From a small-signal perspective, this feedback alters the response of the amplifier, which, rather than behaving as a fully dc-coupled system, now exhibits a zero in its ac transfer function, resulting in a closed-loop high-pass corner at about 700 kHz. VPS INPUT STAGE 125 The outputs from the nine detectors are differential currents, having an average value that is dependent on the signal input level, plus a fluctuation at twice the input frequency. The currents are summed at nodes LGP and LGN in Figure 29. Further currents are added at these nodes, to position the intercept, by slightly raising the output for zero input, and to provide temperature compensation. Since the AD8307 is not lasertrimmed, there is a small uncertainty in both the log slope and the log intercept. These scaling parameters may be adjusted (see below). For zero-signal conditions, all the detector output currents are equal. For a finite input, of either polarity, their difference is converted by the output interface to a single-sided unipolar current nominally scaled 2 A/dB (40 A/decade), at the output pin OUT. An on-chip 12.5 k resistor, R1, converts this current to a voltage of 25 mV/dB. C1 and C2 are effectively in shunt with R1 and form a low-pass filter pole, with a corner frequency of about 5 MHz. The pulse response settles to within 1% of the final value within 300 ns. This integral low-pass filter provides adequate smoothing in many IF applications. At 10.7 MHz, the 2f ripple is 12.5 mV in amplitude, equivalent to 0.5 dB, and only 0.5 mV ( 0.02 dB) at f = 50 MHz. A filter capacitor CFLT added from OUT to ground will lower this corner frequency. Using 1 F, the ripple is maintained to less than 0.5 dB down to input frequencies of 100 Hz. Note that COFS (above) should also be increased in low frequency applications, and will typically be made equal to CFLT. It may be desirable to increase the speed of the output response, with the penalty of increased ripple. One way to do this is simply by connecting a shunt load resistor from OUT to ground, which raises the low pass corner frequency. This also alters the logarithmic slope, for example to 7.5 mV/dB using a 5.36 k resistor, while reducing the 10%-90% rise time to 25 ns. The ripple amplitude for 50 MHz input remains 0.5 mV, but this is now equivalent to 0.07 dB. If a negative supply is available, the output pin may be connected directly to the summing node of an external op amp connected as an inverting-mode transresistance stage. VPS MAIN GAIN STAGES Q1 Q2 16 A AT BALANCE S gm TO LAST DETECTOR 3pF LGP LGM FROM ALL DETECTORS 2 A/dB 0-220 A 25mV/dB 1.25k 1.25k 400mV 8.25k 60k INT 1.25k 1.25k BIAS, 1.2V OFS Q3 36k Q4 48k COFS AVERAGE ERROR CURRENT CHP COM OUT CFLT C1 2.5pF C2 1pF R1 12.5k BIAS Figure 28. Offset Interface and Offset-Nulling Path 60 A COM Figure 29. Simplified Output Interface -12- REV. A AD8307 USING THE AD8307 OUTPUT VOLTAGE - Volts The AD8307 has very high gain and a bandwidth from dc to over 1 GHz, at which frequency the gain of the main path is still over 60 dB. Consequently, it is susceptible to all signals within this very broad frequency range that find their way to the input terminals. It is important to remember that these are quite indistinguishable from the "wanted" signal, and will have the effect of raising the apparent noise floor (that is, lowering the useful dynamic range). For example, while the signal of interest may be an IF of 50 MHz, any of the following could easily be larger than the IF signal at the lower extremities of its dynamic range: 60 Hz hum, picked up due to poor grounding techniques; spurious coupling from a digital clock source on the same PC board; local radio stations; etc. Careful shielding is essential. A ground plane should be used to provide a low impedance connection to the common pin COM, for the decoupling capacitor(s) used at VPS, and as the output ground. It is inadvisable to assume that the ground plane is an equipotential, however, and neither of the inputs should be accoupled directly to the ground plane, but kept separate from it, being returned instead to the low associated with the source. This may mean isolating the low side of an input connector with a small resistance to the ground plane. Basic Connections Figure 31 shows the output versus the input level, in dBm when driven from a terminated 50 generator, for sine inputs at 10 MHz, 100 MHz and 500 MHz; Figure 32 shows the typical logarithmic conformance under the same conditions. Note that +10 dBm corresponds to a sine amplitude of 1 V, equivalent to an rms power of 10 mW in a 50 termination. But if the termination resistor is omitted, the input power is negligible. The use of dBm to define input level therefore needs to be considered carefully in connection with the AD8307. 3 2.5 10MHz 2 1.5 100MHz 1 500MHz 0.5 0 -80 -70 -60 -50 Figure 30 shows the simple connections suitable for many applications. The inputs are ac-coupled by C1 and C2, which should have the same value, say, CC. The coupling time-constant is RIN CC/2, thus forming a high pass corner with a 3 dB attenuation at fHP = 1/(p RIN CC ). In high frequency applications, fHP should be as large as possible, in order to minimize the coupling of unwanted low frequency signals. Conversely, in low frequency applications, a simple RC network forming a low-pass filter should be added at the input for the same reason. For the case where the generator is not terminated, the signal range should be expressed in terms of the voltage response, and extends from -85 dBV to +6 dBV. 0.1 F 4.7 VP, 2.7V - 5.5V AT 8mA C1 = CC NC INPUT -75dBm TO +16dBm RIN 1.1k INP VPS ENB INT -40 -30 -20 -10 INPUT LEVEL - dBm 0 10 20 Figure 31. Log Response at 10 MHz, 100 MHz and 500 MHz 5 4 3 2 500MHz ERROR - dB 1 0 -1 -2 -3 -4 -5 -80 -70 -60 -50 -40 -30 -20 -10 INPUT LEVEL - dBm 0 10 20 10MHz 100MHz RT AD8307 INM COM OFS OUT NC OUTPUT 25mV/dB Figure 32. Logarithmic Law Conformance at 10 MHz, 100 MHz and 500 MHz Input Matching C2 = CC NC = NO CONNECT Figure 30. Basic Connections Where it is necessary to terminate the source at a low impedance, the resistor RT should be added, with allowance for the shunting effect of the basic 1.1 k input resistance (RIN) of the AD8307. For example, to terminate a 50 source a 52.3 1% tolerance resistor should be used. This may be placed on the input side or the log-amp side of the coupling capacitors; in the former case, smaller capacitors can be used for a given frequency range; in the latter case, the effective RIN is lowered directly at the log-amp inputs. Where higher sensitivity is required, an input matching network is valuable. Using a transformer to achieve the impedance transformation also eliminates the need for coupling capacitors, lowers the offset voltage generated directly at the input, and balances the drives to INP and INM. The choice of turns ratio will depend somewhat on the frequency. At frequencies below 50 MHz, the reactance of the input capacitance is much higher than the real part of the input impedance. In this frequency range, a turns ratio of about 1:4.8 will lower the input impedance to 50 while raising the input voltage, and thus lowering the effect of the short circuit noise voltage by the same factor. There will be a small contribution from the input noise current, so the total noise will be reduced by a somewhat smaller factor. The intercept will also be lowered by the turns ratio; for a 50 match, it will be reduced by 20 log10 (4.8) or 13.6 dB. -13- REV. A AD8307 Narrow-Band Matching Table I. Narrow-Band Matching Values Transformer coupling is useful in broadband applications. However, a magnetically-coupled transformer may not be convenient in some situations. At high frequencies, it is often preferable to use a narrow-band matching network, as shown in Figure 33. This has several advantages. The same voltage gain is achieved, providing increased sensitivity, but now a measure of selectively is also introduced. The component count is low: two capacitors and an inexpensive chip inductor. Further, by making these capacitors unequal the amplitudes at INP and INM may be equalized when driving from a single-sided source; that is, the network also serves as a balun. Figure 34 shows the response for a center frequency of 100 MHz; note the very high attenuation at low frequencies. The high-frequency attenuation is due to the input capacitance of the log amp. 0.1 F 4.7 VP, 2.7V - 5.5V AT 8mA 50 INPUT -88dBm TO +3dBm C1 NC INP VPS ENB INT ZIN = 50 LM FC MHz 10 20 50 100 150 200 250 500 10 20 50 100 150 200 250 500 ZIN 45 44 46 50 57 57 50 54 103 102 99 98 101 95 92 114 C1 pF 160 82 30 15 10 7.5 6.2 3.9 100 51 22 11 7.5 5.6 4.3 2.2 C2 pF 150 75 27 13 8.2 6.8 5.6 3.3 91 43 18 9.1 6.2 4.7 3.9 2.0 LM nH 3300 1600 680 330 220 150 100 39 5600 2700 1000 430 260 180 130 47 Voltage Gain (dB) 13.3 13.4 13.4 13.4 13.2 12.8 12.3 10.9 10.4 10.4 10.6 10.5 10.3 10.3 9.9 6.8 Slope and Intercept Adjustments AD8307 INM COM OFS OUT NC OUTPUT 25mV/dB C2 NC = NO CONNECT Figure 33. High Frequency Input Matching Network 14 13 12 11 10 9 GAIN DECIIBELS 8 7 6 5 4 3 2 1 0 -1 60 70 80 90 100 110 120 FREQUENCY - MHz 130 140 150 INPUT Where higher calibration accuracy is needed, the adjustments shown in Figure 35 can be used, either singly or in combination. The log slope is lowered to 20 mV/dB by shunting the nominally 12.5 k on-chip load resistor (see Figure 29) with 50 k, adjusted by VR1. The calibration range is 10% (18 mV/dB to 22 mV/dB), including full allowance for the variability in the value of the internal load. The adjustment may be made by alternately applying two input levels, provided by an accurate signal generator, spaced over the central portion of the log amp's dynamic range, for example -60 dBm and 0 dBm. An AMmodulated signal, at the center of the dynamic range, can also be used. For a modulation depth M, expressed as a fraction, the decibel range between the peaks and troughs over one cycle of the modulation period is given by: dB = 20 log 10 1+ M 1- M (7) Figure 34. Response of 100 MHz Matching Network Table I provides solutions for a variety of center frequencies FC and matching impedances ZIN of nominally 50 and 100 . The unequal capacitor values were chosen to provide a wellbalanced differential drive, and also to allow better centering of the frequency response peak when using standard value components; this generally results in a ZIN that is not exact. The full AD8307 HF input impedance and the inductor losses were included in the modeling. For example, using an rms signal level of -40 dBm with a 70% modulation depth (M = 0.7), the decibel range is 15 dB, as the signal varies from -47.5 dBm to -32.5 dBm. The log intercept is adjustable over a 3 dB range, which is sufficient to absorb the worst-case intercept error in the AD8307 plus some system-level errors. For greater range, set RS to zero. VR2 is adjusted while applying an accurately known CW signal near the lower end of the dynamic range, in order to minimize the effect of any residual uncertainty in the slope. For example, to position the intercept to -80 dBm, a test level of -65 dBm may be applied and VR2 adjusted to produce a dc output of 15 dB above zero at 25 mV/dB, which is +0.3 V. 0.1 F VR2 50k 3dB INPUT -75dBm TO +16dBm C2 = CC NC 32.4k NC = NO CONNECT VR1 50k INP VPS ENB INT 4.7 RS VP, 2.7V - 5.5V AT 8mA C1 = CC AD8307 INM COM OFS OUT FOR VP = 3V, RS = 20k VP = 5V, RS = 51k 20mV/dB 10% Figure 35. Slope and Intercept Adjustments -14- REV. A AD8307 APPLICATIONS 0.1 F VR2 50k 4.7 The AD8307 is a highly versatile and easily applied log amp requiring very few external components. Most applications of this product can be accommodated using the simple connections shown in the preceding section. A few examples of more specialized applications are provided here. Buffered Output VP, 2.7-5.5V RS (SEE FIGURE 36) 2N3904 3dB 25mV/dB INPUT -75dBm TO +16dBm INP VPS ENB INT AD8031 R2 3.01k 10mV/dB VR1 5k 18% R1 2k AD8307 INM COM OFS OUT NC 6.34k RT (OPTIONAL) OUTPUT 50 MIN The output may be buffered, and the slope optionally increased, using an op amp. If the single-supply capability is to be preserved, a suitable component is the AD8031. Like the AD8307, it is capable of operating from a 2.7 V supply and features a railto-rail output capability; it is available in a 5-pin version and in dual form as the 8-pin AD8032. Figure 36 shows how the slope may be increased to 50 mV/dB (1 V per decade), requiring a 5 V supply (90 dB times 50 mV is a 4.5 V swing). VR1 provides a 10% slope adjustment; VR2 provides a 3 dB intercept range. With R2 = 4.99 k, the slope is adjustable to 25 mV/dB, allowing the use of a 2.7 V supply. Setting R2 to 80.6 k, it is raised to 100 mV/dB, providing direct reading in decibels on a digital voltmeter. Since a 90 dB range now corresponds to a 9 V swing, a supply of at least this amount is needed for the op amp. 0.1 F VR2 50k 3dB INP VPS ENB INT 4.7 VP, 2.7V - 5.5V RS FOR VP = 3V, RS = 20k VP = 5V, RS = 51k OUTPUT 50mV/dB 10% R2 30.1k COM NC = NO CONNECT Figure 37. Cable-Driving Log Amp In Figure 38, the capacitor values were chosen for operation in the audio field, providing a corner frequency of 10 Hz, an attenuation of 80 dB/decade above this frequency, and a 1% settling time of 150 ms (0.1% in 175 ms). The residual ripple is 4 mV ( 0.02 dB) when the input to the AD8307 is at 20 Hz. This filter may easily be adapted to other frequencies, by proportional scaling of C5-C7 (e.g., for 100 kHz use 100 pF). Placed ahead of a digital multimeter, the convenient slope scaling of 100 mV/dB requires only a repositioning of the decimal point to read directly in decibels. The supply voltage for the filter must be large enough to support the dynamic range; a minimum of 9 V is needed for most applications; 12 V is recommended. 0.1 F R1 50k C1 10 F VR1 2k INT 4.7 VP 422 OP AMP IS AD8032 SCALE C1 - C8 AS NEEDED (SEE TEXT). NOTE POLARITIES IF TANTALUM CAPACITORS ARE USED. C8 OUTPUT 1F 100mV/dB 7.32k 100k 93k 34k 34k VR2 50k SLOPE 32.4k C7 1F 80.6k INPUT -75dBm TO +16dBm AD8307 INM COM OFS OUT NC 32.4k AD8031 20mV/dB VR1 50k C1 R1 20k COM INPUT 5mV TO 160V rms 4dB NC INP VPS ENB INT C3 2.5nF NC = NO CONNECT AD8307 INM COM OFS OUT C5 1F Figure 36. Log Amp with Buffered Output C1 is optional; it lowers the corner frequency of the low-pass output filter. A value of 0.1 F should be used for applications in which the output is measured on a voltmeter or other low speed device. On the other hand, when C1 is omitted the 10%90% response time is under 200 ns, and is typically 300 ns to 99% of final value. To achieve faster response times, it is necessary to lower the load resistance at the output of the AD8307, then restore the scale using a higher gain in the op amp. Using 8.33 k, the basic slope is 10 mV/dB; this can be restored to 25 mV/dB using a buffer gain of 2.5. The overall 10%-90% response time is under 100 ns. Figure 37 shows how the output current capability can be augmented to drive a 50 load; RT optionally provides reverse termination, which halves the slope to 12.5 mV/dB. Four-Pole Filter C2 10 F C4 1F C6 1F 75k COM Figure 38. Log Amp with Four-Pole Low-Pass Filter In low frequency applications, for example, audio down to 20 Hz, it is useful to employ the buffer amplifier as a multipole low-pass filter, in order to achieve low output ripple while maintaining a rapid response time to changes in signal level. Figure 38 also shows the use of an input attenuator which may optionally be employed here, or in any other of these applications, to produce a useful wide-range ac voltmeter with directdecibel scaling. The basic range of -73 dBm to +17 dBm (that is, 50 V rms to 1.6 V rms, for sine excitations) is shifted, for illustrative purposes, to 5 mV to 160 V rms (at which point the power in R1 is 512 mW). Because the basic input resistance of the AD8307 is not precise, VR1 is used to center the signal range at its input, doubling as a 4 dB intercept adjustment. The low frequency response extends to 15 Hz; a higher corner frequency can be selected as needed, by scaling C1 and C2. The shunt capacitor C3 is used to lower the high frequency bandwidth to about 100 kHz, and thus lower the susceptibility to spurious signals. Other values should be chosen as needed for the coupling and filter capacitors. REV. A -15- AD8307 1 W to 1 kW 50 Power Meter The front-end adaptation shown in Figure 39 provides the measurement of power being delivered from a transmitter final amplifier to an antenna. The range has been set to cover the power range -30 dBm (7.07 mV rms, or 1 W) to +60 dBm (223 V rms, or 1 kW). A nominal voltage attenuation ratio of 158:1 (44 dB) is used; thus the intercept is moved from -84 dBm to -40 dBm and the AD8307, scaled 0.25 V/decade of power, will now read 1.5 V for a power level of 100 mW, 2.0 V at 10 W and 2.5 V at 1 kW. The general expression is: P (dBm) = 40 (VOUT - 1) The required attenuation could be implemented using a capacitive divider, providing a very low input capacitance, but it is difficult to ensure accurate values of small capacitors. A better approach is to use a resistive divider, taking the required precautions to minimize spurious coupling into the AD8307 by placing it in a shielded box, with the input resistor passing through a hole in this box, as indicated in the figure. The coupling capacitors shown here are suitable for f 10 MHz. A capacitor may be added across the input pins of the AD8307 to reduce the response to spurious HF signals which, as already noted, extends to over 1 GHz. The mismatch caused by the loading of this resistor will be trivial; only 0.05% of the power delivered to the load will be absorbed by the measurement system, a maximum of 500 mW at 1 kW. The post-demodulation filtering and slope-calibration arrangements should be chosen from other applications described here, to meet the particular system requirements. The 1 nF capacitor lowers the risk of HF signals entering the AD8307 via the load. TO ANTENNA 100k 1/2W 51pF 0.1 F The AD603 has a very low input referred noise: 1.3 nV/Hz at its 100- input, or 0.9 nV/Hz when matched to 50 , equivalent to 0.4 V rms, or -115 dBm, in a 200 kHz bandwidth. It is also capable of handling inputs in excess of 1.4 V rms, or +16 dBm. It is thus able to cope with a dynamic range of over 130 dB in this particular bandwidth. Now, if the gain control voltage for the X-AMP is derived from the output of the AD8307, the effect will be to raise the gain of this front-end stage when the signal is small and lower it when it is large, but without altering the fundamental logarithmic nature of the response. This gain range is 40 dB, which, combined with the 90 dB range of the AD8307, again corresponds to a 130 dB range. 50 INPUT -105dBm TO +15dBm L1 750nH R2 28k 0.65V NC GPOS GNEG VINP VPOS VOUT VNEG VR1 5k INT 8dB VN , -5V R5 100k 0.15V TO 1.15V * E.G., MURATA SFE10.7MS2G-A NC = NO CONNECT R3 330 R4 464 INP VPS ENB INT R1 187k VP , +5V BANDPASS FILTER* 4.7 0.1 F AD603 C1 150pF COMM FDBK AD8307 INM COM OFS OUT NC 1nF R6 20k R7 80.6k OUTPUT 10mV/dB 0.3V TO 2.3V Figure 40. 120 dB Measurement System 22 NC VP +5V INT 50 INPUT FROM P.A. 1 W TO 1kW VR1 2k 3dB 604 INP VPS ENB INT AD8307 INM COM OFS OUT NC LEADTHROUGH CAPACITORS, 1nF 2k VOUT Figure 40 shows how these two parts can work together to provide state-of-the-art IF measurements in applications such as spectrum/ network analyzers and other high dynamic range instrumentation. To understand the operation, note first that the AD8307 is used to generate an output of about 0.3 V to 2.3 V. This 2 V span is divided by 2 in R5/R6/R7 to provide the 1 V span needed by the AD603 to vary its gain by 40 dB. Note that an increase in the positive voltage applied at GNEG (Pin 2) lowers the gain. This feedback network is tapped to provide a convenient 10 mV/dB scaling at the output node, which may be buffered if necessary. The center of the voltage range fed back to the AD603 is 650 mV and the 20 dB gain range is centered by R1/R2. Note that the intercept calibration of this system benefits from the use of a well regulated 5 V supply. To absorb the insertion loss of the filter and center the full dynamic range, the intercept is adjusted by varying the maximum gain of the AD603, using VR1. Figure 41 shows the AD8307 output over the range -120 dBm to +20 dBm and the deviation from an ideal logarithmic response. The dotted line shows the increase in the noise floor that results when the filter is omitted; the decibel difference is about 10log10(50/0.2) or 24 dB, assuming a 50 MHz bandwidth from the AD603. An L-C filter may be used in place of the ceramic filter used in this example. 51pF 1nF OUTPUT NC = NO CONNECT Figure 39. 1 W to 1 kW 50- Power Meter Measurement System with 120 dB Dynamic Range The dynamic range of the AD8307 can be extended further-- from 90 dB to over 120 dB--by the addition of an X-AMPTM such as the AD603. This type of variable gain amplifier exhibits a very exact exponential gain control characteristic, which is another way of stating that the gain varies by a constant number of decibels for a given change in the control voltage. For the AD603, this scaling factor is 40 dB/V, or 25 mV/dB. It will be apparent that this property of a linear-in-dB response is characteristic of log amps; indeed, the AD8307 exhibits the same scaling factor. X-AMP is a trademark of Analog Devices, Inc. -16- REV. A AD8307 2.50 2.25 2.00 1.75 VOUT - Volts WITHOUT FILTER 2 1 0 ERROR (WITH FILTER) -1 -2 ERROR - dB is 125 ms. (See also Figure 38 for a more elaborate filter). Finally, to improve the law-conformance at very low signal levels and at low frequencies, C4 has been added to the offset compensation loop. +5V 4.7 1.50 1.25 1.00 0.75 0.50 0.25 0 -100 -80 -20 -60 -40 INPUT LEVEL - dBm 0 WITH FILTER 0.1 F C1 10 F VIN 0.5mV TO 20V SINE AMPLITUDE R1 5k NC INP VPS ENB INT C3 750pF FOR SLOPE AND INTERCEPT ADJUSTMENTS SEE FIGURE 35 AD8307 INM COM OFS OUT 20 C2 10 F R2 5k C4 1F C5 1F VOUT 25mV/dB NC = NO CONNECT Figure 41. Results for 120 dB Measurement System Operation at Low Frequencies Figure 42. Connections for Low Frequency Operation DC-Coupled Applications The AD8307 provides excellent logarithmic conformance at signal frequencies that may be arbitrarily low, depending only on the values used for the input coupling capacitors. It may also be desirable to add a low-pass input filter in order to desensitize the log amp to HF signals. Figure 42 shows a simple arrangement, providing coupling with an attenuation of 20 dB; the intercept is shifted up by this attenuation, from -84 dBm to -64 dBm, and the input range is now 0.5 mV to 20 V (sine amplitude). A high pass 3 dB corner frequency of nominally 3 Hz is set by the 10 F coupling capacitors C1 and C2, which are preferably tantalum electrolytics (note the polarity) and a low pass 3 dB corner frequency of 200 kHz (set by C3 and the effective resistance at the input of 1 k). The -1% amplitude error points occur at 20 Hz and 30 kHz. These are readily altered to suit other applications by simple scaling. When C3 is zero, the low pass corner is at 200 MHz. Note that the lower end of the dynamic range is improved by this capacitor, which provides essentially an HF short circuit at the input, thus significantly lowering the wideband noise; the noise reduction is about 2 dB compared to the case when the AD8307 is driven from a 50 source. To ensure that the output is free of post-demodulation ripple, it is necessary to lower the low-pass filter time-constant. This is provided by C5; with the value shown, the output time-constant R1 4.7 +5V C1 0.1 F R2 3.3k VIN TEMP C2 1F It may occasionally be necessary to provide response to dc inputs. Since the AD8307 is internally dc-coupled, there is no fundamental reason why this is precluded. However, there is a practical constraint, which is that its inputs must be positioned about 2 V above the COM potential for proper biasing of the first stage. If it happens that the source is a differential signal at this level, it may be directly connected to the input. For example, a microwave detector can be ac-coupled at its RF input and its baseband load then automatically provided by the "floating" RIN and CIN of the AD8307, at about VP/2. Usually, the source will be a single-sided ground-referenced signal, and it will thus be necessary to provide a negative supply for the AD8307. This can be achieved as shown in Figure 43. The output is now referenced to this negative supply, and it is necessary to provide an output interface that performs a differential-to-single-sided conversion. This is the purpose of the AD830. The slope may be arranged to be 20 mV/dB, when the output ideally runs from zero, for a dc input of 10 V, to +2.2 V for an input of 4 V. The AD8307 is fundamentally insensitive to the sign of the input signal, but with this biasing scheme, the maximum negative input is constrained to about -1.5 V. The transfer function after trimming and with R7 = 0, is VOUT = (0.4 V) log10 (VIN /10 V) VR2 50k R5 * +5V FOR 20mV/dB +10V FOR 50mV/dB +15V FOR 100mV/dB -5V VP INT NC VN R7 Y2 R7,R8: SEE TEXT VOUT INP VPS ENB INT * 51k FOR 20mV/dB INM COM OFS OUT 5k FOR 100mV/dB 20mV/dB AD8307 AD830 X1 X2 Y1 AD589 R3 1k Q1 2N3904 VR1 2k C3 0.1 F -2V R6 32.4k VR3 50k R9 250 R8 -5V NC = NO CONNECT Figure 43. Connections for DC-Coupled Applications REV. A -17- AD8307 The intercept can be raised, for example, to 100 V, with the rationale that the dc precision does not warrant operation in the first decade (from 10 V-100 V). Likewise, the slope can be raised to 50 mV/dB, using R7 = 3 k, R8 = 2 k, or to 100 mV/ dB, to simplify decibel measurements on a DVM, using R7 = 8 k, R8 = 2 k, which raises the maximum output to +11 V, thus requiring a +15 V supply for the AD830. The output may be made to swing in a negative direction by simply reversing Pins 1 and 2. Low-pass filtering capacitor C3 sets the output rise time to about 1 ms. 6.0 5.5 5.0 4.5 4.0 VOUT - V 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 10 100 1m 10m VIN VOUT - Volts requires adjusting the output to 0.68 V; for the 100 mV/dB scaling, this becomes 3.4 V. If a 100 V intercept is preferred (usefully lowering the maximum output voltage), these become 0.28 V and 1.4 V respectively. Finally, the slope must be adjusted. This can be performed by applying a low frequency square wave to the main input, having precisely determined upper and lower voltage levels, provided by a programmable waveform generator. A suitable choice is a 100 Hz square wave with levels of 10 mV and 1 V. The output will be a low-pass filtered square wave, and its amplitude should be 0.8 V, for 20 mV/dB scaling, or 4 V for 100 mV/dB scaling. Operation Above 500 MHz 1.0 ERROR - dB 0.5 0 -0.5 -1.0 The AD8307 is not intended for use above 500 MHz. However, it does provide useful performance at higher frequencies. Figure 45 shows a plot of the logarithmic output of the AD8307 for an input frequency of 900 MHz. The device shows good logarithmic conformance from -50 dBm to -10 dBm. There is a "bump" in the transfer function at -5 dBm, but if this is acceptable, the device is usable over a 60 dB dynamic range (-50 dBm to +10 dBm). 2 1.8 100m 1 10 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 -60 -50 -40 -30 -20 PIN - dBm -10 0 10 Figure 44. Ideal Output and Law-Conformance Error for the DC-Coupled AD8307 at 50 mV/dB Figure 44 shows the output and the law-conformance error in the absence of noise and input offset, for the 50 mV/dB option. Note in passing that the error ripple for dc excitation is about twice that for the more usual sinusoidal excitation. In practice, both the noise and the internal offset voltage will degrade the accuracy in the first decade of the dynamic range. The latter is now manually nulled, by VR1, using a simple method that ensures very low residual offsets. A temporary ac signal, typically a sine wave of 100 mV in amplitude at a frequency of about 100 Hz, is applied via the capacitor at node TEMP; this has the effect of disturbing the offset-nulling voltage. The output voltage is then viewed on an oscilloscope and VR1 is adjusted until the peaks of the (frequency-doubled) waveform are exactly equal in amplitude. This procedure can provide an input null down to about 10 V; the temperature drift is very low, though not specified since the AD8307 is not principally designed to operate as a baseband log amp, and in ac modes this offset is continuously and automatically nulled. Next, it is necessary to set the intercept. This is the purpose of VR2, which should be adjusted after VR1. The simplest method is to short the input and adjust VR2 for an output of 0.3 V, corresponding to the noise floor. For more exacting applications, a temporary sinusoidal test voltage of 1 mV in amplitude, at about 1 MHz, should be applied, which may require the use of a temporary onboard input attenuator. For 20 mV/dB scaling, a 10 V dc intercept (which is 6 dB below the ac intercept) Figure 45. Output vs. Input Level for a 900 MHz Input Signal Evaluation Board An evaluation board, carefully laid out and tested to demonstrate the specified high speed performance of the AD8307 is available. Figure 46 shows the schematic of the evaluation board. For ordering information, please refer to the Ordering Guide. Figures 47 and 49 show the component-side and solder-side silkscreens of the evaluation board. The component-side and solder-side layouts are shown in Figures 48 and 50. For connection to external instruments, side-launched SMA type connectors are provided. Space is also provided on the board for the installation of SMB or SMC type connectors. When using the top-mount SMA connector, it is recommended that the stripline on the outside 1/8" of the board edge be removed (i.e., scraped using a blade) as this unused stripline acts as an open stub, which could degrade the overall performance of the evaluation board/device combination at high frequencies. -18- REV. A AD8307 +VS C3 10 F C4 0.1 F A B LK1 INPUT C2 0.1 F R1 52.3 C1 0.1 F INP VPS ENB INT LK2 R2 50k AD8307 INM COM OFS OUT LOG OUTPUT LK4 LK5 C5 0.1 F LK3 R3 12.5k C6 1nF Figure 49. Solder Side Silkscreen Figure 46. Evaluation Board Schematic Figure 47. Component Side Silkscreen Figure 50. Board Layout (Solder Side) Link and Trim Options There are a number of link options and trim potentiometers on the evaluation board which should be set for the required operating setup before using the board. The functions of these link options and trim potentiometers are described in detail in Table II. Figure 48. Board Layout (Component Side) REV. A -19- AD8307 Table II. Evaluation Board Link Options Link No. LK1 Default Position A Function Power Up/Power Down. When this link is in position B, the ENB pin is connected to ground, putting the AD8307 in power-down mode. Placing this link in position A connects the ENB pin to the positive supply, thereby putting the AD8307 in normal operating mode. Intercept Adjust. When Pin LK2 is left open, the AD8307 has a nominal logarithmic intercept of -84 dBm. Putting LK2 in place connects the wiper of potentiometer R2 to Pin 5 (INT). By varying the voltage on INT, the position of the intercept can be adjusted. The intercept varies by about 8 dB/V. Slope Adjust. When this link is open, the nominal slope of the output is 25 mV/dB. Putting this link in place connects a ground referenced 12.5 k load resistor (R3) to the logarithmic output. The parallel combination of this resistor and an internal 12.5 k resistor reduces the logarithmic slope to 12.5 mV/dB. R3 may be adjusted for other scaling factors. Corner Frequency of Low-Pass Demodulating Filter. When this link is open, the corner frequency of the low pass post demodulation filter has a nominal value of 4 MHz. This is set by an on-chip load impedance of 12.5 k and an on-chip load capacitance of 3.5 pF. The load capacitance (e.g., of an oscilloscope probe) must be added to this capacitance, and will lower the internal video bandwidth, to a nominal 1 MHz for a total of 13.5 pF. Putting LK4 in place connects an external load capacitance (C5) of 0.1 F to the output, reducing the corner frequency of the low-pass filter to about 125 Hz. For large values of C5, the corner frequency can be calculated using the equation, f 12.7 Hz/C5 (F). Offset Control Loop. When this link is open, the internal offset control loop gives the circuit an overall high pass corner frequency of about 1 MHz. With LK5 link in place, a 1 nF capacitor (C6) is connected to the OFS pin, reducing the high pass corner frequency to allow accurate operation down to 10 kHz. To reduce the minimum operational frequency even further, a larger capacitor can replace C6 (e.g., a 1 F capacitor allows operation down to 10 Hz). Note that that external capacitor C6 has no effect on the minimum signal frequency for input levels that exceed the offset voltage (typically 400 V). The range for such signals extends down to dc (for signals applied directly to the input pins). C3065a-0-3/99 PRINTED IN U.S.A. LK2 Open LK3 Open LK4 Open LK5 Open OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 8-Lead Plastic DIP (N-8) 0.430 (10.92) 0.348 (8.84) 8 5 8-Lead SOIC (SO-8) 0.1968 (5.00) 0.1890 (4.80) 0.280 (7.11) 0.240 (6.10) 1 4 PIN 1 0.210 (5.33) MAX 0.160 (4.06) 0.115 (2.93) 0.060 (1.52) 0.015 (0.38) 0.130 (3.30) MIN SEATING PLANE 0.325 (8.25) 0.300 (7.62) 0.195 (4.95) 0.115 (2.93) 0.1574 (4.00) 0.1497 (3.80) 8 1 5 4 0.2440 (6.20) 0.2284 (5.80) PIN 1 0.0098 (0.25) 0.0040 (0.10) 0.0688 (1.75) 0.0532 (1.35) 0.0196 (0.50) 0.0099 (0.25) 45 0.022 (0.558) 0.100 0.070 (1.77) 0.014 (0.356) (2.54) 0.045 (1.15) BSC 0.015 (0.381) 0.008 (0.204) 0.0500 0.0192 (0.49) SEATING (1.27) 0.0098 (0.25) PLANE BSC 0.0138 (0.35) 0.0075 (0.19) 8 0 0.0500 (1.27) 0.0160 (0.41) -20- REV. A |
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