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Final Electrical Specifications
LTC1562 Very Low Noise, Low Distortion Active RC Quad Universal Filter
March 1998
FEATURES
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DESCRIPTION
The LTC(R)1562 is a low noise, low distortion active RC filter with rail-to-rail inputs and outputs, optimized for a center frequency (fO) of 10kHz to 150kHz. It contains four independent 2nd order filter blocks, which can be cascaded in any combination, such as one 8th order or two 4th order filters. Each block's response is programmed with three external resistors for center frequency, Q and gain, using simple design formulas. Each 2nd order block provides lowpass and bandpass outputs. Highpass response is available if an external capacitor replaces one of the resistors. Allpass and elliptic responses can also be realized. The LTC1562 is designed for applications where dynamic range is important. For example, by cascading 2nd order sections in pairs, the user can configure the IC as a dual 4th order Butterworth lowpass filter with 94dB signal-to-noise ratio from a single 5V power supply. Low level signals can exploit the built-in gain capability of the LTC1562. Varying the gain of a section can achieve a dynamic range as high as 118dB with a 5V supply. Other cutoff frequency ranges can be provided upon request. Please contact LTC Marketing.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Four 2nd Order Filter Sections in 16-Pin PDIP or 20-Pin SSOP, 10kHz to 150kHz Center Frequency Butterworth, Chebyshev, Elliptic, or Equiripple Delay Response Lowpass, Bandpass, Highpass Responses 103dB Typical S/N, 5V Supply (Q = 1) 97dB Typical S/N, Single 5V Supply (Q = 1) 96dB S/(N + THD) at 5V Supply, 20kHz Input Rail-to-Rail Input and Output Voltages DC Accurate to 3mV (Typ) 0.5% Typical Center Frequency Accuracy "Zero-Power" Shutdown Mode Single or Dual Supply, 5V to 10V Total Resistor-Programmable fO, Q, Gain
APPLICATIONS
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High Resolution Systems (14 Bits to 18 Bits) Antialiasing Filters Smoothing or Reconstruction Filters Data Communications, Equalizers Dual or I-and-Q Channels (Two Matched 4th Order Filters in One Package) Linear Phase Filtering Replacing LC Filter Modules
TYPICAL APPLICATION
Dual 4th Order 100kHz Butterworth Lowpass Filter
RIN1 10k VIN2 RQ1, 5.62k R21, 10k 5V 0.1F R23, 10k RIN3 10k VIN1 RIN2, 10k 1 2 3 4 5 6 7 RQ3, 5.62k 8 INV B BP B LP B INV C BP C LP C 16 15 RQ2, 13k 14 R22, 10k VOUT2 -5V VOUT1 DC ACCURATE, NONINVERTING, UNITY-GAIN, RAIL-TO-RAIL INPUT AND OUTPUTS. PEAK SNR 100dB WITH 5V SUPPLIES
1562 TA01
10 0 -10 -20
GAIN (dB)
-30 -40 -50 -60 -70 -80 10k 100k FREQUENCY (Hz) 1M
1562 TA02
- 13 V + LTC1562 V 0.1F 12 AGND SHDN 11 LP D LP A 10 R24, 10k BP D BP A RQ4, 13k 9 INV D INV A
RIN4, 10k
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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Frequency Response
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LTC1562
ABSOLUTE MAXIMUM RATINGS
Total Supply Voltage (V + to V -) .............................. 11V Maximum Input Voltage at Any Pin ....................(V - - 0.3V) V (V + + 0.3V) Storage Temperature Range ................. - 65C to 150C Operating Temperature Range LTC1562C................................................ 0C to 70C LTC1562I ............................................ - 40C to 85C Lead Temperature (Soldering, 10 sec).................. 300C
PACKAGE/ORDER INFORMATION
ORDER PART NUMBER
TOP VIEW INV B BP B LP B V -* V+ SHDN V -* LP A BP A INV A 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 INV C BP C LP C V -* V- AGND V -* LP D BP D INV D
LTC1562CG LTC1562ACG LTC1562IG LTC1562AIG
G PACKAGE 20-LEAD PLASTIC SSOP *G PACKAGE PINS 4, 7, 14, 17 ARE SUBSTRATE/SHIELD CONNECTIONS AND MUST BE TIED TO V - TJMAX = 150C, JA = 136C/W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL VS IS PARAMETER Total Supply Voltage Supply Current
VS = 5V, outputs unloaded, TA = 25C, SHDN pin to logic "low" (pages 3, 4), unless otherwise noted. AC specs are for a single 2nd order section, RIN = R2 = RQ =10k 0.1%, fO = 100kHz.
CONDITIONS VS = 2.375V, RL = 5k, CL = 30pF, Outputs at 0V VS = 5V, RL = 5k, CL = 30pF, Outputs at 0V VS = 2.375V, RL = 5k, CL = 30pF, Outputs at 0V VS = 5V, RL = 5k, CL = 30pF, Outputs at 0V Output Voltage Swing VOS DC Offset Magnitude, LP Outputs DC AGND Reference Point Center Frequency (f O) Error (Note 1) LTC1562 (Note 2) LTC1562A (Note 3) HL LP Passband Gain VS = 2.375V, RL = 5k, CL = 30pF VS = 5V, RL = 5k, CL = 30pF VS = 2.375V, Input at AGND Voltage VS = 5V, Input at AGND Voltage VS = Single 5V Supply VS = 5V, LP Output RL = 5k, CL = 30pF VS = 5V, LP Output RL = 5k, CL = 30pF VS = 2.375V, fIN = 10kHz, LP Output RL = 5k, CL = 30pF
q q q q q q q
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TOP VIEW INV B BP B LP B V+ SHDN LP A BP A INV A 1 2 3 4 5 6 7 8 N PACKAGE 16-LEAD PDIP 16 INV C 15 BP C 14 LP C 13 V - 12 AGND 11 LP D 10 BP D 9 INV D
ORDER PART NUMBER LTC1562CN
(CONSULT MARKETING, ENGINEERING SAMPLES ONLY)
TJMAX = 150C, JA = 90C/W
MIN 4.75
TYP 17.3 19
MAX 10.5 19.5 21.5 23.5 25.5
UNITS V mA mA mA mA VP-P VP-P
4.2 9.5
4.6 9.8 3 3 2.5 0.5 0.3 1.2 + 0.1 15 15
mV mV V % % dB
0
+ 0.05
LTC1562
VS = 5V, outputs unloaded, TA = 25C, SHDN pin to logic "low" (pages 3, 4), unless otherwise noted. AC specs are for a single 2nd order section, RIN = R2 = RQ =10k 0.1%, fO = 100kHz.
SYMBOL HB PARAMETER BP Passband Gain Q Accuracy Wideband Output Noise Input-Referred Noise, Gain = 100 THD Total Harmonic Distortion, LP Output CONDITIONS VS = 2.375V, fIN = fO, LP Output RL = 5k, CL = 30pF VS = 2.375V, LP Output RL = 5k, CL = 30pF VS = 2.375V, BW = 200kHz, Input AC GND VS = 5V, BW = 200kHz, Input AC GND BW = 200kHz, f O = 100kHz, Q = 1, Input AC GND fIN = 20kHz, 2.8VP-P. LP and BP Outputs RL = 5k, CL = 30pF fIN = 100kHz, 2.8VP-P. LP and BP Outputs RL = 5k, CL = 30pF Shutdown Supply Current Shutdown-Input Logic Threshold Shutdown-Input Bias Current Shutdown Delay Shutdown Recovery Delay Inverting Input Bias Current, Each Biquad The q denotes specifications that apply over the full operating temperature range. Note 1: fO change from 5V to 2.375 supplies is - 0.15% typical, fO temperature coefficient, - 40C to 85C, is 25ppm/C typical. SHDN Pin to 0V SHDN Pin Steps from 0V to V + SHDN Pin Steps from V + to 0V SHDN Pin to V + SHDN Pin to V +, VS = 2.375V
q
ELECTRICAL CHARACTERISTICS
MIN
TYP + 0.2 +3 24 24 4.5 - 96 - 78 1.5 1.0 2.5 - 10 20 100 5
MAX + 0.5
UNITS dB % VRMS VRMS VRMS dB dB
5
A A V A s s pA
- 20
Note 2: This data sheet reflects initial production limits that will be tightened in the final data sheet. Note 3: The LTC1562A, with tighter guaranteed fO, will soon be available. Contact LTC Marketing.
PIN FUNCTIONS
Power Supply Pins: The V + and V - pins should be bypassed with 0.1F capacitors to an adequate analog ground or ground plane. These capacitors should be connected as closely as possible to the supply pins. In the 20-lead SSOP package, the additional pins 4, 7, 14 and 17 are internally connected to V - (Pin 16) and should also be tied to the same point as Pin 16 for best shielding. Low noise linear supplies are recommended. Switching supplies are not recommended as they will lower the filter dynamic range. Analog Ground (AGND): The AGND pin is the midpoint of a resistive voltage divider, developing a potential halfway between the V + and V - pins, with an equivalent series resistance nominally 7k. This serves as an internal ground reference. Filter performance will reflect the quality of the analog signal ground and an analog ground plane surrounding the package is recommended. The analog ground plane should be connected to any digital ground at a single point. For dual supply operation, the AGND pin should be connected to the ground plane. For single supply operation, the AGND pin should be bypassed to the ground plane with at least a 0.1F capacitor (at least 1F for best AC performance). Shutdown (SHDN): When the SHDN input goes high or is open-circuited, the LTC1562 enters a "zero-power" shutdown state and only junction leakage currents flow. The AGND pin and the amplifier outputs (see Figure 1) assume a high impedance state and the amplifiers effectively disappear from the circuit. (If an input signal is applied to a complete filter circuit while the LTC1562 is in shutdown, some signal will normally flow to the output through passive components around the inactive op amps.) A small pull-up current source at the SHDN input defaults the LTC1562 to the shutdown state if the SHDN pin is left floating. Therefore, the user must connect the SHDN pin
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LTC1562
PIN FUNCTIONS
to a logic "low" (0V for 5V supplies, V - for 5V total supply) for normal operation of the LTC1562. (This convention permits true "zero-power" shutdown since not even the driving logic must deliver current while the part is in shutdown.) INV A, INV B, INV C, INV D: Each of the INV pins is a virtualground summing point for the corresponding 2nd order section. For each section, all three external resistors RIN, R2, RQ connect to the INV pin as shown in Figure 1 and described further in the Applications Information. Note that the INV pins are sensitive internal nodes of the filter and will readily receive any unintended signals that are capacitively coupled into them. Capacitance to the INV nodes will also affect the frequency response of the filter sections. For these reasons, printed circuit connections to the INV pins must be kept as short as possible, less than one inch (2.5cm) total and surrounded by a ground plane. LP A, LP B, LP C, LP D: Output Pins. Provide a lowpass, bandpass or other response depending on external circuitry (see Applications Information section). Each LP pin also connects to the R2 resistor of the corresponding 2nd order filter section (see Figure 1 and Applications Information). Each output is designed to drive a nominal net load of 5k and 30pF, which includes the loading due to the external R2. Distortion performance improves when the outputs are loaded as lightly as possible. BP A, BP B, BP C, BP D: Output Pins. Provide a bandpass, highpass or other response depending on external circuitry (see Applications Information section). Each BP pin also connects to the RQ resistor of the corresponding 2nd order filter section (see Figure 1 and Applications Information). Each output is designed to drive a nominal net load of 5k and 30pF, which includes the loading due to the external RQ. Distortion performance improves when the outputs are loaded as lightly as possible.
BLOCK DIAGRA SM
INV BP LP INV BP LP
1 sCR1
1 = 2 * 100kHz CR1
LP R2
INV RQ
RIN
+ -
VIN
Figure 1. Equivalent Circuit of a Single 2nd Order Section (Inside Dashed Line) Shown in a Simple Application. External Resistors R2, RIN and RQ in This Example Set up a Lowpass Response from VIN to the LP Output and a Bandpass Response from VIN to the BP Output
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+
-
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C
V+ V-
V+ SHUTDOWN SWITCH A B
2ND ORDER SECTIONS
SHDN
SHUTDOWN SWITCH V-
AGND
D
C
BP
1562 F01
INV
BP
LP
INV
BP
LP
1562 F02
Figure 2. Overall Block Diagram Showing Four 3-Terminal 2nd Order Sections
LTC1562
APPLICATIONS INFORMATION
Functional Description The LTC1562 contains four matched, 2nd order, 3-terminal universal continuous-time filter blocks, each with a virtual-ground input node (INV) and two rail-to-rail outputs. In the most basic application, one such block and three external resistors provide 2nd order lowpass and bandpass filtering functions simultaneously (Figure 1). The three external resistors program fO, Q and gain. A combination of internal precision components and an external resistor R2 sets the center frequency fO of each 2nd order block. The LTC1562 is trimmed at manufacture so that fO will be 100kHz 0.5% if the external resistor R2 is exactly 10k. However, lowpass/bandpass filtering is only one specific application for the 2nd order building blocks in the LTC1562. Highpass response results if the external input resistor RIN of Figure 1 is replaced by a capacitor CIN (which sets only gain, not critical frequencies) as described below. Responses with zeroes (e.g., elliptic or notch responses) are available by feedforward connections with multiple 2nd order blocks (see Typical Applications). Moreover, the virtual-ground input gives each 2nd order section the built-in capability for analog operations such as gain (preamplification), summing and weighting of multiple inputs, or accepting current or charge signals directly. These Operational FilterTM frequency-selective building blocks are nearly as versatile as operational amplifiers. Setting fO, Q and Gain Standard all-pole transfer functions characterize the response of each 2nd order filter section. The responses from VIN in Figure 1 to the LP and BP outputs are, respectively, The external resistors R2, RIN and RQ set the filter parameters O = 2fO, Q, HL and HB as follows:
10k = 100kHz C R2 2 R1R2 1
HLP (s) =
2 s2 + O / Q s + O
(
2 -HLO
)
HBP (s) =
-HB O / Q s
2 s2 + O / Q s + O
(
(
)
)
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fO =
(
)
100kHz or, R2 = 10k fO RQ R R2 Q= HL = HB = Q RIN RIN R1R2
2
(
)
Note that R1 (=10k) and C (=159pF) are internal to the LTC1562 while R2, RIN and RQ are external. The usual design procedure is to first determine R2 from the required fO, then determine RQ to set Q once R2 is known, and finally determine RIN to set gain. The fO range is approximately 10kHz to 150kHz, limited mainly by the magnitudes of the external resistors required. As shown above, R2 varies with the inverse square of fO. This relationship desensitizes fO to R2's tolerance (by a factor of 2 incrementally), but it also implies that R2 has a wider range than fO. (RQ and RIN also tend to scale with R2.) At high fO these resistors fall below 5k, heavily loading the outputs of the LTC1562 and leading to increased THD and other effects. At the other extreme, a lower fO limit of 10kHz reflects an arbitrary upper resistor limit of 1M. The LTC1562's MOS input circuitry can accommodate higher resistor values than this, but junction leakage current from the input protection circuitry may cause DC errors. The 2nd order transfer functions HLP(s), HBP(s) and HHP(s) (below) are all inverting so that, for example, at DC the lowpass gain is - HL. If two such sections are cascaded, these phase inversions cancel. Thus, the filter in the application schematic on the first page of this data sheet is a dual DC preserving, noninverting, rail-to-rail lowpass filter, approximating two "straight wires with frequency selectivity."
Operational Filter is a trademark of Linear Technology Corporation.
5
LTC1562
APPLICATIONS INFORMATION
Highpass Option If a capacitor of value CIN replaces the resistor RIN, then a standard all-pole highpass response becomes available between VIN and the "BP" output.
HHP (s) =
2 s2 + O / Q s + O
(
-HHs 2
)
Here, the passband gain HH = CIN/C where C is the internal capacitance, nominally 159pF. The expressions for fO and Q remain as above. Two Bandpass Options There are two distinct ways to obtain a bandpass response HBP(s) from the LTC1562. In the basic circuit of Figure 1, where the only external components are resistors, the BP output has a bandpass response from VIN. Alternatively, with an input capacitor CIN replacing RIN, the BP output has a highpass response as described above, but simultaneously the LP output now has a bandpass response, with the same HBP(s) form as above. The bandpass gain parameter becomes HB = (RQ/R1)(CIN/159pF) where again R1 = 10k. fO and Q are controlled by R2 and RQ as always. Relative Signal Swings The signal swings in each 2nd order section must be scaled so that neither output overloads (saturates), even if it is not used as a signal output. (Filter literature often calls this the "dynamics" issue.) For the LTC1562, the ratio of peak signal swings, LP output to BP output, is approximately equal to the ratio,
( ) = (100kHz) fO HBP(jO )
HLP jO
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Thus the "LP" output of the LTC1562 tends to have the higher swing of the two when fO is below 100kHz, and "BP" the higher swing when fO is above 100kHz. When an unused output has a larger signal swing than the output of interest, the section's gain or input amplitude must be scaled to avoid overdriving the unused output. The LTC1562 can still be used with high performance in such situations as long as this constraint is followed. The following cases are the most convenient because the relative-signal-swing issue does not arise: the unused output's swing is naturally the lower of the two. Lowpass response (all-resistor form, "LP" output) with fO < 100kHz Bandpass response (capacitor-input form, "LP" output) with fO < 100kHz Bandpass response (all-resistor form, "BP" output) with fO > 100kHz Highpass response (capacitor-input form, "BP" output) with fO > 100kHz Low Level or Wide Range Input Signals The LTC1562 contains a built-in capability for low noise amplification of low level signals. The RIN resistor in each 2nd order section controls the block's gain. When set for unity passband gain, a 2nd order section can deliver an output signal more than 100dB above the noise level. If low level inputs require further dynamic range, reducing the value of RIN boosts the signal gain while reducing the input-referred noise. This feature can increase the SNR for low level signals. Varying or switching RIN (CIN for the highpass option) is also an efficient way to effect automatic gain control (AGC). From a system viewpoint, this technique boosts the ratio of maximum signal to minimum noise, for a typical 2nd order lowpass response (Q = 1, fO = 100kHz), to 118dB.
LTC1562
TYPICAL APPLICATIONS
RFF1, 12.1k CIN1, 87pF 1 RIN1, 19.6k RQ1, 13k R21, 8.87k 5V 0.1F 2 3 4 5 6 R23, 8.87k RIN3, 69.8k CIN3, 47pF RQ3, 28k 7 8 RIN2, 8.06k INV C BP C LP C V- 16 15 14 13 12 11 10 9 R24, 17.8k RQ4, 6.98k 0.1F RQ2, 8.87k R22, 12.1k -5V
5V 0.1F R23, 10k VIN
8th Order 100kHz Lowpass Elliptic Filter
RIN1 10k
VIN
INV B BP B LP B
V + LTC1562 SHDN LP A BP A INV A
RIN4, 7.32k RFF2, 17.8k WITH VIN = 1.5VRMS, fIN 30kHz SIGNAL/(NOISE + THD) = 88dB OUTPUT DC OFFSET = 5mV TYP VOUT
1562 TA03
ALL RESISTORS = 1% METAL FILM VS = 5V TOTAL OUTPUT NOISE = 46VRMS
Amplitude Response
10 0 -10 -20 10 0 -10 -20
GAIN (dB)
-40 -50 -60 -70 -80 -90 1k 10k 100k FREQUENCY (Hz) 1M
1562 TA04
GAIN (dB)
-30
RIN1A 6.19k VIN1
RIN1B 3.83k CIN1 680pF
5V 0.1F R23, 10k RIN3A 6.19k VIN3 CIN3 680PF ALL RESISTORS = 1% METAL FILM VS = 5V RIN3B 3.83k
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(Pinout shown for the 16-pin PDIP) 8th Order 100kHz Lowpass Butterworth Filter
RIN2, 10k 1 RQ1, 5.11k R21, 10k 2 3 4 5 6 7 RQ3, 9.09k 8 INV B BP B LP B V + LTC1562 SHDN LP A BP A INV A INV C BP C LP C V- 16 15 14 13 12 11 10 9 RQ4, 25.5k 0.1F R24, 10k RQ2, 6.04k R22, 10k -5V
AGND LP D BP D INV D
AGND LP D BP D INV D
RIN4, 10k RIN3 ,10k ALL RESISTORS = 1% METAL FILM VS = 5V TOTAL OUTPUT NOISE = 40VRMS WITH VIN = 1.5VRMS, SIGNAL/(NOISE + THD) 90dB OUTPUT DC OFFSET = 10mV TYP
VOUT
1562 TA05
Amplitude Response
-30 -40 -50 -60 -70 -80 -90 1k 10k 100k FREQUENCY (Hz) 1M
1562 TA06
Quad 3-Pole 100kHz Butterworth Lowpass Filter
VOUT1 1 RQ1, 10k R21, 10k 2 3 4 5 6 7 RQ3, 10k 8 INV B BP B LP B V + LTC1562 SHDN LP A BP A INV A INV C BP C LP C V- VOUT2 16 15 14 13 12 11 10 9 R24, 10k RQ4, 10k RIN4B 3.83k RIN4A 6.19k VIN4 CIN4 680pF
1562 TA07
RIN2B 3.83k RQ2, 10k R22, 10k -5V 0.1F
RIN2A 6.19k CIN2 680pF
VIN2
AGND LP D BP D INV D
VOUT3
VOUT4
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LTC1562
TYPICAL APPLICATIONS
8th Order Highpass 0.05dB Ripple Chebyshev Filter fCUTOFF = 30kHz
CIN1 150pF CIN 1 RQ1, 10.2k R21, 35.7k 5V 0.1F 2 3 4 5 6 CIN3 150pF R23, 107k RQ3, 54.9k 7 8 INV B BP B LP B INV C BP C LP C 16 15 14 RQ2, 22.1k R22, 66.5k CIN2 150pF -5V 0.1F
- 13 V + LTC1562 V 12 AGND SHDN 11 LP D LP A 10 BP D BP A 9 INV D INV A
TOTAL OUTPUT NOISE = 40VRMS
Amplitude Response
10 0 -10 -20
GAIN (dB)
-40 -50 -60 -70 -80 -90 1k 10k 100k FREQUENCY (Hz) 1M
1562 TA09
GAIN (dB)
-30
RELATED PARTS
PART NUMBER LTC1560-1 DESCRIPTION 5-Pole Elliptic Lowpass, fC = 1MHz/0.5MHz COMMENTS No External Components, SO8
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 q (408) 432-1900 FAX: (408) 434-0507 q TELEX: 499-3977 q www.linear-tech.com
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(Pinout shown for the 16-pin PDIP)
2nd Order 30kHz Highpass Cascaded with 6th Order 138kHz Lowpass
CIN1 150pF VIN 1 RQ1, 30.1k R21, 110k 5V 0.1F 2 3 4 5 6 RIN2, 5.23k INV B BP B LP B INV C BP C LP C 16 15 14 RQ2, 5.11k R22, 5.23k -5V 0.1F
R24, 127k RQ4, 98.9k
CIN4 150pF
R23, 5.23k RQ3, 14k
7 8
- 13 V + LTC1562 V 12 AGND SHDN 11 LP D LP A 10 BP D BP A 9 INV D INV A
R24, 5.23k RQ4, 3.74k VOUT
1562 TA10
VOUT
1562 TA08
RIN3, 8.06k
RIN4, 3.4k
ALL RESISTORS = 1% METAL FILM
Amplitude Response
20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 10 100 FREQUENCY (kHz) 400
1562 TA11
1562i LT/TP 0398 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1998

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