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LTC1278 12-Bit, 500ksps Sampling A/D Converter with Shutdown
FEATURES
s s
DESCRIPTIO
s s s s s s s s s
s
Single Supply 5V or 5V Operation Two Speed Grades,500ksps (LTC1278-5) 400ksps (LTC1278-4) 70dB S/(N + D) and 74dB THD at Nyquist No Missing Codes Over Temperature 75mW (Typ) Power Dissipation Power Shutdown with Instant Wake-Up Internal Reference Can Be Overdriven Externally Internal Synchronized Clock; No Clock Required High Impedance Analog Input 0V to 5V or 2.5V Input Range New Flexible, Friendly Parallel Interface to DSPs and FIFOs 24-Pin Narrow PDIP and SW Packages
The LTC(R)1278 is a 1.6s, 500ksps, sampling 12-bit A/D converter that draws only 75mW from a single 5V or 5V supplies. This easy-to-use device comes complete with a 200ns sample-and-hold, a precision reference and an internally trimmed clock. Unipolar and bipolar conversion modes add to the flexibility of the ADC. The low power dissipation is made even more attractive by a 8.5mW power-down feature. Instant wake-up from shutdown allows the converter to be powered down even during brief inactive periods. The LTC1278 converts 0V to 5V unipolar inputs from a single 5V supply and 2.5V bipolar inputs from 5V supplies. Maximum DC specs include 1LSB INL and 1LSB DNL. Outstanding guaranteed AC performance includes 70dB S/(N + D) and 78dB THD at the input frequency of 100kHz over temperature. The internal clock is trimmed for 1.6s conversion time. The clock automatically synchronizes to each sample command, eliminating problems with asynchronous clock noise found in competitive devices. A separate convert start input and a data ready signal (BUSY) ease connections to FIFOs, DSPs and microprocessors.
APPLICATI
s s s s s
S
High Speed Data Acquisition Digital Signal Processing Multiplexed Data Acquisition Systems Audio and Telecom Processing Spectrum Analysis
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATI
2.42V REFERENCE OUTPUT
Single 5V Supply, 500kHz, 12-Bit Sampling A/D Converter
LTC1278-5 ANALOG INPUT 1 A AVDD (0V TO 5V) 2 IN VREF VSS 3 AGND BUSY 0.1F 4 D11(MSB) CS 5 D10 RD 6 D9 CONVST 7 D8 SHDN 8 D7 DVDD 9 D6 D0 10 12-BIT D5 D1 PARALLEL 11 D4 D2 BUS 12 DGND D3 24 23 22 21 20 19 18 17 16 15 14 13 10F P CONTROL LINES CONVERSION START INPUT POWER DOWN INPUT 5V
Effective Bits and Signal-to-(Noise + Distortion) vs Input Frequency
12 74 68 NYQUIST FREQUENCY 62 56
+
+
10F
0.1F
EFFECTIVE NUMBER OF BITS
11 10 9 8 7 6 5 4 3 2 1 0 10k fSAMPLE = 500kHz 100k INPUT FREQUENCY (Hz) 1M
LTC1278 * TA01
U
S/(N+D) (dB)
2M
LT1278 G4
UO
UO
1
LTC1278 ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW AIN VREF AGND D11(MSB) D10 D9 D8 D7 D6 1 2 3 4 5 6 7 8 9 24 AVDD 23 VSS 22 BUSY 21 CS 20 RD 19 CONVST 18 SHDN 17 DVDD 16 D0 15 D1 14 D2 13 D3 SW PACKAGE 24-LEAD PLASTIC SO WIDE
AVDD = DVDD = VDD (Notes 1, 2)
Supply Voltage (VDD) .............................................. 12V Negative Supply Voltage (VSS) Bipolar Operation Only .......................... - 6V to GND Total Supply Voltage (VDD to VSS) Bipolar Operation Only ....................................... 12V Analog Input Voltage (Note 3) Unipolar Operation ................... - 0.3V to VDD + 0.3V Bipolar Operation............... VSS - 0.3V to VDD + 0.3V Digital Input Voltage (Note 4) Unipolar Operation ................................- 0.3V to 12V Bipolar Operation........................... VSS - 0.3V to 12V Digital Output Voltage Unipolar Operation ................... - 0.3V to VDD + 0.3V Bipolar Operation................ VSS - 0.3V to VDD + 0.3V Power Dissipation ............................................. 500mW Operating Temperature Range LTC1278-4C, LTC1278-5C ..................... 0C to 70C LTC1278-4I ....................................... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC1278-4CN LTC1278-5CN LTC1278-4IN LTC1278-4CSW LTC1278-5CSW LTC1278-4ISW
D5 10 D4 11 DGND 12 N PACKAGE 24-LEAD PDIP
TJMAX = 110C, JA = 100C/W (N) TJMAX = 110C, JA = 130C/W (SW)
Consult factory for Military grade parts.
CO VERTER CHARACTERISTICS
PARAMETER Resolution (No Missing Codes) Integral Linearity Error Differential Linearity Error Offset Error Gain Error Gain Error Tempco IOUT(REF) = 0 (Note 8) (Note 7) CONDITIONS
With Internal Reference (Notes 5, 6)
MIN
q q q q
LTC1278-4/LTC1278-5 TYP MAX 1 1 4 6 15 10 45
UNITS Bit LSB LSB LSB LSB LSB ppm/C
12
q
A ALOG I PUT
SYMBOL PARAMETER VIN IIN CIN
(Note 5)
CONDITIONS 4.95V VDD 5.25V (Unipolar) 4.75V VDD 5.25V, - 5.25V VSS - 2.45V (Bipolar) CS = High Between Conversions (Sample Mode) During Conversions (Hold Mode)
q q q
LTC1278-4/LTC1278-5 MIN TYP MAX 0 to 5 2.5 1 45 5
UNITS V V A pF pF
Analog Input Range (Note 9) Analog Input Leakage Current Analog Input Capacitance
2
U
W
U
U
WW
W
U
U
U
LTC1278
DY A IC ACCURACY
SYMBOL S/(N + D) THD PARAMETER Signal-to-Noise Plus Distortion Ratio Total Harmonic Distortion First 5 Harmonics Peak Harmonic or Spurious Noise IMD Intermodulation Distortion Full Power Bandwidth Full Linear Bandwidth (S/(N + D) 68dB)
I TER AL REFERE CE CHARACTERISTICS
PARAMETER VREF Output Voltage VREF Output Tempco VREF Line Regulation VREF Load Regulation CONDITIONS IOUT = 0 IOUT = 0 4.95V VDD 5.25V - 5.25V VSS - 4.95V 0V |IOUT| 1mA
DIGITAL I PUTS A D DIGITAL OUTPUTS
SYMBOL PARAMETER VIH VIL IIN CIN VOH High Level Input Voltage Low Level Input Voltage Digital Input Current Digital Input Capacitance High Level Output Voltage VDD = 4.95V IO = - 10A IO = - 200A VDD = 4.95V IO = 160A IO = 1.6mA VOUT = 0V to VDD, CS High CS High (Note 9 ) VOUT = 0V VOUT = VDD CONDITIONS VDD = 5.25V VDD = 4.95V VIN = 0V to VDD
VOL
Low Level Output Voltage
IOZ COZ ISOURCE ISINK
High Z Output Leakage D11 to D0 High Z Output Capacitance D11 to D0 Output Source Current Output Sink Current
U
U
U
WU
(Note 5)
CONDITIONS 100kHz Input Signal 250kHz Input Signal 100kHz Input Signal 250kHz Input Signal 100kHz Input Signal 250kHz Input Signal fIN1 = 99.37kHz, fIN2 = 102.4kHz fIN1 = 249.37kHz, fIN2 = 252.4kHz
q q q
LTC1278-4/LTC1278-5 MIN TYP MAX 70 72 70 - 80 - 74 - 84 - 74 - 82 - 70 4 350 - 78 - 78
UNITS dB dB dB dB dB dB dB dB MHz kHz
U
U
(Note 5)
LTC1278-4/LTC1278-5 MIN TYP MAX 2.400
q
UNITS V ppm/C LSB/V LSB/V LSB/mA
2.420 10 0.01 0.01 2
2.440 45
(Note 5)
LTC1278-4/LTC1278-5 MIN TYP MAX
q q q
UNITS V V A pF V V V V A pF mA mA
2.4 0.8 10 5 4.7
q
4 0.05 0.10
q q q
0.4 10 15
- 10 10
3
LTC1278
POWER REQUIRE E TS
SYMBOL PARAMETER VDD VSS IDD ISS PD Positive Supply Voltage (Notes 10, 11) Negative Supply Voltage (Note 10) Positive Supply Current Negative Supply Current Power Dissipation
TI I G CHARACTERISTICS
SYMBOL fSAMPLE(MAX) tSAMPLE(MIN) tCONV tACQ t1 t2 t3 t4 t5 PARAMETER Maximum Sampling Frequency Minimum Throughput Time (Acquisition Time Plus Conversion Time) Conversion Time Acquisition Time CS to RD Setup Time CS to CONVST Setup Time SHDN to CONVST Wake-Up Time CONVST Low Time CONVST to BUSY Delay
t6 t7 t8
Data Ready Before BUSY Wait Time RD After BUSY Data Access Time After RD
t9
Bus Relinquish Time Commercial Industrial
q q q q
t10 t11 t12
RD Low Time CONVST High Time Aperture Delay of Sample-and-Hold
4
UW
(Note 5)
CONDITIONS Unipolar Bipolar Bipolar Only fSAMPLE = 500ksps SHDN = 0V fSAMPLE = 500ksps, VSS = - 5V fSAMPLE = 500ksps SHDN = 0V
q q q q q
LTC1278-4/LTC1278-5 MIN TYP MAX 4.95 4.75 - 2.45 15.0 1.7 0.12 75.0 8.5 5.25 5.25 - 5.25 29.5 3.0 0.30 150 15
UNITS V V V mA mA mA mW mW
UW
(Note 5)
CONDITIONS LTC1278-4 LTC1278-5 LTC1278-4 LTC1278-5 LTC1278-4 LTC1278-5 (Notes 9, 10) (Notes 9, 10) (Note 10) (Notes 10, 12) CL = 100pF Commercial Industrial CL = 100pF Mode 2, (see Figure 14) (Note 9) CL = 20pF (Note 9) Commercial Industrial CL = 100pF Commercial Industrial
q q q q q q q q q q q q q
LTC1278-4/LTC1278-5 MIN TYP MAX 400 500 2.5 2.0 2.0 1.6 200 0 20 350 40 40 110 130 140 2.3 1.85
UNITS kHz s s s s ns ns ns ns ns ns ns ns ns ns
20 - 20
40 50 90 110 120 125 150 170 75 85 90
ns ns ns ns ns ns ns ns ns ns ns
70
q q
20 20 20 t8 40
30
(Note 9) (Notes 9, 12) Jitter < 50ps
15
ns
LTC1278
TI I G CHARACTERISTICS
The q indicates specifications which apply over the full operating temperature range; all other limits and typicals TA = 25C. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: All voltage values are with respect to ground with DGND and AGND wired together (unless otherwise noted). Note 3: When these pin voltages are taken below VSS (ground for unipolar mode) or above VDD, they will be clamped by internal diodes. This product can handle input currents greater than 60mA below VSS (ground for unipolar mode) or above VDD without latch-up. Note 4: When these pin voltages are taken below VSS (ground for unipolar mode), they will be clamped by internal diodes. This product can handle input currents greater than 60mA below VSS (ground for unipolar mode) without latch-up. These pins are not clamped to VDD. Note 5: AVDD = DVDD = VDD = 5V, (VSS = - 5V for bipolar mode), fSAMPLE = 400kHz (LTC1278-4), 500kHz (LTC1278-5), tr = tf = 5ns unless otherwise specified. Note 6: Linearity, offset and full-scale specifications apply for unipolar and bipolar modes.
TYPICAL PERFORMANCE CHARACTERISTICS
Integral Nonlinearity vs Output Code
1.0 fSAMPLE = 500kHz
0.5
0.5
DNL ERROR (LSB)
EFFECTIVE NUMBER OF BITS
INL ERROR (LSB)
0
-0.5
-1.0 0 512 1024 1536 2048 2560 3072 3584 4096 CODE
LT1278 G1
UW
UW
(Note 5)
Note 7: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 8: Bipolar offset is the offset voltage measured from - 1/2LSB when the output code flickers between 0000 0000 0000 and 1111 1111 1111. Note 9: Guaranteed by design, not subject to test. Note 10: Recommended operating conditions. Note 11: AIN must not exceed VDD or fall below VSS by more than 50mV for specified accuracy. Therefore the minimum supply voltage for the unipolar mode is 4.95V. The minimum for the bipolar mode is 4.75V, - 2.45V. Note 12: The falling CONVST edge starts a conversion. If CONVST returns high at a bit decision point during the conversion it can create small errors. For best performance ensure that CONVST returns high either within 120ns after conversion start (i.e., before the first bit decision) or after BUSY rises (i.e., after the last bit test). See mode 1a and 1b (Figures 12 and 13) timing diagrams.
Differential Nonlinearity vs Output Code
1.0
12
ENOBs and S/(N + D) vs Input Frequency
74 68 NYQUIST FREQUENCY 62 56
S/(N+D) (dB)
fSAMPLE = 500kHz
11 10 9 8 7 6 5 4 3 2 1 fSAMPLE = 500kHz 100k INPUT FREQUENCY (Hz) 1M
0
-0.5
-1.0 0 512 1024 1536 2048 2560 3072 3584 4096 CODE
LT1278 G2
0 10k
2M
LT1278 G4
5
LTC1278 TYPICAL PERFORMANCE CHARACTERISTICS
S/(N + D) vs Input Frequency and Amplitude
80 VIN = 0dB
SIGNAL TO NOISE RATIO (dB)
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
SIGNAL/(NOISE + DISTORTION) (dB)
70 60 50 40 30 20 10 fSAMPLE = 500kHz 0 1k 10k 100k 1M INPUT FREQUENCY (Hz) 10M
LTC1278 G10
VIN = -20dB
VIN = -60dB
Spurious Free Dynamic Range vs Input Frequency
0
SPURIOUS FREE DYNAMIC RANGE (dB)
-10 -20
fSAMPLE = 500kHz
-40 -50 -60 -70 -80 -90
AMPLITUDE (dB)
-30
ACQUISITION TIME (ns)
-100 10k
100k INPUT FREQUENCY (Hz)
AMPLITUDE OF POWER SUPPLY FEEDTHROUGH (dB)
Supply Current vs Temperature
20 fSAMPLE = 500kHz 15
-40 -60 -80 -100 -120 1k 10k 100k 1M
LTC1278 G7
REFERENCE VOLTAGE (V)
SUPPLY CURRENT (mA)
10
5
0 -55
-25
75 0 25 50 TEMPERATURE (C)
6
UW
1M 2M
LTLTC1278 G11
Signal-to-Noise Ratio (without Harmonics) vs Input Frequency
80 70 60 50 40 30 20 10 0 fSAMPLE = 500kHz 1k 10k 100k 1M INPUT FREQUENCY (Hz) 10M
LTC1278 G5
Distortion vs Input Frequency
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 10k 2ND HARMONIC THD fSAMPLE = 500kHz
3RD HARMONIC
100k INPUT FREQUENCY (Hz)
1M
2M
LT1278 G6
Intermodulation Distortion Plot
0 -20 -40 -60 -80 fSAMPLE = 500kHz fIN1 = 96.80kHz fIN2 = 101.68kHz
4500 4000 3500 3000 2500 2000 1500 1000
Acquisition Time vs Source Impedance
-100 -120 0 50k 100k 150k FREQUENCY (Hz) 200k 250k
LTC1278 G8
500 0 10 100 1k RSOURCE () 10k
LTC1278 G9
Power Supply Feedthrough vs Ripple Frequency
0 -20 fSAMPLE = 500kHz 2.435 2.430 2.425 2.420 2.415 2.410 2.405
Reference Voltage vs Load Current
VSS (VRIPPLE = 10mV) DGND (VRIPPLE = 0.1V)
AVDD (VRIPPLE = 1mV)
100
125
-5
-4
RIPPLE FREQUENCY (Hz)
LTC1278 G3
-3 0 -2 -1 LOAD CURRENT (mA)
1
2
LTC1278 G12
LTC1278
PI FU CTIO S
AIN (Pin 1): Analog Input. 0V to 5V (Unipolar), 2.5V (Bipolar). VREF (Pin 2): 2.42V Reference Output. Bypass to AGND (10F tantalum in parallel with 0.1F ceramic). AGND (Pin 3): Analog Ground. D11 to D4 (Pins 11 to 4): Three-State Data Outputs. D11 is the Most Significant Bit. DGND (Pin 12): Digital Ground. D3 to D0 (Pins 13 to 16): Three-State Data Outputs. DVDD (Pin 17 ): Digital Power Supply, 5V. Tie to AVDD pin. SHDN (Pin 18): Power Shutdown. CONVST (Pin 19): Conversion Start Signal. This active low signal starts a conversion on its falling edge (to recognize CONVST, CS has to be low). RD (Pin 20): READ Input. This enables the output drivers when CS is low. CS (Pin 21): The CHIP SELECT input must be low for the ADC to recognize CONVST and RD inputs. BUSY (Pin 22): The BUSY output shows the converter status. It is low when a conversion is in progress. VSS (Pin 23): Negative Supply. - 5V for bipolar operation. Bypass to AGND with 0.1F ceramic. Analog ground for unipolar operation. AVDD (Pin 24): Positive Supply, 5V. Bypass to AGND (10F tantalum in parallel with 0.1F ceramic).
FU CTIO AL BLOCK DIAGRA
AIN
CSAMPLE AVDD ZEROING SWITCH 2.42V REF VREF 12-BIT CAPACITIVE DAC COMPARATOR DVDD VSS (0V FOR UNIPOLAR MODE OR -5V FOR BIPOLAR MODE)
12 AGND DGND SUCCESSIVE APPROXIMATION REGISTER 12 OUTPUT LATCHES * * *
LTC1278 * BD
INTERNAL CLOCK
CONTROL LOGIC
SHDN CONVST
RD
W
U
U
U
U
U
D11 D0
CS
BUSY
7
LTC1278
TEST CIRCUITS
Load Circuits for Access Timing
5V 3k DBN 3k DGND A) HIGH-Z TO VOH (t8) AND VOL TO VOH (t6) CL DBN CL DGND B) HIGH-Z TO VOL (t8) AND VOH TO VOL (t6)
LTC1278 TA08
Load Circuits for Output Float Delay
5V 3k DBN 3k DGND A) VOH TO HIGH-Z 10pF DBN 10pF DGND B) VOL TO HIGH-Z
1278 * TA08
TI I G DIAGRA S
CS to RD Setup Timing CS to CONVST Setup Timing SHDN to CONVST Wake-Up Timing
CS t1 RD
LTC1278 * TC01
APPLICATIONS INFORMATION
CONVERSION DETAILS The LTC1278 uses a successive approximation algorithm and an internal sample-and-hold circuit to convert an analog signal to a 12-bit parallel output. The ADC is complete with a precision reference and an internal clock. The control logic provides easy interface to microprocessors and DSPs. (Please refer to the Digital Interface section for the data format.) Conversion start is controlled by the CS and CONVST inputs. At the start of conversion the successive approximation register (SAR) is reset. Once a conversion cycle has begun it cannot be restarted. During conversion, the internal 12-bit capacitive DAC output is sequenced by the SAR from the most significant bit (MSB) to the least significant bit (LSB). Referring to Figure 1, the AIN input connects to the sample-and-hold capacitor during the acquire phase, and the comparator
SAMPLE AIN HOLD CDAC DAC VDAC S A R CSAMPLE SAMPLE SI
8
U
W
W
U
U
UW
CS t2 CONVST
LTC1278 * TC02
SHDN t3 CONVST
LTC1278 * TC03
- +
COMPARATOR
12-BIT LATCH
LTC1278 F1
Figure 1. AIN Input
offset is nulled by the feedback switch. In this acquire phase, a minimum delay of 200ns will provide enough time for the sample-and-hold capacitor to acquire the analog signal. During the convert phase, the comparator feedback switch opens, putting the comparator into the
LTC1278
APPLICATIONS INFORMATION
compare mode. The input switch switches CSAMPLE to ground, injecting the analog input charge onto the summing junction. This input charge is successively compared with the binary-weighted charges supplied by the capacitive DAC. Bit decisions are made by the high speed comparator. At the end of a conversion, the DAC output balances the AIN input charge. The SAR contents (a 12-bit data word) which represent the AIN are loaded into the 12-bit output latches. DYNAMIC PERFORMANCE The LTC1278 has excellent high speed sampling capability. FFT (Fast Fourier Transform) test techniques are used to test the ADC's frequency response, distortion and noise at the rated throughput. By applying a low distortion sine wave and analyzing the digital output using an FFT algorithm, the ADC's spectral content can be examined for frequencies outside the fundamental. Figure 2 shows a typical LTC1278 FFT plot.
0 -20 fSAMPLE = 500kHz 5V fIN = 97.045kHz
EFFECTIVE NUMBER OF BITS
AMPLITUDE (dB)
-40 -60 -80
-100 -120 0 50k 100k 150k FREQUENCY (Hz) 200k 250k
LTC1278 F2
Figure 2. LTC1278 Nonaveraged, 4096 Point FFT Plot
Signal-to-Noise Ratio The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input frequency to the RMS amplitude of all other frequency components at the A/D output. The output is band limited to frequencies from above DC and below half the sampling frequency. Figure 2 shows a typical spectral content with
U
W
U
U
a 500kHz sampling rate and a 100kHz input. The dynamic performance is excellent for input frequencies up to the Nyquist limit of 250kHz. Effective Number of Bits The Effective Number of Bits (ENOBs) is a measurement of the resolution of an ADC and is directly related to the S/(N + D) by the equation: N = [S/(N + D) - 1.76]/6.02 where N is the Effective Number of Bits of resolution and S/(N + D) is expressed in dB. At the maximum sampling rate of 500kHz the LTC1278 maintains very good ENOBs up to the Nyquist input frequency of 250kHz. Refer to Figure 3.
12 11 10 9 8 7 6 5 4 3 2 1 0 10k fSAMPLE = 500kHz 100k INPUT FREQUENCY (Hz) 1M NYQUIST FREQUENCY
74 68 62 56
S/(N+D) (dB)
2M
LT1278 G4
Figure 3. Effective Bits and Signal-to-Noise + Distortion vs Input Frequency
Total Harmonic Distortion Total Harmonic Distortion (THD) is the ratio of the RMS sum of all harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is expressed as:
THD = 20log
V22 + V32 + V42 ... + VN2 V1
where V1 is the RMS amplitude of the fundamental frequency and V2 through VN are the amplitudes of the second through Nth harmonics. THD versus input
9
LTC1278
APPLICATIONS INFORMATION
frequency is shown in Figure 4. The LTC1278 has good distortion performance up to the Nyquist frequency and beyond.
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 10k 2ND HARMONIC THD
-100 -120 0 50k 100k 150k FREQUENCY (Hz) 200k 250k
LTC1278 G8
fSAMPLE = 500kHz
AMPLITUDE (dB)
3RD HARMONIC
100k INPUT FREQUENCY (Hz)
1M
2M
LT1278 G6
Figure 4. Distortion vs Input Frequency
Intermodulation Distortion If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) in addition to THD. IMD is the change in one sinusoidal input caused by the presence of another sinusoidal input at a different frequency. If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer function can create distortion products at sum and difference frequencies of mfa nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa + fb) and (fa - fb) while the 3rd order IMD terms include (2fa + fb), (2fa - fb), (fa + 2fb), and (fa - 2fb). If the two input sine waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula:
IMD (fa fb) = 20log
Amplitude at (fa fb) Amplitude at fa
Figure 5 shows the IMD performance at a 100kHz input.
10
U
W
U
U
0 -20 -40 -60 -80 fSAMPLE = 500kHz fIN1 = 96.80kHz fIN2 = 101.68kHz
Figure 5. Intermodulation Distortion Plot
Peak Harmonic or Spurious Noise The peak harmonic or spurious noise is the largest spectral component excluding the input signal and DC. This value is expressed in decibels relative to the RMS value of a full-scale input signal. Full Power and Full Linear Bandwidth The full power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full-scale input signal. The full linear bandwidth is the input frequency at which the S/(N + D) has dropped to 68dB (11 effective bits). The LTC1278 has been designed to optimize input bandwidth, allowing ADC to undersample input signals with frequencies above the converter's Nyquist Frequency. The noise floor stays very low at high frequencies; S/(N + D) becomes dominated by distortion at frequencies far beyond Nyquist. Driving the Analog Input The analog input of the LTC1278 is easy to drive. It draws only one small current spike while charging the sampleand-hold capacitor at the end of conversion. During conversion the analog input draws no current. The only requirement is that the amplifier driving the analog input must settle after the small current spike before the next
LTC1278
APPLICATI
S I FOR ATIO
conversion starts. Any op amp that settles in 200ns to small current transients will allow maximum speed operation. If slower op amps are used, more settling time can be provided by increasing the time between conversions. Suitable devices capable of driving the ADC's AIN input include the LT1360, LT1220, LT1223 and LT1224 op amps. Internal Reference The LTC1278 has an on-chip, temperature compensated, curvature corrected, bandgap reference, which is factory trimmed to 2.42V. It is internally connected to the DAC and is available at Pin 2 to provide up to 1mA current to an external load. For minimum code transition noise the reference output should be decoupled with a capacitor to filter wideband noise from the reference (10F tantalum in parallel with a 0.1F ceramic). The VREF pin can be driven with a DAC or other means to provide input span adjustment in bipolar mode. The VREF pin must be driven to at least 2.45V to prevent conflict with the internal reference. The reference should be driven to no more than 4.8V to keep the input span within the 5V supplies. Figure 6 shows an LT1006 op amp driving the reference pin. (In the unipolar mode, the input span is already 0V to 5V with the internal reference so driving the reference is not recommended, since the input span will exceed the supply and codes will be lost at the full scale.) Figure 7 shows a typical reference, the LT1019A-2.5 connected to the LTC1278. This will provide an improved drift (equal to the maximum 5ppm/C of the LT1019A-2.5) and a 2.582V full scale.
INPUT RANGE 1.033VREF(OUT) 5V
OUTPUT CODE
OUTPUT CODE
+
LT1006
VREF(OUT) 2.45V 3 10F
LTC1278 AIN VREF AGND
-
Figure 6. Driving the VREF with the LT1006 Op Amp
U
INPUT RANGE 2.58V (= 1.033 x VREF) 5V 5V VIN VOUT LT1019A-2.5 GND 3 10F -5V LTC1278 AIN VREF AGND
LTC1278 F7
W
U
UO
Figure 7. Supplying a 2.5V Reference Voltage to the LTC1278 with the LT1019A-2.5
UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT Figure 8a shows the ideal input/output characteristics for the LTC1278. The code transitions occur midway between successive integer LSB values (i.e., 0.5LSB, 1.5LSB, 2.5LSB, ... FS - 1.5LSB). The output code is naturally binary with 1LSB = FS/4096 = 5V/4096 = 1.22mV. Figure 8b shows the input/output transfer characteristics for the bipolar mode in two's complement format.
111...111 111...110 111...101 111...100 1LSB = FS = 5V 4096 4096
000...011 000...010 000...001 000...000 0V
UNIPOLAR ZERO
1 LSB INPUT VOLTAGE (V)
FS - 1LSB
LTC1278 F8a
Figure 8a. LTC1278 Unipolar Transfer Characteristics
011...111 011...110 BIPOLAR ZERO
000...001 000...000 111...111 111...110
100...001 -5V
LTC1278 F6
100...000 -FS/2
FS = 5V 1LSB = FS/4096 -1 0V 1 LSB LSB INPUT VOLTAGE (V) FS/2 - 1LSB
LTC1278 * F8b
Figure 8b. LTC1278 Bipolar Transfer Characteristics
11
LTC1278
APPLICATI
R1 50 V1
S I FOR ATIO
+
A1
AIN R4 100 LTC1278
-
R2 10k R3 10k
FULL-SCALE ADJUST AGND
LTC1278 F9a
ADDITIONAL PINS OMITTED FOR CLARITY 20LSB TRIM RANGE
Figure 9a. Full-Scale Adjust Circuit
ANALOG INPUT 0V TO 5V 5V R9 20 R1 10k R2 10k
+
AIN
10k
-
R4 100k R5 4.3k FULL-SCALE 5V ADJUST R3 100k R7 100k R6 400 LTC1278
R8 10k OFFSET ADJUST
Figure 9b. LTC1278 Unipolar Offset and Full-Scale Adjust Circuit
Unipolar Offset and Full-scale Error Adjustments In applications where absolute accuracy is important, then offset and full-scale errors can be adjusted to zero. Offset error must be adjusted before full-scale error. Figure 9a shows the extra components required for full-scale error adjustment. If both offset and full-scale adjustments are needed, the circuit in Figure 9b can be used. For zero offset error apply 0.61mV (i.e., 1/2LSB) at the input and adjust the offset trim until the LTC1278 output code flickers between 0000 0000 0000 and 0000 0000 0001. For zero full-scale error apply an analog input of 4.99817V (i.e., FS - 1 1/2LSB or last code transition) at the input and adjust R5 until the LTC1278 output code flickers between 1111 1111 1110 and 1111 1111 1111. Bipolar Offset and Full-scale Error Adjustments Bipolar offset and full-scale errors are adjusted in a similar fashion to the unipolar case. Again, bipolar offset must be adjusted before full-scale error. Bipolar offset error adjustment is achieved by trimming the offset of the op amp
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ANALOG INPUT R1 10k R2 10k
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AIN
-
R4 100k R5 4.3k FULL-SCALE 5V ADJUST R3 100k R7 100k R6 200 LTC1278
R8 20k OFFSET ADJUST -5V
LTC1278 F9c
Figure 9c. LTC1278 Bipolar Offset and Full-Scale Adjust Circuit
LTC1278 F9b
driving the analog input of the LTC1278 while the input voltage is 1/2LSB below ground. This is done by applying an input voltage of - 0.61mV (- 0.5LSB) to the input in Figure 9c and adjusting the R8 until the ADC output code flickers between 0000 0000 0000 and 1111 1111 1111. For full-scale adjustment, an input voltage of 2.49817V (FS - 1.5LSBs) is applied to the input and R5 is adjusted until the output code flickers between 0111 1111 1110 and 0111 1111 1111. BOARD LAYOUT AND BYPASSING Wire wrap boards are not recommended for high resolution or high speed A/D converters. To obtain the best performance from the LTC1278, a printed circuit board is required. Layout for the printed circuit board should ensure that digital and analog signal lines are separated as much as possible. In particular, care should be taken not to run any digital track alongside an analog signal track or underneath the ADC. The analog input should be screened by AGND. High quality tantalum and ceramic bypass capacitors should be used at the AVDD and VREF pins as shown in Figure 10. For the bipolar mode, a 0.1F ceramic provides adequate bypassing for the VSS pin. The capacitors must be located as close to the pins as possible. The traces connecting the pins and the bypass capacitors must be kept short and should be made as wide as possible. Input signal leads to AIN and signal return leads from AGND (Pin 3) should be kept as short as possible to minimize input noise coupling. In applications where this is not possible, a shielded cable between source and ADC is recommended.
LTC1278
APPLICATI
S I FOR ATIO
Also, since any potential difference in grounds between the signal source and ADC appears as an error voltage in series with the input signal, attention should be paid to reducing the ground circuit impedances as much as possible. A single point analog ground separate from the logic system ground should be established with an analog ground plane at Pin 3 (AGND) or as close as possible to the ADC. Pin 12 (DGND) and all other analog grounds should be connected to this single analog ground point. No other digital grounds should be connected to this analog ground point. Low impedance analog and digital power supply common returns are essential to low noise operation of the ADC and the foil width for these tracks should be as wide as possible. In applications where the ADC data outputs and control signals are connected to a continuously active microprocessor bus, it is possible to get errors in conversion results. These errors are due to feedthrough from the microprocessor to the successive approximation comparator. The problem can be eliminated by forcing the microprocessor into a WAIT state during conversion or by using three-state buffers to isolate the ADC data bus.
1 AIN AGND ANALOG INPUT CIRCUITRY VREF 2 10F 0.1F 10F LTC1278
+ -
3
ANALOG GROUND PLANE
Figure 10. Power Supply Grounding Practice
ACTIVE HIGH ENABLE THREE-STATE OUTPUTS DB11....DB0 BUSY D Q CONVERSION START (RISING EDGE TRIGGER)
RD
CS
CONVST
SHDN
Figure 11. Internal Logic for Control Inputs CS, RD, CONVST and SHDN
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DIGITAL INTERFACE The A/D converter is designed to interface with microprocessors as a memory mapped device. The CS and RD control inputs are common to all peripheral memory interfacing. A separate CONVST is used to initiate a conversion. Internal Clock The A/D converter has an internal clock that eliminates the need of synchronization between the external clock and the CS and RD signals found in other ADCs. The internal clock is factory trimmed to achieve a typical conversion time of 1.6s. No external adjustments are required, and with the typical acquisition time of 250ns, throughput performance of 500ksps is assured. Power Shutdown The LTC1278 provides a shutdown feature that will save power when the ADC is in inactive periods. To power down the ADC, Pin 18 (SHDN) needs to be driven low. When in power shutdown mode, the LTC1278 will not start a conversion even though the CONVST goes low. All the
DIGITAL SYSTEM AVDD 24 0.1F DVDD 17 DGND 12 GROUND CONNECTION TO DIGITAL CIRCUITRY
LTC1278 F10
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FLIP FLOP CLEAR
LTC1278 F11
13
LTC1278
APPLICATI
S I FOR ATIO
power is off except the Internal Reference which is still active and provides 2.42V output voltage to the other circuitry. In this mode the ADC draws 8.5mW instead of 75mW (for minimum power, the logic inputs must be within 600mV of the supply rails). The wake-up time from the power shutdown to active state is 350ns. Timing and Control Conversion start and data read operations are controlled by three digital inputs: CS, CONVST and RD. Figure 11 shows the logic structure associated with these inputs. A logic "0" for CONVST will start a conversion after the ADC has been selected (i.e., CS is low). Once initiated it cannot be restarted until the conversion is complete. Converter status is indicated by the BUSY output, and this is low while conversion is in progress. Figures 12 through 16 show several different modes of operation. In modes 1a and 1b (Figures 12 and 13) CS and RD are both tied low. The falling CONVST starts the conversion. The data outputs are always enabled and data can be latched with the BUSY rising edge. Mode 1a shows operation with a narrow low going CONVST pulse. Mode 1b shows high going CONVST pulse.
CS = RD = 0 t4 SAMPLE N CONVST t5 BUSY t6 DATA DATA (N-1) DB11 TO DB0 tCONV
Figure 12. Mode 1a. CONVST Starts a Conversion. Data Ouputs Always Enabled. (CONVST =
CS = RD = 0 t11 SAMPLE N CONVST t5 BUSY t6 DATA DATA (N-1) DB11 TO DB0 DATA N DB11 TO DB0 DATA (N + 1) DB11 TO DB0
LTC1278 F13
tCONV SAMPLE N + 1
Figure 13. Mode 1b. CONVST Starts a Conversion. Data Outputs Always Enabled. (CONVST =
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In mode 2 (Figure 14) CS is tied low. The falling CONVST signal again starts the conversion. Data outputs are in three-state until read by MPU with the RD signal. Mode 2 can be used for operation with a shared MPU databus. In Slow memory and ROM modes (Figures 15 and 16) CS is tied low and CONVST and RD are tied together. The MPU starts conversion and read the output with the RD signal. Conversions are started by the MPU or DSP (no external sample clock). In Slow memory mode the processor takes RD (= CONVST) low and starts the conversion. BUSY goes low forcing the processor into a WAIT state. The previous conversion result appears on the data outputs. When the conversion is complete, the new conversion results appear on the data outputs; BUSY goes high releasing the processor, and the processor takes RD (= CONVST) back high and reads the new conversion data. In ROM mode, the processor takes RD (= CONVST) low which starts a conversion and reads the previous conversion result. After the conversion is complete, the processor can read the new result (which will initiate another conversion).
SAMPLE N + 1 DATA N DB11 TO DB0 DATA (N + 1) DB11 TO DB0
LTC1278 F12
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t5
)
LTC1278
APPLICATI
CS = 0
S I FOR ATIO
t11 tCONV t4 SAMPLE N
CONVST t5 BUSY t7 RD t8 DATA DATA N DB11 TO DB0 DATA (N + 1) DB11 TO DB0
LTC1278 F14
t10
Figure 14. Mode 2. CONVST Starts a Conversion. Data is Read by RD
CS = 0 SAMPLE N RD = CONVST t5 BUSY t8 DATA
tCONV SAMPLE N + 1
t6 DATA (N - 1) DB11 TO DB0 DATA N DB11 TO DB0 DATA N DB11 TO DB0 DATA (N + 1) DB11-DB0
LTC1278 F15
Figure 15. Slow Memory Mode
CS = 0 SAMPLE N RD = CONVST t5 BUSY t8 DATA
tCONV SAMPLE N + 1
t9
DATA (N - 1) DB11 TO DB0
Figure 16. ROM Mode Timing
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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SAMPLE N + 1 t9
t9 DATA N DB11 TO DB0
LTC1278 F16
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LTC1278
PACKAGE DESCRIPTIO
0.300 - 0.325 (7.620 - 8.255)
0.009 - 0.015 (0.229 - 0.381)
(
+0.035 0.325 -0.015 +0.889 8.255 -0.381
)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
0.291 - 0.299** (7.391 - 7.595)
0.010 - 0.029 x 45 (0.254 - 0.737)
0.093 - 0.104 (2.362 - 2.642)
0 - 8 TYP
0.009 - 0.013 (0.229 - 0.330)
NOTE 1
0.050 (1.270) TYP
0.016 - 0.050 (0.406 - 1.270) NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
0.014 - 0.019 (0.356 - 0.482) TYP
RELATED PARTS
PART NUMBER LTC1274/LTC1277 LTC1279 LTC1400 LTC1409 LTC1415 LTC1419 DESCRIPTION 12-Bit, 10mW, 100ksps A/D Converters with 1A Shutdown 12-Bit, 600ksps Sampling A/D Converter with Shutdown 12-Bit, 400ksps Serial A/D Converter 12-Bit, 800ksps Sampling A/D Converter with Shutdown 12-Bit, 1.25Msps Sampling A/D Converter with Shutdown 14-Bit, 800ksps Sampling A/D Converter with Shutdown COMMENTS Complete with Clock Reference 70dB SINAD at Nyquist, Low Power Complete High Speed 12-Bit ADC in SO-8 Fast, Complete Low Power ADC Single 5V Supply, Low Power: 55mW 81.5dB SINAD, Low Power: 150mW
1278fa LT/GP 0998 REV A 2K * PRINTED IN USA
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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Dimensions in inches (millimeters) unless otherwise noted. N Package 24-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
1.265* (32.131) MAX 24 23 22 21 20 19 18 17 16 15 14 13
0.255 0.015* (6.477 0.381)
1 0.130 0.005 (3.302 0.127) 0.020 (0.508) MIN
2
3
4
5
6
7
8
9
10
11
12
0.045 - 0.065 (1.143 - 1.651)
0.065 (1.651) TYP 0.018 0.003 (0.457 0.076)
N24 1197
0.125 (3.175) MIN
0.100 0.010 (2.540 0.254)
SW Package 24-Lead Plastic Small Outline (Wide 0.300)
(LTC DWG # 05-08-1620)
0.598 - 0.614* (15.190 - 15.600) 24 23 22 21 20 19 18 17 16 15 14 13
0.037 - 0.045 (0.940 - 1.143) NOTE 1 0.394 - 0.419 (10.007 - 10.643)
0.004 - 0.012 (0.102 - 0.305) 1 2 3 4 5 6 7 8 9 10 11 12
S24 (WIDE) 0996
(c) LINEAR TECHNOLOGY CORPORATION 1994

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