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FEATURES 12.5 MHz Master Clock Frequency 0 V to +2.5 V or 1.25 V Input Range Single Bit Output Stream 90 dB Dynamic Range Power Supplies: AVDD, DVDD: +5 V 5% On-Chip 2.5 V Voltage Reference 28-Lead TSSOP
CMOS Sigma-Delta Modulator AD7720
FUNCTIONAL BLOCK DIAGRAM
AVDD AGND DVDD DGND REF1
AD7720
2.5V REFERENCE
REF2
VIN(+) VIN(-)
SIGMA-DELTA MODULATOR
DATA SCLK
MZERO GC
CLOCK CIRCUITRY
XTAL1/MCLK XTAL2
BIP
CONTROL LOGIC
DVAL RESETO RESET
STBY
GENERAL DESCRIPTION
This device is a 7th order sigma-delta modulator that converts the analog input signal into a high speed 1-bit data stream. The part operates from a +5 V supply and accepts a differential input range of 0 V to +2.5 V or 1.25 V centered about a commonmode bias. The analog input is continuously sampled by the analog modulator, eliminating the need for external sample and hold circuitry. The input information is contained in the output stream as a density of ones. The original information can be reconstructed with an appropriate digital filter. The part provides an accurate on-chip 2.5 V reference. A reference input/output function is provided to allow either the internal reference or an external system reference to be used as the reference source for the part. The device is offered in a 28-lead TSSOP package and designed to operate from -40C to +85C.
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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., 1997
AD7720-SPECIFICATIONS1 REF2 = +2.5 V; T = T
(AVDD = +5 V
A
5%; DVDD = +5 V 5%; AGND = DGND = 0 V, fMCLK = 12.5 MHz, MIN to TMIN, unless otherwise noted)
Test Conditions/Comments When Tested with Ideal FIR Filter as in Figure 1
Parameter STATIC PERFORMANCE Resolution Differential Nonlinearity Integral Nonlinearity Precalibration Offset Error Precalibration Gain Error2 Postcalibration Offset Error3 Postcalibration Gain Error2, 3 Offset Error Drift Gain Error Drift Unipolar Mode Bipolar Mode ANALOG INPUTS Signal Input Span (VIN(+) - VIN(-)) Bipolar Mode Unipolar Mode Maximum Input Voltage Minimum Input Voltage Input Sampling Capacitance Input Sampling Rate Differential Input Impedance REFERENCE INPUTS REF1 Output Voltage REF1 Output Voltage Drift REF1 Output Impedance Reference Buffer Offset Voltage Using Internal Reference REF2 Output Voltage REF2 Output Voltage Drift Using External Reference REF2 Input Impedance External Reference Voltage Range DYNAMIC SPECIFICATIONS4 Bipolar Mode Signal to (Noise + Distortion)5 Total Harmonic Distortion5 Spurious Free Dynamic Range Unipolar Mode Signal to (Noise + Distortion)5 Total Harmonic Distortion5 Spurious Free Dynamic Range Intermodulation Distortion AC CMRR Overall Digital Filter Response 0 kHz-90.625 kHz 96.92 kHz 104.6875 kHz to 12.395 MHz CLOCK MCLK Duty Ratio VMCLKH, MCLK High Voltage VMCLKL, MCLK Low Voltage
B Version 16 1 2 6 0.6 1.5 0.3 1 1 0.5
Units Bits LSB max LSB typ mV typ % FSR typ mV typ % FSR typ LSB/C typ LSB/C typ LSB/C typ
Guaranteed Monotonic
REF2 Is an Ideal Reference, REF1 = AGND
VREF2/2 0 to VREF2 AVDD 0 2 2 fMCLK 109/(8 fMCLK) 2.32 to 2.62 60 3 12 2.32 to 2.62 60 109/(16 fMCLK) 2.32 to 2.62
V max V max V V pF typ MHz k typ V min/max ppm/C typ k typ mV max V min/max ppm/C typ
BIP = VIH BIP = VIL
Offset Between REF1 and REF2
REF1 = AGND k typ V min/max Applied to REF1 or REF2 When Tested with Ideal FIR Filter as in Figure 1 BIP = VIH, VCM = 2.5 V, VIN(+) = VIN(-) = 1.25 V p-p or VIN(-) = 1.25 V, VIN(+) = 0 V to 2.5 V Input BW = 0 kHz-90.625 kHz Input BW = 0 kHz-90.625 kHz Input BW = 0 kHz-90.625 kHz BIP = VIL, VIN(-) = 0 V, VIN(+) = 0 V to 2.5 V Input BW = 0 kHz-90.625 kHz Input BW = 0 kHz-97.65 kHz Input BW = 0 kHz-97.65 kHz VIN(+) = VIN(-) = 2.5 V p-p, VCM = 1.25 V to 3.75 V, 20 kHz See Figure 1 for Characteristics of FIR Filter
90 86/84.5 -90/-88 -90 88 84.5/83 -89/-87 -90 -93 96 0.005 -3 90 45 to 55 4 0.4
dB typ dB min dB max dB max dB typ dB min dB max dB max dB typ dB typ
dB max dB min dB typ % max V min V max For Specified Operation MCLK Uses CMOS Logic
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AD7720
Parameter LOGIC INPUTS VIH, Input High Voltage VIL, Input Low Voltage IINH, Input Current CIN, Input Capacitance LOGIC OUTPUTS VOH, Output High Voltage VOL, Output Low Voltage POWER SUPPLIES AVDD DVDD IDD (Total for AVDD, DVDD) Active Mode Standby Mode B Version 2 0.8 10 10 2.4 0.4 4.75/5.25 4.75/5.25 43 25 Units V min V max A max pF max V min V max V min/V max V min/V max Digital Inputs Equal to 0 V or DVDD mA max A max |IOUT| 200 A |IOUT| 1.6 mA Test Conditions/Comments
NOTES 1 Operating temperature range is as follows: B Version: -40C to +85C. 2 Gain Error excludes reference error. The modulator gain is calibrated w.r.t. the voltage on the REF2 pin. 3 Applies after calibration at temperature of interest. 4 Measurement Bandwidth = 0.5 x fMCLK; Input Level = -0.05 dB. 5 TA = +25C to +85C/TA = TMIN to TMAX. Specifications subject to change without notice.
BIT STREAM
90.625kHz 120dB DECIMATE BY 32
90.625kHz 90dB DECIMATE BY 2 16-BIT OUTPUT
FILTER 1
292.969kHz
FILTER 2
104.687kHz
BANDWIDTH = 90.625 kHz TRANSITION = 292.969kHz ATTENUATION = 120dB COEFFICIENTS = 384
BANDWIDTH = 90.625 kHz TRANSITION = 104.687kHz ATTENUATION = 90dB COEFFICIENTS = 151
Figure 1. Digital Filter (Consists of 2 FIR Filters). This filter is implemented on the AD7722.
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AD7720 TIMING CHARACTERISTICS (AVDD = +5 V
Parameter fMCLK t1 t2 t3 t4 t5 t6 t7
NOTE Guaranteed by design.
5%; DVDD = +5 V
5%; AGND = DGND = 0 V, REF2 = +2.5 V unless otherwise noted)
Limit at TMIN, TMAX (B Version) 100 15 67 0.45 x tMCLK 0.45 x tMCLK 15 10 10 20 x tMCLK
Units kHz min MHz max ns min ns min ns min ns min ns min ns min ns max
Conditions/Comments Master Clock Frequency 12.5 MHz for Specified Performance Master Clock Period Master Clock Input High Time Master Clock Input Low Time Data Hold Time After SCLK Rising Edge RESET Pulsewidth RESET Low Time Before MCLK Rising DVAL High Delay after RESET Low
IOL 1.6mA TO OUTPUT PIN
+1.6V CL 50pF IOH 200 A
Figure 2. Load Circuit for Access Time and Bus Relinquish Time
t1 t2 t3 t4
DATA (O)
SCLK (O)
NOTE: O SIGNIFIES AN OUTPUT
Figure 3. Data Timing
MCLK (I)
t6
RESET (I)
t5 t7
DVAL (O)
NOTE: I SIGNIFIES AN INPUT O SIGNIFIES AN OUTPUT
Figure 4. RESET Timing
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ABSOLUTE MAXIMUM RATINGS 1
(TA = +25C unless otherwise noted)
PIN CONFIGURATION
REF2 1 AGND 2 NC 3 STBY 4 DVAL 5 DGND 6 GC 7
28 27 26 25 24
DVDD to DGND . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V AVDD to AGND . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V AVDD to DVDD . . . . . . . . . . . . . . . . . . . . -0.3 V to +0.3 V AGND to DGND . . . . . . . . . . . . . . . . . . . . -0.3 V to +0.3 V Digital Input Voltage to DGND . . -0.3 V to DVDD + 0.3 V Analog Input Voltage to AGND . . . -0.3 V to AVDD + 0.3 V Input Current to Any Pin Except Supplies2 . . . . . . . 10 mA Operating Temperature Range Industrial (B Version) . . . . . . . . . . . . . . . -40C to +85C Storage Temperature Range . . . . . . . . . . . . -65C to +150C Maximum Junction Temperature . . . . . . . . . . . . . . . +150C TSSOP Package JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 120C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . +215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220C
NOTES 1 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 listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 Transient currents of up to 100 mA will not cause SCR latchup.
AVDD REF1 AGND AVDD AGND VIN(+)
AD7720
23
TOP VIEW 22 RESET BIP 8 (Not to Scale) 21 VIN(-) MZERO 9 DATA 10 SCLK 11 RESETO 12 NC 13 AGND 14
20 19 18 17 16 15
AGND DVDD AGND XTAL2 XTAL1/MCLK DGND
NC = NO CONNECT
ORDERING GUIDE
Model AD7720BRU
Temperature Range -40C to +85C
Package Description 28-Lead Thin Shrink Small Outline
Package Option RU-28
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 AD7720 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
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AD7720
PIN FUNCTION DESCRIPTIONS
Pin No. 1
Mnemonic REF2
Function
2, 14, 18, 20, 24, 26 3, 13 4 5
6, 15 7 8 9
10 11 12 16
17 19 21, 23
25, 28 22
27
Reference Input/Output. REF2 connects to the output of an internal buffer amplifier used to drive the sigma-delta modulator. When REF2 is used as an input, REF1 must be connected to AGND. AGND Ground reference point for analog circuitry. NC No Connect. STBY Standby, Logic Input. When STBY is high, the device is placed in a low power mode. When STBY is low, the device is powered up. DVAL Data Valid Logic Output. A logic high on DVAL indicates that the data bit stream from the AD7720 is an accurate digital representation of the analog voltage at the input to the sigma-delta modulator. The DVAL pin is set low for 20 MCLK cycles if the analog input is overranged. DGND Ground reference for the digital circuitry. GC Digital Control Input. When GC is high, the gain error of the modulator can be calibrated. BIP Analog Input Range Select, Logic Input. A logic low on this input selects unipolar mode. A logic high selects bipolar mode. MZERO Digital Control Input. When MZERO is high, the modulator inputs are internally grounded, i.e., tied to AGND in unipolar mode and REF2 in bipolar mode. MZERO allows on-chip offsets to be calibrated out. MZERO is low for normal operation. DATA Modulator Bit Stream. The digital bit stream from the sigma-delta modulator is output at DATA. SCLK Serial Clock, Logic Output. The bit stream from the modulator is valid on the rising edge of SCLK. RESETO Reset Logic Output. The signal applied to the RESET pin is made available as an output at RESETO. XTAL1/MCLK CMOS Logic Clock Input. The XTAL1/MCLK pin interfaces the device's internal oscillator circuit to an external crystal or an external clock. A parallel resonant, fundamental-frequency, microprocessor-grade crystal and a 1 M resistor should be connected between the MCLK and XTAL pins with two capacitors connected from each pin to ground. Alternatively, the XTAL1/MCLK pin can be driven with an external CMOS-compatible clock. The part is specified with a 12.5 MHz master clock. XTAL2 Oscillator Output. The XTAL2 pin connects the internal oscillator output to an external crystal. If an external clock is used, XTAL2 should be left unconnected. DVDD Digital Supply Voltage, +5 V 5%. VIN(-), VIN(+) Analog Input. In unipolar operation, the analog input range on VIN(+) is VIN(-) to (VIN(-) + VREF); for bipolar operation, the analog input range on VIN+ is (VIN(-) VREF/2). The absolute analog input range must lie between 0 and AVDD. The analog input is continuously sampled and processed by the analog modulator. AVDD Analog Positive Supply Voltage, +5 V 5%. RESET Reset Logic Input. RESET is an asynchronous input. When RESET is taken high, the sigma-delta modulator is reset by shorting the integrator capacitors in the modulator. DVAL goes low for 20 MCLK cycles while the modulator is being reset. REF1 Reference Input/Output. REF1 connects via a 3 k resistor to the output of the internal 2.5 V reference, and to the input of a buffer amplifier that drives the sigma-delta modulator. This pin can also be overdriven with an external 2.5 V reference.
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AD7720
TERMINOLOGY (IDEAL FIR FILTER USED WITH AD7720 [FIGURE 1]) Integral Nonlinearity
This is the maximum deviation of any code from a straight line passing through the endpoints of the transfer function. The endpoints of the transfer function are zero scale (not to be confused with bipolar zero), a point 0.5 LSB below the first code transition (100 . . . 00 to 100 . . . 01 in bipolar mode and 000 . . . 00 to 000 . . . 01 in unipolar mode) and full scale, a point 0.5 LSB above the last code transition (011 . . . 10 to 011 . . . 11 in bipolar mode and 111 . . . 10 to 111 . . . 11 in unipolar mode). The error is expressed in LSBs.
Differential Nonlinearity
fundamental. Noise plus distortion is the rms sum of all of the nonfundamental signals and harmonics to half the output word rate (fMCLK/128), excluding dc. Signal-to-(Noise + Distortion) is dependent on the number of quantization levels used in the digitization process; the more levels, the smaller the quantization noise. The theoretical Signal-to-(Noise + Distortion) ratio for a sine wave input is given by Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB where N is the number of bits.
Total Harmonic Distortion
This is the difference between the measured and the ideal 1 LSB change between two adjacent codes in the ADC.
Common-Mode Rejection Ratio
Total Harmonic Distortion (THD) is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7720, THD is defined as THD = 20 log (V 22 +V 32 +V 42 +V 52 +V 62) V1
The ability of a device to reject the effect of a voltage applied to both input terminals simultaneously--often through variation of a ground level--is specified as a common-mode rejection ratio. CMRR is the ratio of gain for the differential signal to the gain for the common-mode signal.
Unipolar Offset Error
where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic.
Spurious Free Dynamic Range
Unipolar offset error is the deviation of the first code transition from the ideal VIN(+) voltage which is (VIN(-) + 0.5 LSB) when operating in the unipolar mode.
Bipolar Offset Error
This is the deviation of the midscale transition (111 . . . 11 to 000 . . . 00) from the ideal VIN(+) voltage which is (VIN(-) -0.5 LSB) when operating in the bipolar mode.
Gain Error
Spurious free dynamic range is the difference, in dB, between the peak spurious or harmonic component in the ADC output spectrum (up to fMCLK/128 and excluding dc) and the rms value of the fundamental. Normally, the value of this specification will be determined by the largest harmonic in the output spectrum of the FFT. For input signals whose second harmonics occur in the stop band region of the digital filter, a spur in the noise floor limits the spurious free dynamic range.
Intermodulation Distortion
The first code transition should occur at an analog value 1/2 LSB above minus full scale. The last code transition should occur for an analog value 3/2 LSB below the nominal full scale. Gain error is the deviation of the actual difference between first and last code transitions and the ideal difference between first and last code transitions.
Signal-to-(Noise + Distortion)
Signal-to-(Noise + Distortion) is measured signal-to-noise at the output of the ADC. The signal is the rms magnitude of the
With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb where m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are those for which neither m or n are equal to zero. For example, the second order terms include (fa + fb) and (fa - fb), while the third order terms include (2fa + fb), (2fa - fb), (fa + 2fb) and (fa - 2fb).
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AD7720-Typical Characteristics
(AVDD = DVDD = 5.0 V, TA = +25 C; CLKIN = 12.5 MHz, AIN = 20 kHz, Bipolar Mode; VIN(+) = 0 V to 2.5 V, VIN(-) = 1.25 V unless otherwise noted)
110
84 85 AIN = 1/5 * BW 86 -85
100
-90 SNR -95
90 SFDR
87 dB 88 89 90
dB
80 S/ (N+D) 70
dB
-100
-105 SFDR -110 THD 0 20 40 60 80 INPUT FREQUENCY - kHz 100
60
91
50 -40
-30
-20 -10 INPUT LEVEL - dB
0
92
0
50 100 150 200 250 OUTPUT DATA RATE - kSPS
300
-115
Figure 5. S/(N+D) and SFDR vs. Analog Input Level
Figure 6. S/(N+D) vs. Output Sample Rate
Figure 7. SNR, THD, and SFDR vs. Input Frequency
-85
84 85 SNR VIN (+) = VIN(-) = 1.25Vpk-pk VCM = 2.5V
dB
92.0 91.5
-90
86 87
-95
dB
AIN = 1/5 * BW VIN (+) = VIN(-) = 1.25Vpk-pk VCM = 2.5V
91.0 90.5
dB
-100
THD
88 89
90.0 89.5 89.0 88.5 88.0 -50
-105 SFDR -110
90 91 92
-115
0
20 40 60 80 INPUT FREQUENCY - kHz
100
0
50 100 150 200 250 OUTPUT DATA RATE - kSPS
300
0 50 TEMPERATURE - C
100
Figure 8. SNR, THD, and SFDR vs. Input Frequency
Figure 9. S/(N+D) vs. Output Sample Rate
Figure 10. SNR vs. Temperature
-94 -96 THD
5000 4500
1.0 0.8
FREQUENCY OF OCCURENCE
-98 -100 -102
dB
4000 3500 3000 2500 2000 1500 1000 500
DNL ERROR - LSB
VIN(+) = VIN(-) CLKIN = 12.5MHz 8k SAMPLES
0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0
-104 -106 -108 -110 -112 -114 -116 -50 -25 0 25 50 TEMPERATURE - C 75
3RD
4TH
2ND 100 0 n-3 n-2 n-1 n n+1 CODES n+2 n+3
0
20000
40000 CODE
65535
Figure 11. THD vs. Temperature
Figure 12. Histogram of Output Codes with DC Input
Figure 13. Differential Nonlinearity
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AD7720
1.0 0.8 0.6
INL ERROR - LSB
0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 20000 40000 CODE 65535
Figure 14. Integral Nonlinearity Error
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130 0 FREQUENCY - MHz 6.25
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130 0 FREQUENCY - kHz 393.295 kHz
dB
Figure 15. Modulator Output (0 Hz to MCLK/2)
dB
Figure 18. Modulator Output (0 to 393.295 kHz)
0 -20 -40 -60 CLKIN = 12.5MHz SNR = 90.1dB S/(N+D) = 89.2dB SFDR = -99.5dB THD = -96.6dB 2ND = -100.9dB 3RD = -106.0dB 4TH = -99.5dB
0 -20 -40 -60
dB
AIN = 90kHz CLKIN = 12.5 MHz SNR = 89.6dB S/(N+D) = 89.6dB SFDR = -108.0dB
dB
-80 -100 -120 -140 -154 0E+0 10E+3 20E+3 30E+3 40E+3
-80 -100 -120 -140
50E+3 60E+3
70E+3 80E+3 90E+3 98E+3
-154 0E+0
10E+3
20E+3 30E+3
40E+3
50E+3 60E+3
70E+3 80E+3 90E+3 98E+3
Figure 16. 16K Point FFT
Figure 19. 16K Point FFT
0 -20 -40 -60 XTAL = 12.288MHz SNR = 89.0dB S/(N+D) = 87.8dB SFDR = -94.3dB THD = -93.8dB 2ND = -94.3dB 3RD = -108.5dB 4TH = -105.7dB
0 -20 -40 -60 AIN = 90kHz XTAL = 12.288MHz SNR = 88.1dB S/(N+D) = 88.1dB SFDR = -103.7dB
dB
-80 -100 -120 -140 -154 0E+0 10E+3 20E+3 30E+3 40E+3
dB
-80 -100 -120 -140
50E+3 60E+3
70E+3
80E+3 90E+3 96E+3
-154 0E+0
10E+3
20E+3 30E+3
40E+3
50E+3 60E+3
70E+3
80E+3 90E+3 96E+3
Figure 17. 16K Point FFT
Figure 20. 16K Point FFT
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AD7720
CIRCUIT DESCRIPTION Sigma-Delta ADC
A
500 VIN(+)
B
The AD7720 ADC employs a sigma-delta conversion technique that converts the analog input into a digital pulse train. The analog input is continuously sampled by a switched capacitor modulator at twice the rate of the clock input frequency (2 x fMCLK). The digital data that represents the analog input is in the one's density of the bit stream at the output of the sigmadelta modulator. The modulator outputs the bit stream at a data rate equal to fMCLK. Due to the high oversampling rate, which spreads the quantization noise from 0 to fMCLK/2, the noise energy contained in the band of interest is reduced (Figure 21a). To reduce the quantization noise further, a high order modulator is employed to shape the noise spectrum, so that most of the noise energy is shifted out of the band of interest (Figure 21b).
2pF 2pF
A
500 VIN(-)
B
AC GROUND
B A B
MCLK
A
Figure 22. Analog Input Equivalent Circuit
Since the AD7720 samples the differential voltage across its analog inputs, low noise performance is attained with an input circuit that provides low differential mode noise at each input. The amplifiers used to drive the analog inputs play a critical role in attaining the high performance available from the AD7720. When a capacitive load is switched onto the output of an op amp, the amplitude will momentarily drop. The op amp will try to correct the situation and, in the process, hits its slew rate limit. This nonlinear response, which can cause excessive ringing, can lead to distortion. To remedy the situation, a low pass RC filter can be connected between the amplifier and the input to the AD7720 as shown in Figure 23. The external capacitor at each input aids in supplying the current spikes created during the sampling process. The resistor in this diagram, as well as creating the pole for the antialiasing, isolates the op amp from the transient nature of the load.
QUANTIZATION NOISE BAND OF INTEREST fMCLK/2
a.
NOISE SHAPING fMCLK/2
BAND OF INTEREST
b.
R
Figure 21. Sigma-Delta ADC
USING THE AD7720 ADC Differential Inputs
ANALOG INPUT R
VIN(+) C
The AD7720 uses differential inputs to provide common-mode noise rejection (i.e., the converted result will correspond to the differential voltage between the two inputs). The absolute voltage on both inputs must lie between AGND and AVDD. In the unipolar mode, the full-scale input range (VIN(+) - VIN(-)) is 0 V to VREF. In the bipolar mode configuration, the full-scale analog input range is VREF2/2. The bipolar mode allows complementary input signals. Alternatively, VIN(-) can be connected to a dc bias voltage to allow a single-ended input on VIN(+) equal to VBIAS VREF2/2.
Differential Inputs
VIN(-) C
Figure 23. Simple RC Antialiasing Circuit
The differential input impedance of the AD7720 switched capacitor input varies as a function of the MCLK frequency, given by the equation: ZIN = 109/(8 fMCLK) k Even though the voltage on the input sampling capacitors may not have enough time to settle to the accuracy indicated by the resolution of the AD7720, as long as the sampling capacitor charging follows the exponential curve of RC circuits, only the gain accuracy suffers if the input capacitor is switched away too early. An alternative circuit configuration for driving the differential inputs to the AD7720 is shown in Figure 24.
The analog input to the modulator is a switched capacitor design. The analog input is converted into charge by highly linear sampling capacitors. A simplified equivalent circuit diagram of the analog input is shown in Figure 22. A signal source driving the analog input must be able to provide the charge onto the sampling capacitors every half MCLK cycle and settle to the required accuracy within the next half cycle.
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AD7720
R 100 C 2.7nF VIN(+) C 2.7nF VIN(-) C 2.7nF
R 100
The AD7720 can operate with its internal reference or an external reference can be applied in two ways. An external reference can be connected to REF1, overdriving the internal reference. However, there will be an error introduced due to the offset of the internal buffer amplifier. For lowest system gain errors when using an external reference, REF1 is grounded (disabling the internal buffer) and the external reference is connected to REF2. In all cases, since the REF2 voltage connects to the analog modulator, a 220 nF capacitor must connect directly from REF2 to AGND. The external capacitor provides the charge required for the dynamic load presented at the REF2 pin (Figure 26).
Figure 24. Differential Input with Antialiasing
A capacitor between the two input pins sources or sinks charge to allow most of the charge that is needed by one input to be effectively supplied by the other input. This minimizes undesirable charge transfer from the analog inputs to and from ground. The series resistor isolates the operational amplifier from the current spikes created during the sampling process and provides a pole for antialiasing. The 3 dB cutoff frequency (f3 dB) of the antialias filter is given by Equation 1, and the attenuation of the filter is given by Equation 2. f3 dB = 1/(2 REXT CEXT)
Attenuation = 20 log 1/ 1+ f / f 3 dB
A
REF2 220nF
B
4pF
B
4pF
A
(1)
SWITCHED-CAP DAC REF
(
)
2
(2)
MCLK
A
B
A
B
The choice of the filter cutoff frequency will depend on the amount of roll off that is acceptable in the passband of the digital filter and the required attenuation at the first image frequency. The capacitors used for the input antialiasing circuit must have low dielectric absorption to avoid distortion. Film capacitors such as Polypropylene, Polystyrene or Polycarbonate are suitable. If ceramic capacitors are used, they must have NPO dielectric.
Applying the Reference
Figure 26. REF2 Equivalent Circuit
The AD780 is ideal to use as an external reference with the AD7720. Figure 27 shows a suggested connection diagram.
+5V
1 NC 2 +VIN
O/P 8 SELECT NC 7 VOUT 6 TRIM 5 220nF 22 F
REF2
1F 22nF
3 TEMP 4 GND
REF1
The reference circuitry used in the AD7720 includes an on-chip +2.5 V bandgap reference and a reference buffer circuit. The block diagram of the reference circuit is shown in Figure 25. The internal reference voltage is connected to REF1 via a 3 k resistor and is internally buffered to drive the analog modulator's switched capacitor DAC (REF2). When using the internal reference, connect 100 nF between REF1 and AGND. If the internal reference is required to bias external circuits, use an external precision op amp to buffer REF1.
AD780
Figure 27. External Reference Circuit Connection
Input Circuits
Figures 28 and 29 show two simple circuits for bipolar mode operation. Both circuits accept a single-ended bipolar signal source and create the necessary differential signals at the input to the ADC. The circuit in Figure 28 creates a 0 V to 2.5 V signal at the VIN(+) pin to form a differential signal around an initial bias voltage of 1.25 V. For single-ended applications, best THD performance is obtained with VIN(-) set to 1.25 V rather than 2.5 V. The input to the AD7720 can also be driven differentially with a complementary input as shown in Figure 29. In this case, the input common-mode voltage is set to 2.5 V. The 2.5 V p-p full-scale differential input is obtained with a 1.25 V p-p signal at each input in antiphase. This configuration minimizes the required output swing from the amplifier circuit and is useful for single supply applications.
1V
COMPARATOR REFERENCE BUFFER
REF1 100nF 3k 2.5V REFERENCE
SWITCHED-CAP DAC REF
REF2
Figure 25. Reference Circuit Block Diagram
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AD7720
12pF 1k 1k
XTAL 1M MCLK
AIN = 1.25V
1/2 OP275
1nF 1k
VIN(+)
VIN(-) 12pF 1k 1nF DIFFERENTIAL INPUT = 2.5V p-p VIN(-) BIAS VOLTAGE = 1.25V REF1 1k
Figure 30. Crystal Oscillator Connection
1/2 OP275
374k
100nF
374k
10nF REF2 220nF
An external clock must be free of ringing and have a minimum rise time of 5 ns. Degradation in performance can result as high edge rates increase coupling that can generate noise in the sampling process. The connection diagram for an external clock source (Figure 31) shows a series damping resistor connected between the clock output and the clock input to the AD7720. The optimum resistor will depend on the board layout and the impedance of the trace connecting to the clock input.
Figure 28. Single-Ended Analog Input for Bipolar Mode Operation
12pF AIN = 0.625V 1k 1k
CLOCK CIRCUITRY
25-150 MCLK
Figure 31. External Clock Oscillator Connection
VIN(-) 1nF 12pF
1/2 OP275
1k 1k
DIFFERENTIAL INPUT = 2.5V p-p COMMON MODE VOLTAGE = 2.5V VIN(+) 1nF
A low phase noise clock should be used to generate the ADC sampling clock because sampling clock jitter effectively modulates the input signal and raises the noise floor. The sampling clock generator should be isolated from noisy digital circuits, grounded and heavily decoupled to the analog ground plane. The sampling clock generator should be referenced to the analog ground plane in a split ground system. However, this is not always possible because of system constraints. In many cases, the sampling clock must be derived from a higher frequency multipurpose system clock that is generated on the digital ground plane. If the clock signal is passed between its origin on a digital plane to the AD7720 on the analog ground plane, the ground noise between the two planes adds directly to the clock and will produce excess jitter. The jitter can cause unwanted degradation in the signal-to-noise ratio and also produce unwanted harmonics. This can be somewhat remedied by transmitting the sampling clock signal as a differential one, using either a small RF transformer or a high speed differential driver and receiver such as PECL. In either case, the original master system clock should be generated from a low phase noise crystal oscillator.
1/2 OP275
R
REF1 R
OP07
100nF
REF2 220nF
Figure 29. Single-Ended to Differential Analog Input Circuit for Bipolar Mode Operation
The 1 nF capacitors at each ADC input store charge to aid the amplifier settling as the input is continuously switched. A resistor in series with the drive amplifier output and the 1 nF input capacitor may also be used to create an antialias filter.
Clock Generation
The AD7720 contains an oscillator circuit to allow a crystal or an external clock signal to generate the master clock for the ADC. The connection diagram for use with the crystal is shown in Figure 30. Consult the crystal manufacturer's recommendation for the load capacitors.
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AD7720
Offset and Gain Calibration
The analog inputs of the AD7720 can be configured to measure offset and gain errors. Pins MZERO and GC are used to configure the part. Before calibrating the device, the part should be reset so that the modulator is in a known state at calibration. When MZERO is taken high, the analog inputs are tied to AGND in unipolar mode and VREF in bipolar mode. After taking MZERO high, 1000 MCLK cycles should be allowed for the circuitry to settle before the bit stream is read from the device. The ideal ones density is 50% when bipolar operation is selected and 37.5% when unipolar mode is selected. When GC is taken high, VIN(-) is tied to ground while VIN(+) is tied to VREF. Again, 1000 MCLK cycles should be allowed for the circuitry to settle before the bit stream is read. The ideal ones density is 62.5%. The calibration results apply only for the particular analog input mode (unipolar/bipolar) selected when performing the calibration cycle. On changing to a different analog input mode, a new calibration must be performed. Before calibrating, ensure that the supplies have settled and that the voltage on the analog input pins is between the supply voltages.
Standby
common-mode rejection of the part will remove common-mode noise on these inputs. The analog and digital supplies to the AD7720 are independent and separately pinned out to minimize coupling between analog and digital sections of the device. The printed circuit board that houses the AD7720 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes which can easily be separated. A minimum etch technique is generally best for ground planes as it gives the best shielding. Digital and analog ground planes should only be joined in one place. If the AD7720 is the only device requiring an AGND-to-DGND connection, the ground planes should be connected at the AGND and DGND pins of the AD7720. If the AD7720 is in a system where multiple devices require AGND-to-DGND connections, the connection should still be made at one point only, a star ground point that should be established as close as possible to the AD7720. Avoid running digital lines under the device as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7720 to avoid noise coupling. The power supply lines to the AD7720 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will reduce the effects of feedthrough through the board. A microstrip technique is by far the best, but is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes while signals are placed on the other side. Good decoupling is important when using high resolution ADCs. All analog and digital supplies should be decoupled to AGND and DGND respectively, with 100 nF ceramic capacitors in parallel with 10 F tantalum capacitors. To achieve the best from these decoupling capacitors, they should be placed as close as possible to the device, ideally right up against the device. In systems where a common supply voltage is used to drive both the AVDD and DVDD of the AD7720, it is recommended that the system's AVDD supply is used. This supply should have the recommended analog supply decoupling between the AVDD pin of the AD7720 and AGND and the recommended digital supply decoupling capacitor between the DVDD pins and DGND.
The part can be put into a low power standby mode by taking STBY high. During standby, the clock to the modulator is turned off and bias is removed from all analog circuits.
Reset
The RESET pin is used to reset the modulator to a known state. When RESET is taken high, the integrator capacitors of the modulator are shorted and DVAL goes low and remains low until 20 MCLK cycles after RESET is deasserted. However, an additional 1000 MCLK cycles should be allowed before reading the modulator bit stream as the modulator circuitry needs to settle after the reset.
DVAL
The DVAL pin is used to indicate that an overrange input signal has resulted in invalid data at the modulator output. As with all single bit DAC high order sigma-delta modulators, large overloads on the inputs can cause the modulator to go unstable. The modulator is designed to be stable with signals within the input bandwidth that exceed full scale by 20%. When instability is detected by internal circuits, the modulator is reset to a stable state and DVAL is held low for 20 clock cycles.
Grounding and Layout
Since the analog inputs are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent
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AD7720
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
28-Lead Thin Shrink Small Outline (RU-28)
0.386 (9.80) 0.378 (9.60)
28
15
0.177 (4.50) 0.169 (4.30)
1
14
PIN 1 0.006 (0.15) 0.002 (0.05)
0.256 (6.50) 0.246 (6.25)
0.0433 (1.10) MAX 0.0079 (0.20) 0.0035 (0.090)
SEATING PLANE
0.0256 (0.65) 0.0118 (0.30) BSC 0.0075 (0.19)
8 0
0.028 (0.70) 0.020 (0.50)
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C3235-8-10/97
PRINTED IN U.S.A.

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