Datasheet AD7656A-1 (Analog Devices) - 14

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Data Sheet AD7656A-1 TERMINOLOGY Integral Nonlinearity (INL)
The ratio depends on the number of quantization levels in the The INL is the maximum deviation from a straight line passing digitization process: the more levels, the smal er the quantization through the endpoints of the ADC transfer function. The noise. The theoretical SINAD ratio for an ideal N-bit converter endpoints of the transfer function are zero scale at a ½ LSB below with a sine wave input is given by the first code transition and full scale at ½ LSB above the last SINAD = (6.02 N + 1.76) dB code transition. Therefore, SINAD is 98 dB for a 16-bit converter.
Differential Nonlinearity (DNL) Total Harmonic Distortion (THD)
The DNL is the difference between the measured and the ideal The THD is the ratio of the rms sum of the harmonics to the 1 LSB change between any two adjacent codes in the ADC. fundamental. For the AD7656A-1, it is defined as
Bipolar Zero-Scale Error
2 2 2 2 2 The bipolar zero-scale error is the deviation of the midscale V + + + + 2 V3 V4 V5 V6 THD ) dB ( = 20 log transition (all 1s to all 0s) from the ideal VIN voltage, that is, 1 V AGND − 1 LSB. where:
Bipolar Zero-Scale Error Matching
V1 is the rms amplitude of the fundamental. The bipolar zero-scale error matching is the difference in V2, V3, V4, V5, and V6 are the rms amplitudes of the second bipolar zero-code error between any two input channels. through sixth harmonics.
Positive Full-Scale Error Peak Harmonic or Spurious Noise
The positive ful -scale error is the deviation of the last code The peak harmonic or spurious noise is the ratio of the rms transition (011 … 110 to 011 … 111) from the ideal (4 × VREF − value of the next largest component in the ADC output 1 LSB or 2 × VREF − 1 LSB) after adjusting for the bipolar zero- spectrum (up to fSAMPLE/2, excluding dc) to the rms value of scale error. the fundamental. Normally, the value of this specification is
Positive Full-Scale Error Matching
determined by the largest harmonic in the spectrum, but for The positive ful -scale error matching is the difference in ADCs where the harmonics are buried in the noise floor, it is positive ful -scale error between any two input channels. determined by a noise peak.
Negative Full-Scale Error Intermodulation Distortion (IMD)
The negative full-scale error is the deviation of the first code With inputs consisting of sine waves at two frequencies, fa and transition (10 … 000 to 10 … 001) from the ideal (−4 × V fb, any active device with nonlinearities creates distortion REF + 1 LSB or −2 × V products at the sum and difference frequencies of mfa ± nfb, REF + 1 LSB) after adjusting for the bipolar zero scale error. where m, n = 0, 1, 2, 3. Intermodulation distortion terms are those for which neither m nor n are equal to 0. For example, the
Negative Full-Scale Error Matching
second-order terms include (fa + fb) and (fa − fb), and the The negative full-scale error matching is the difference in third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and negative full-scale error between any two input channels. (fa − 2fb).
Track-and-Hold Acquisition Time
The AD7656A-1 is tested using the CCIF standard in which two The track-and-hold amplifier returns to track mode at the end input frequencies near the maximum input bandwidth are used. of the conversion. The track-and-hold acquisition time is the In this case, the second-order terms are usually distanced in time required for the output of the track-and-hold amplifier to frequency from the original sine waves, and the third-order reach its final value, within ±1 LSB, after the end of the conversion. terms are usual y at a frequency close to the input frequencies. See the Track-and-Hold section for more details. As a result, the second- and third-order terms are specified
Signal-to-Noise Ratio (SNR)
separately. The calculation of the intermodulation distortion is SNR is the ratio of the rms value of the measured output signal per the THD specification, where it is the ratio of the rms sum to the rms sum of all other spectral components below the of the individual distortion products to the rms amplitude of Nyquist frequency. The value for SNR is expressed in decibels. the sum of the fundamentals and is expressed in decibels.
Signal-to-Noise-and-Distortion (SINAD) Ratio Channel-to-Channel Isolation
The SINAD ratio is the measured ratio of signal-to-noise-and- Channel-to-channel isolation is a measure of the level of crosstalk distortion at the output of the ADC. The signal is the rms between any two channels. It is measured by applying a ful -scale, amplitude of the fundamental. Noise is the sum of all 100 kHz sine wave signal to all unselected input channels and nonfundamental signals up to half the sampling frequency determining the degree to which the signal attenuates in the (fSAMPLE/2, excluding dc). selected channel with a 30 kHz signal. Rev. 0 | Page 13 of 28 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS POWER SUPPLY SEQUENCING THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TERMINOLOGY THEORY OF OPERATION CONVERTER DETAILS Track-and-Hold Amplifiers Analog Input ADC TRANSFER FUNCTION INTERNAL/EXTERNAL REFERENCE TYPICAL CONNECTION DIAGRAM DRIVING THE ANALOG INPUTS INTERFACE OPTIONS Parallel Interface (SER/PAR/SEL = 0) SOFTWARE SELECTION OF ADCS Changing the Analog Input Range (H/S SEL = 0) Changing the Analog Input Range (H/S SEL = 1) Serial Interface (SER/PAR/SEL = 1) SERIAL READ OPERATION DAISY-CHAIN MODE (DCEN = 1, SER/PAR/SEL = 1) Standby/Partial Power-Down Modes of Operation(SER/PAR/SEL = 0 or SER/PAR/SEL = 1) APPLICATION HINTS LAYOUT OUTLINE DIMENSIONS ORDERING GUIDE