Datasheet AD7864 (Analog Devices) - 10

AD7864 TERMINOLOGY Channel-to-Channel Isolation Signal-to-(Noise + Distortion) Ratio
Channel-to-channel isolation is a measure of the level of This is the measured ratio of signal-to-(noise + distortion) at crosstalk between channels. It is measured by applying a full- the output of the ADC. The signal is the rms amplitude of the scale 50 kHz sine wave signal to all nonselected input channels fundamental. Noise is the rms sum of all nonfundamental signals and determining how much that signal is attenuated in the up to half the sampling frequency (fS/2), excluding dc. The ratio selected channel. The figure given is the worst case across all depends on the number of quantization levels in the digitization four channels. process; the more levels, the smaller the quantization noise. The theoretical signal-to-(noise + distortion) ratio for an ideal N-bit
Relative Accuracy
converter with a sine wave input is given by Relative accuracy, or endpoint nonlinearity, is the maximum deviation from a straight line passing through the endpoints of Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB the ADC transfer function. Thus, for a 12-bit converter, this is 74 dB.
Differential Nonlinearity Total Harmonic Distortion (THD)
This is the difference between the measured and the ideal 1 LSB THD is the ratio of the rms sum of harmonics to the change between any two adjacent codes in the ADC. fundamental. For the AD7864, it is defined as
Positive Full-Scale Error
V 2 + V 2 + V 2 + V 2 + V 2 2 3 4 5 6 THD( ) dB = 20 log This is the deviation of the last code transition (01...110 to 01...111) V1 from the ideal, 4 × VREF − 3/2 LSB (AD7864-1, ±10 V), or 2 × VREF − 3/2 LSB (AD7864-1, ±5 V range), or VREF − 3/2 LSB where V1 is the rms amplitude of the fundamental, and V2, V3, (AD7864-3, ±2.5 V range), after the bipolar offset error has V4, V5, and V6 are the rms amplitudes of the second through the been adjusted out. fifth harmonics.
Positive Full-Scale Error (AD7864-2, 0 V to 2.5 V and 0 V to 5 V) Peak Harmonic or Spurious Noise
This is the deviation of the last code transition (11...110 to 11...111) Peak harmonic or spurious noise is defined as the ratio of the from the ideal 2 × VREF − 3/2 LSB (AD7864-2, 0 V to 5 V range) rms value of the next largest component in the ADC output or VREF − 3/2 LSB (AD7864-2, 0 V to 2.5 V range), after the spectrum (up to fS/2 and excluding dc) to the rms value of the unipolar offset error has been adjusted out. fundamental. Normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for parts
Bipolar Zero Error (AD7864-1, ±10 V/±5 V, AD7864-3, ±2.5 V)
where the harmonics are buried in the noise floor, it is a noise peak. This is the deviation of the midscale transition (all 0s to all 1s) from the ideal, AGND − 1/2 LSB.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
Unipolar Offset Error (AD7864-2, 0 V to 2.5 V and 0 V to 5 V)
fb, any active device with nonlinearities creates distortion products This is the deviation of the first code transition (00...000 to at sum and difference frequencies of mfa ± nfb, where m, n = 0, 00...001) from the ideal, AGND + 1/2 LSB. 1, 2, 3, and so on. Intermodulation terms are those for which
Negative Full-Scale Error (AD7864-1, ±10 V/±5 V, and
neither m nor n are equal to zero. For example, second-order
AD7864-3, ±2.5 V)
terms include (fa + fb) and (fa − fb), whereas third-order terms This is the deviation of the first code transition (10...000 to include (2 fa + fb), (2 fa − fb), (fa + 2 fb), and (fa − 2 fb). 10...001) from the ideal, −4 × VREF + 1/2 LSB (AD7864-1, ±10 V), The AD7864 is tested using the CCIF standard, where two input −2 × VREF + 1/2 LSB (AD7864-1, ±5 V range) or −VREF + 1/2 LSB frequencies near the top end of the input bandwidth are used. (AD7864-3, ±2.5 V range), after bipolar zero error has been In this case, the second- and third-order terms are of different adjusted out. significance. The second-order terms are usually distanced in
Track-and-Hold Acquisition Time
frequency from the original sine waves, whereas the third-order Track-and-hold acquisition time is the time required for the terms are usually at a frequency close to the input frequencies. As output of the track-and-hold amplifier to reach its final value, a result, the second- and third-order terms are specified separately. within ±1/2 LSB, after the end of a conversion (the point at The calculation of the intermodulation distortion is as per the which the track-and-hold returns to track mode). It also applies THD specification where it is the ratio of the rms sum of the to situations where there is a step input change on the input individual distortion products to the rms amplitude of the funda- voltage applied to the selected VINxA/VINxB input of the AD7864. mental expressed in decibels. Rev. D | Page 9 of 28 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TERMINOLOGY THEORY OF OPERATION CONVERTER DETAILS Track-and-Hold Amplifiers Reference CIRCUIT DESCRIPTION ANALOG INPUT AD7864-1 AD7864-2 AD7864-3 SELECTING A CONVERSION SEQUENCE TIMING AND CONTROL Reading Between Each Conversion in the Conversion Sequence Reading After the Conversion Sequence USING AN EXTERNAL CLOCK STANDBY MODE OPERATION ACCESSING THE OUTPUT DATA REGISTERS OFFSET AND FULL-SCALE ADJUSTMENT POSITIVE FULL-SCALE ADJUST NEGATIVE FULL-SCALE ADJUST DYNAMIC SPECIFICATIONS SIGNAL-TO-NOISE RATIO (SNR) EFFECTIVE NUMBER OF BITS INTERMODULATION DISTORTION AC LINEARITY PLOTS MEASURING APERTURE JITTER MICROPROCESSOR INTERFACING AD7864 TO ADSP-2100/ADSP-2101/ADSP-2102 INTERFACE AD7864 TO TMS320C5x INTERFACE AD7864 TO MC68HC000 INTERFACE VECTOR MOTOR CONTROL MULTIPLE AD7864S IN A SYSTEM OUTLINE DIMENSIONS ORDERING GUIDE