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AD7866BRU Datasheet(PDF) 8 Page - Analog Devices |
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AD7866BRU Datasheet(HTML) 8 Page - Analog Devices |
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8 / 24 page  REV. A –8– AD7866 TERMINOLOGY Integral Nonlinearity This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints of the transfer function are zero scale, a point 1 LSB below the first code transition, and full scale, a point 1 LSB above the last code transition. Differential Nonlinearity This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Offset Error This applies to Straight Binary output coding. It is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal, i.e., AGND + 1 LSB. Offset Error Match This is the difference in Offset Error between the two channels. Gain Error This applies to Straight Binary output coding. It is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (i.e., VREF – 1 LSB) after the offset error has been adjusted out. Gain Error Match This is the difference in Gain Error between the two channels. Zero Code Error This applies when using the twos complement output coding option, in particular with the 2 VREF input range as –VREF to +VREF biased about the VREF point. It is the deviation of the midscale transition (all 1s to all 0s) from the ideal VIN voltage, i.e., VREF – 1 LSB. Zero Code Error Match This refers to the difference in Zero Code Error between the two channels. Positive Gain Error This applies when using the twos complement output coding option, in particular with the 2 VREF input range as –VREF to +VREF biased about the VREF point. It is the deviation of the last code transition (011 . . . 110) to (011 . . . 111) from the ideal (i.e., +VREF – 1 LSB) after the Zero Code Error has been adjusted out. Negative Gain Error This applies when using the twos complement output coding option, in particular with the 2 VREF input range as –VREF to +VREF biased about the VREF point. It is the deviation of the first code transition (100 . . . 000) to (100 . . . 001) from the ideal (i.e., –VREF + 1 LSB) after the Zero Code Error has been adjusted out. Track-and-Hold Acquisition Time The track-and-hold amplifier returns into track mode after the end of conversion. Track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within ±1/2 LSB, after the end of conversion. Signal-to-(Noise + Distortion) Ratio (SNDR) This is the measured ratio of signal-to-(noise + distortion) at the output of the A/D converter. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental sig- nals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantiza- tion noise. The theoretical signal-to-(noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by: Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB Thus, for a 12-bit converter, this is 74 dB. Total Harmonic Distortion (THD) Total harmonic distortion is the ratio of the rms sum of har- monics to the fundamental. For the AD7866, it is defined as: THD db VV VVV V ()= +++ + 20 2 2 3 2 4 2 5 2 6 2 1 log 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 harmonics. Peak Harmonic or Spurious Noise Peak harmonic, or spurious noise, is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is deter- mined by the largest harmonic in the spectrum. But for ADCs where the harmonics are buried in the noise floor, it will be a noise peak. Intermodulation Distortion 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, and so on. Intermodulation distortion terms are those for which neither m nor 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). The AD7866 is tested using the CCIF standard where two input frequencies near the top end of the input bandwidth are used. In this case, the second order terms are usually distanced in frequency from the original sine waves while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified sepa- rately. The calculation of the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dB. Channel-to-Channel Isolation Channel-to-channel isolation is a measure of the level of crosstalk between channels. It is measured by applying a full-scale (2 VREF), 455 kHz sine wave signal to all unselected input channels and determining how much that signal is attenuated in the selected channel with a 10 kHz signal (0 V to VREF). The figure given is the worst-case across all four channels for the AD7866. PSR (Power Supply Rejection) See the Performance Curves section. |
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