Datasheet AD7888 (Analog Devices) - 7

AD7888 TERMINOLOGY Peak Harmonic or Spurious Noise Integral Nonlinearity
Peak harmonic or spurious noise is defined as the ratio of the This is the maximum deviation from a straight line passing rms value of the next largest component in the ADC output through the endpoints of the ADC transfer function. The end- spectrum (up to fS/2 and excluding dc) to the rms value of the points of the transfer function are zero scale, a point 1/2 LSB fundamental. Normally, the value of this specification is deter- below the first code transition, and full scale, a point 1/2 LSB mined by the largest harmonic in the spectrum, but for ADCs above the last code transition. where the harmonics are buried in the noise floor, it will be a noise peak.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
Intermodulation Distortion
change between any two adjacent codes in the ADC. With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion
Offset Error
This is the deviation of the first code transition (00 . 000) to products at sum and difference frequencies of mfa ± nfb where (00 . 001) from the ideal, i.e., AGND + 0.5 LSB. m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are those for which neither m nor n is equal to zero. For example,
Offset Error Match
the second order terms include (fa + fb) and (fa – fb), while the This is the difference in offset error between any two channels. third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
Gain Error
(fa – 2fb). This is the deviation of the last code transition (111 . 110) to The AD7888 is tested using the CCIF standard where two (111 . 111) from the ideal (i.e., VREF – 1.5 LSB) after the input frequencies near the top end of the input bandwidth are offset error has been adjusted out. used. In this case, the second order terms are usually distanced
Gain Error Match
in frequency from the original sine waves while the third order This is the difference in gain error between any two channels. terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified sepa-
Track/Hold Acquisition Time
rately. The calculation of the intermodulation distortion is as The track/hold amplifier returns into track mode at the end of per the THD specification where it is the ratio of the rms sum of conversion. Track/Hold acquisition time is the time required for the individual distortion products to the rms amplitude of the the output of the track/hold amplifier to reach its final value, sum of the fundamentals expressed in dBs. within ± 1/2 LSB, after the end of conversion.
Channel-to-Channel Isolation Signal to (Noise + Distortion) Ratio
Channel-to-channel isolation is a measure of the level of crosstalk This is the measured ratio of signal to (noise + distortion) at the between channels. It is measured by applying a full-scale 25 kHz output of the A/D converter. The signal is the rms amplitude of sine wave signal to all nonselected input channels and determin- the fundamental. Noise is the sum of all nonfundamental sig- ing how much that signal is attenuated in the selected channel. nals up to half the sampling frequency (fS/2), excluding dc. The The figure given is the worst case across all four or eight chan- ratio is dependent on the number of quantization levels in the nels for the AD7888. digitization process; the more levels, the smaller the quantiza- tion noise. The theoretical signal to (noise + distortion) ratio for
PSR (Power Supply Rejection)
an ideal N-bit converter with a sine wave input is given by: Variations in power supply will affect the full-scale transition, but not the converter’s linearity. Power supply rejection is the Signal to (Noise + Distortion) = (6.02 N + 1.76) dB maximum change in the full-scale transition point due to a Thus for a 12-bit converter, this is 74 dB. change in power-supply voltage from the nominal value.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of harmonics to the fundamental. For the AD7888, it is defined as: V 2 2 2 2 2 2 + V3 + V4 + V5 + V THD (dB) = log 6 V1 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. –6– REV. C