Datasheet AD7858, AD7858L (Analog Devices) - 8

AD7858/AD7858L TERMINOLOGY1 Total Harmonic Distortion Integral Nonlinearity
Total harmonic distortion (THD) is the ratio of the rms sum of This is the maximum deviation from a straight line passing harmonics to the fundamental. For the AD7858/AD7858L, it is through the endpoints of the ADC transfer function. The end- defined as: points of the transfer function are zero scale, a point 1/2 LSB 2 2 2 2 2 below the first code transition, and full scale, a point 1/2 LSB (V + V + V + V + V ) THD (dB) = 20 log 2 3 4 5 6 above the last code transition. V1
Differential Nonlinearity
where V1 is the rms amplitude of the fundamental and V2, V3, This is the difference between the measured and the ideal 1 LSB V4, V5, and V6 are the rms amplitudes of the second through the change between any two adjacent codes in the ADC. sixth harmonics.
Total Unadjusted Error Peak Harmonic or Spurious Noise
This is the deviation of the actual code from the ideal code tak- Peak harmonic or spurious noise is defined as the ratio of the ing all errors into account (Gain, Offset, Integral Nonlinearity, and rms value of the next largest component in the ADC output other errors) at any point along the transfer function. spectrum (up to fS/2 and excluding dc) to the rms value of the
Unipolar Offset Error
fundamental. Normally, the value of this specification is deter- This is the deviation of the first code transition (00 . 000 to mined by the largest harmonic in the spectrum, but for parts 00 . 001) from the ideal AIN(+) voltage (AIN(–) + 1/2 LSB). where the harmonics are buried in the noise floor, it will be a noise peak.
Positive Full-Scale Error
This is the deviation of the last code transition from the ideal
Intermodulation Distortion
AIN(+) voltage (AIN(–) + Full Scale – 1.5 LSB) after the offset With inputs consisting of sine waves at two frequencies, fa and error has been adjusted out. fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa ± nfb where
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of crosstalk between m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are the channels. It is measured by applying a full-scale 25 kHz those for which neither m nor n are equal to zero. For example, signal to the other seven channels and determining how much the second order terms include (fa + fb) and (fa – fb), while the that signal is attenuated in the channel of interest. The figure third order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and given is the worst case for all channels. (fa – 2fb). Testing is performed using the CCIF standard where two input
Track/Hold Acquisition Time
The track/hold amplifier returns into track mode and the end of frequencies near the top end of the input bandwidth are used. In conversion. Track/hold acquisition time is the time required for this case, the second order terms are usually distanced in fre- the output of the track/hold amplifier to reach its final value, quency from the original sine waves, while the third order terms within ± 1/2 LSB, after the end of conversion. are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately.
Signal to (Noise + Distortion) Ratio
The calculation of the intermodulation distortion is as per the This is the measured ratio of signal to (noise + distortion) at the THD specification where it is the ratio of the rms sum of the output of the A/D converter. The signal is the rms amplitude of individual distortion products to the rms amplitude of the sum the fundamental. Noise is the sum of all nonfundamental signals of the fundamentals expressed in dBs. up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitiza- tion process; the more levels, the smaller the quantization 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. 1AIN(+) refers to the positive input of the pseudo differential pair, and AIN(–) refers to the negative analog input of the pseudo differential pair or to AGND depending on the channel configuration. –8– REV. B