Datasheet AD7819 (Analog Devices) - 6

AD7819 TERMINOLOGY
The AD7819 is tested using the CCIF standard, where two
Signal to (Noise + Distortion) Ratio
input frequencies near the top end of the input bandwidth are This is the measured ratio of signal to (noise + distortion) at the used. In this case, the second and third order terms are of different output of the A/D converter. The signal is the rms amplitude of significance. The second order terms are usually distanced in the fundamental. Noise is the rms sum of all nonfundamental frequency from the original sine waves, while the third order signals up to half the sampling frequency (f terms are usually at a frequency close to the input frequencies. S/2), excluding dc. The ratio is dependent upon the number of quantization levels As a result, the second and third order terms are specified sepa- in the digitization process; the more levels, the smaller the quan- rately. The calculation of the intermodulation distortion is as tization noise. The theoretical signal to (noise + distortion) per the THD specification where it is the ratio of the rms sum ratio for an ideal N-bit converter with a sine wave input is given of the individual distortion products to the rms amplitude of by: the fundamental expressed in dBs. Signal to (Noise + Distortion) = (6.02N + 1.76) dB
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum Thus for an 8-bit converter, this is 50 dB. deviation from a straight line passing through the endpoints of
Total Harmonic Distortion
the ADC transfer function. Total harmonic distortion (THD) is the ratio of the rms sum of
Differential Nonlinearity
harmonics to the fundamental. For the AD7819 it is defined as: This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. 2 2 2 2 2 V +V +V +V +V THD (dB) = 20 log 2 3 4 5 6
Offset Error
V1 This is the deviation of the first code transition (0000 . 000) where V to (0000 . 001) from the ideal, i.e., AGND + 1 LSB. 1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the
Offset Error Match
sixth harmonics. This is the difference in Offset Error between any two channels.
Peak Harmonic or Spurious Noise Gain Error
Peak harmonic or spurious noise is defined as the ratio of the This is the deviation of the last code transition (1111 . 110) rms value of the next largest component in the ADC output to (1111 . 111) from the ideal, i.e., VREF – 1 LSB, after the spectrum (up to fS/2 and excluding dc) to the rms value of the offset error has been adjusted out. fundamental. Normally, the value of this specification is deter-
Gain Error Match
mined by the largest harmonic in the spectrum, but for parts This is the difference in Gain Error between any two channels. where the harmonics are buried in the noise floor, it will be a noise peak.
Track/Hold Acquisition Time
Track/hold acquisition time is the time required for the output
Intermodulation Distortion
of the track/hold amplifier to reach its final value, within With inputs consisting of sine waves at two frequencies, fa and ±1/2 LSB, after the end of conversion (the point at which the fb, any active device with nonlinearities will create distortion track/hold returns to track mode). It also applies to situations products at sum and difference frequencies of mfa ± nfb where where a change in the selected input channel takes place or m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which where there is a step input change on the input voltage applied neither m nor n are equal to zero. For example, the second order to the selected V input of the AD7819. It means that the user terms include (fa + fb) and (fa – fb), while the third order terms IN must wait for the duration of the track/hold acquisition time include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). after the end of conversion or after a step input change to VIN before starting another conversion, to ensure that the part operates to specification. REV. B –5–