Datasheet AD7892 (Analog Devices) - 9

AD7892 TERMINOLOGY Relative Accuracy Signal to (Noise + Distortion) Ratio
Relative accuracy or endpoint nonlinearity is the maximum This is the measured ratio of signal to (noise + distortion) at the deviation from a straight line passing through the endpoints of output of the A/D converter. The signal is the rms amplitude of the ADC transfer function. the fundamental. Noise is the rms sum of all nonfundamental
Differential Nonlinearity
signals up to half the sampling frequency (fS/2), excluding dc. This is the difference between the measured and the ideal The ratio is dependent upon the number of quantization levels 1 LSB change between any two adjacent codes in the ADC. in the digitization process; the more levels, the smaller the quan-
Positive Full-Scale Error (AD7892-1)
tization noise. The theoretical signal to (noise + distortion) This is the deviation of the last code transition (01 . 110 to ratio for an ideal N-bit converter with a sine wave input is given 01 . 111) from the ideal 4 by: × REF IN – 3/2 LSB (±10 V range) or 2 × REF IN – 3/2 LSB (± 5 V range) after the bipolar zero Signal to (Noise + Distortion) = (6.02 N + 1.76) dB error has been adjusted out. Thus for a 12-bit converter, this is 74 dB.
Positive Full-Scale Error (AD7892-2) Total Harmonic Distortion
This is the deviation of the last code transition (11 . 110 to Total harmonic distortion (THD) is the ratio of the rms sum of 11 . 111) from the ideal (REF IN – 3/2 LSB) after the unipo- harmonics to the fundamental. For the AD7892, it is defined lar offset error has been adjusted out. as:
Positive Full-Scale Error (AD7892-3)
This is the deviation of the last code transition (01 . 110 to 2 2 2 2 2 V +V +V +V +V 01 . 111) from the ideal (REF IN – 3/2 LSB) after the bipolar THD (dB) = 20 log 2 3 4 5 6 zero error has been adjusted out. V 1
Bipolar Zero Error (AD7892-1, AD7892-3)
where V1 is the rms amplitude of the fundamental and V2, V3, This is the deviation of the midscale transition (all 1s to all 0s) V4, V5 and V6 are the rms amplitudes of the second through the from the ideal (AGND – 1/2 LSB). sixth harmonics.
Unipolar Offset Error (AD7892-2) Peak Harmonic or Spurious Noise
This is the deviation of the first code transition (00 . 000 to Peak harmonic or spurious noise is defined as the ratio of the 00 . 001) from the ideal (AGND + 1/2 LSB). rms value of the next largest component in the ADC output
Negative Full-Scale Error (AD7892-1)
spectrum (up to fS/2 and excluding dc) to the rms value of the This is the deviation of the first code transition (10 . 000 to fundamental. Normally, the value of this specification is deter- 10 . 001) from the ideal –4 mined by the largest harmonic in the spectrum, but for parts × REF IN + 1/2 LSB (±10 V range) or –2 where the harmonics are buried in the noise floor, it will be a × REF IN + 1/2 LSB (±5 V range) after bipolar zero error has been adjusted out. noise peak.
Negative Full-Scale Error (AD7892-3) Intermodulation Distortion
This is the deviation of the first code transition (10 . 000 to With inputs consisting of sine waves at two frequencies, fa and 10 . 001) from the ideal – REF IN + 1/2 LSB after bipolar fb, any active device with nonlinearities will create distortion zero error has been adjusted out. products at sum and difference frequencies of mfa ± nfb where m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for
Track/Hold Acquisition Time
which neither m nor n are equal to zero. For example, the sec- Track/Hold acquisition time is the time required for the output ond order terms include (fa + fb) and (fa – fb), while the third of the track/hold amplifier to reach its final value, within ±1/2 LSB, order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and after the end of conversion (the point at which the track/hold (fa – 2fb). returns to track mode). It also applies to situations where there is a step input change on the input voltage applied to the V The AD7892 is tested using two input frequencies away from IN input of the AD7892. It means that the user must wait for the the bottom end of the input bandwidth. In this case, the second duration of the track/hold acquisition time after the end of con- and third order terms are of different significance. The second version or after a step input change to V order terms are usually distanced in frequency from the original IN before starting another conversion, to ensure that the part operates to specification. 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 separately. 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 fundamental expressed in dBs. –8– REV. C