Datasheet AD7893 (Analog Devices) - 6

AD7893 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 1 LSB The ratio is dependent upon the number of quantization levels change between any two adjacent codes in the ADC. in the digitization process; the more levels, the smaller the quan-
Positive Full-Scale Error (AD7893-10)
tization noise. The theoretical signal to (noise + distortion) ratio This is the deviation of the last code transition (01 . 110 to for an ideal N-bit converter with a sine wave input is given by: 01 . 111) from the ideal 4 × REF IN – 1 LSB (AD7893-10 Signal to (Noise + Distortion) = (6.02 N + 1.76) dB ±10 V range) after the Bipolar Zero Error has been adjusted out. Thus for a 12-bit converter, this is 74 dB.
Positive Full-Scale Error (AD7893-3) Total Harmonic Distortion
This is the deviation of the last code transition (01 . 110 to Total harmonic distortion (THD) is the ratio of the rms sum of 01 . 111) from the ideal (REF IN – 1 LSB) after the harmonics to the fundamental. For the AD7893, it is defined as: Bipolar Zero Error has been adjusted out.
Positive Full-Scale Error (AD7893-5)
V 2 2 2 2 2 This is the deviation of the last code transition (11 . 110 to +V +V +V +V THD(dB ) = 20 log 2 3 4 5 6 11 . 111) from the ideal (2 × REF IN – 1 LSB) after the Uni- V 1 polar Offset Error has been adjusted out. where V1 is the rms amplitude of the fundamental and V2, V3,
Positive Full-Scale Error (AD7893-2)
V4, V5 and V6 are the rms amplitudes of the second through the This is the deviation of the last code transition (11 . 110 to sixth harmonics. 11 . 111) from the ideal (REF IN – 1 LSB) after the Unipolar
Peak Harmonic or Spurious Noise
Offset Error has been adjusted out. Peak harmonic or spurious noise is defined as the ratio of the
Bipolar Zero Error (AD7893-10,
6
10 V; AD7893-3,
6
2.5 V)
rms value of the next largest component in the ADC output This is the deviation of the midscale transition (all 0s to all 1s) spectrum (up to fS/2 and excluding dc) to the rms value of the from the ideal 0 V (AGND). fundamental. Normally, the value of this specification is deter-
Unipolar Offset Error (AD7893-2, AD7893-5)
mined by the largest harmonic in the spectrum, but for parts This is the deviation of the first code transition (00 . 000 to where the harmonics are buried in the noise floor, it will be a 00 . 001) from the ideal 1 LSB. noise peak.
Negative Full-Scale Error (AD7893-10) 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 –4 × REF IN + 1 LSB (AD7893-10 fb, any active device with nonlinearities will create distortion ± 10 V range) after Bipolar 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
Negative Full-Scale Error (AD7893-3)
which neither m nor n are equal to zero. For example, the second This is the deviation of the first code transition (10 . 000 to order terms include (fa + fb) and (fa – fb), while the third order 10 . 001) from the ideal (–REF IN + 1 LSB) after Bipolar terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). Zero Error has been adjusted out. The AD7893 is tested using the CCIF standard where two
Track/Hold Acquisition Time
input frequencies near the top end of the input bandwidth are Track/Hold acquisition time is the time required for the output used. In this case, the second and third order terms are of differ- of the track/hold amplifier to reach its final value, within ent significance. The second order terms are usually distanced ±1/2 LSB, after the end of conversion (the point at which the in frequency from the original sine waves, while the third order track/hold returns to track mode). It also applies to situations terms are usually at a frequency close to the input frequencies. where there is a step input change on the input voltage applied As a result, the second and third order terms are specified sepa- to the VIN input of the AD7893. This means that the user must rately. The calculation of the intermodulation distortion is per wait for the duration of the track/hold acquisition time after the the THD specification where it is the ratio of the rms sum of the end of conversion or after a step input change to VIN before individual distortion products to the rms amplitude of the fun- starting another conversion, to ensure that the part operates to damental expressed in dBs. specification. REV. E –5–