AD7893
TERMINOLOGY
Relative Accuracy
Signal to (Noise + Distortion) Ratio
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (fS/2), excluding dc.
The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the quan-
tization noise. The theoretical signal to (noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by:
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Positive Full-Scale Error (AD7893-10)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal 4 × REF IN – 1 LSB (AD7893-10
±10 V range) after the Bipolar Zero Error has been adjusted out.
Signal to (Noise + Distortion) = (6.02 N + 1.76) dB
Thus for a 12-bit converter, this is 74 dB.
Positive Full-Scale Error (AD7893-3)
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7893, it is defined as:
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (REF IN – 1 LSB) after the
Bipolar Zero Error has been adjusted out.
Positive Full-Scale Error (AD7893-5)
V22 +V32 +V42 +V52 +V62
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (2 × REF IN – 1 LSB) after the Uni-
polar Offset Error has been adjusted out.
THD(dB) = 20 log
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.
Positive Full-Scale Error (AD7893-2)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (REF IN – 1 LSB) after the Unipolar
Offset Error has been adjusted out.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to fS/2 and excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is deter-
mined by the largest harmonic in the spectrum, but for parts
where the harmonics are buried in the noise floor, it will be a
noise peak.
Bipolar Zero Error (AD7893-10, ؎10 V; AD7893-3, ؎2.5 V)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (AGND).
Unipolar Offset Error (AD7893-2, AD7893-5)
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 1 LSB.
Negative Full-Scale Error (AD7893-10)
Intermodulation Distortion
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal –4 × REF IN + 1 LSB (AD7893-10
±10 V range) after Bipolar Zero Error has been adjusted out.
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the second
order terms include (fa + fb) and (fa – fb), while the third order
terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).
Negative Full-Scale Error (AD7893-3)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–REF IN + 1 LSB) after Bipolar
Zero Error has been adjusted out.
The AD7893 is tested using the CCIF standard where two
input frequencies near the top end of the input bandwidth are
used. In this case, the second and third order terms are of differ-
ent significance. The second order terms are usually distanced
in frequency from the original 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 sepa-
rately. The calculation of the intermodulation distortion is per
the THD specification where it is the ratio of the rms sum of the
individual distortion products to the rms amplitude of the fun-
damental expressed in dBs.
Track/Hold Acquisition Time
Track/Hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
±1/2 LSB, after the end of conversion (the point at which the
track/hold returns to track mode). It also applies to situations
where there is a step input change on the input voltage applied
to the VIN input of the AD7893. This means that the user must
wait for the duration of the track/hold acquisition time 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. E
–5–
ADI [ ADI ]
