AD7892
TERMINOLOGY
Signal to (Noise + Distortion) Ratio
Relative Accuracy
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 (f
S
/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:
Signal to
(Noise
+ Distortion) =
(6.02
N +
1.76)
dB
Thus for a 12-bit converter, this is 74 dB.
Total Harmonic Distortion
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
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 (AD7892-1)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal 4
×
REF IN – 3/2 LSB (± 10 V range)
or 2
×
REF IN – 3/2 LSB (± 5 V range) after the bipolar zero
error has been adjusted out.
Positive Full-Scale Error (AD7892-2)
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7892, it is defined
as:
V
2
+V
3
+V
4
+V
5
+V
6
THD
(dB)
=
20 log
V
1
2
2
2
2
2
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (REF IN – 3/2 LSB) after the unipo-
lar offset error has been adjusted out.
Positive Full-Scale Error (AD7892-3)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (REF IN – 3/2 LSB) after the bipolar
zero error has been adjusted out.
Bipolar Zero Error (AD7892-1, AD7892-3)
where
V
1
is the rms amplitude of the fundamental and
V
2
,
V
3
,
V
4
,
V
5
and
V
6
are the rms amplitudes of the second through the
sixth harmonics.
Peak Harmonic or Spurious Noise
This is the deviation of the midscale transition (all 1s to all 0s)
from the ideal (AGND – 1/2 LSB).
Unipolar Offset Error (AD7892-2)
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 f
S
/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.
Intermodulation Distortion
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal (AGND + 1/2 LSB).
Negative Full-Scale Error (AD7892-1)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal –4
×
REF IN + 1/2 LSB (± 10 V
range) or –2
×
REF IN + 1/2 LSB (± 5 V range) after bipolar
zero error has been adjusted out.
Negative Full-Scale Error (AD7892-3)
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 sec-
ond order terms include (fa + fb) and (fa – fb), while the third
order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
(fa – 2fb).
The AD7892 is tested using two input frequencies away from
the bottom end of the input bandwidth. In this case, the second
and third order terms are of different 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 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.
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal – REF IN + 1/2 LSB after bipolar
zero error has been adjusted out.
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 V
IN
input of the AD7892. It means that the user must wait for the
duration of the track/hold acquisition time after the end of con-
version or after a step input change to V
IN
before starting another
conversion, to ensure that the part operates to specification.
–8–
REV. C
AD [ ANALOG DEVICES ]
