AD7811/AD7812
TERMINOLOGY
Signal to (Noise + Distortion) Ratio
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
quantization noise. The theoretical signal to (noise + distor-
tion) ratio for an ideal N-bit converter with a sine wave input
is given by:
Signal to (Noise + Distortion)
= (6.02N + 1.76)
dB
Thus for a 10-bit converter, this is 62 dB.
Total Harmonic Distortion
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 inter-
modulation 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.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a full-
scale 20 kHz sine wave signal to all nonselected input channels
and determining how much that signal is attenuated in the selected
channel. The figure given is the worst case across all four or
eight channels for the AD7811 and AD7812 respectively.
Relative Accuracy
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7811 and AD7812
it is defined as:
THD
(dB)
=
20 log
2
2
V
22
+V
3
+V
4
+V
52
+V
62
Relative accuracy, or endpoint nonlinearity, is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
Differential Nonlinearity
V
1
This is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
Offset Error
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 first code transition (0000 . . . 000)
to (0000 . . . 001) from the ideal, i.e., AGND + 1 LSB.
Offset Error Match
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
determined 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 difference in Offset Error between any two channels.
Gain Error
This is the deviation of the last code transition (1111 . . . 110)
to (1111 . . . 111) from the ideal, i.e., V
REF
– 1 LSB, after the
offset error has been adjusted out.
Gain Error Match
This is the difference in Gain Error between any two channels.
Track/Hold Acquisition Time
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 (2fa + fb), (2fa – fb), (fa + 2fb) and
(fa – 2fb).
The AD7811 and AD7812 are 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 different significance. The second order terms are
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 a change in the selected input channel takes place or
where there is a step input change on the input voltage applied
to the selected V
IN
input of the AD7811 or AD7812. It means
that the user must wait for the duration of the track/hold acquisi-
tion time after the end of conversion or after a channel change/
step input change to V
IN
before starting another conversion, to
ensure that the part operates to specification.
–6–
REV. B
AD [ ANALOG DEVICES ]
