REV. A–8–
AD7866
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are zero scale, a point 1 LSB
below the first code transition, and full scale, a point 1 LSB above
the last code transition.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
This applies to Straight Binary output coding. It is the deviation
of the first code transition (00 . . . 000) to (00 . . . 001) from the
ideal, i.e., AGND + 1 LSB.
Offset Error Match
This is the difference in Offset Error between the two channels.
Gain Error
This applies to Straight Binary output coding. It is the deviation
of the last code transition (111 . . . 110) to (111 . . . 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 the two channels.
Zero Code Error
This applies when using the twos complement output coding
option, in particular with the 2 V
REF
input range as –V
REF
to
+V
REF
biased about the V
REF
point. It is the deviation of the
midscale transition (all 1s to all 0s) from the ideal V
IN
voltage,
i.e., V
REF
– 1 LSB.
Zero Code Error Match
This refers to the difference in Zero Code Error between the
two channels.
Positive Gain Error
This applies when using the twos complement output coding
option, in particular with the 2 V
REF
input range as –V
REF
to
+V
REF
biased about the V
REF
point. It is the deviation of the last
code transition (011 . . . 110) to (011 . . . 111) from the ideal
(i.e., +V
REF
– 1 LSB) after the Zero Code Error has been
adjusted out.
Negative Gain Error
This applies when using the twos complement output coding
option, in particular with the 2 V
REF
input range as –V
REF
to
+V
REF
biased about the V
REF
point. It is the deviation of the first
code transition (100 . . . 000) to (100 . . . 001) from the ideal
(i.e., –V
REF
+ 1 LSB) after the Zero Code Error has been
adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns into track mode after the
end of conversion. Track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±1/2 LSB, after the end of conversion.
Signal-to-(Noise + Distortion) Ratio (SNDR)
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 sum of all nonfundamental sig-
nals up to half the sampling frequency (f
S
/2), excluding dc. The
ratio is dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the quantiza-
tion 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 (THD)
Total harmonic distortion is the ratio of the rms sum of har-
monics to the fundamental. For the AD7866, it is defined as:
THD db
VVVVV
V
()
=
++++
20
2
2
3
2
4
2
5
2
6
2
1
log
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
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 ADCs
where the harmonics are buried in the noise floor, it will be a
noise peak.
Intermodulation Distortion
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, and so on. Intermodulation distortion 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 AD7866 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 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 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
sum of the fundamentals expressed in dB.
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
(2 V
REF
), 455 kHz sine wave signal to all unselected input
channels and determining how much that signal is attenuated in the
selected channel with a 10 kHz signal (0 V to V
REF
). The figure
given is the worst-case across all four channels for the AD7866.
PSR (Power Supply Rejection)
See the Performance Curves section.
