AD7829-1
Rev. 0 | Page 8 of 20
TERMINOLOGY
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 decibels (dB).
Signal-to-(Noise + Distortion) Ratio
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 +
distortion) ratio for an ideal N-bit converter with a sine wave
input is given by
Channel-to-Channel Isolation
A measure of the level of crosstalk between channels. It is
measured by applying a full-scale 20 kHz sine wave signal to
one input channel and determining how much that signal is
attenuated in each of the other channels. The figure given is
the worst case across all eight channels of the AD7829-1.
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
Thus, for an 8-bit converter, this is 50 dB.
Relative Accuracy or Endpoint Nonlinearity
The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function.
Total Harmonic Distortion (THD)
The ratio of the rms sum of harmonics to the fundamental.
For the AD7829-1 it is defined as
Differential Nonlinearity
The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
1
6
54
32
V
VVVVV
THD
22222
log20)dB(
++++
=
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.
The deviation of the 128th code transition (01111111) to
(10000000) from the ideal, that is, V
MID
.
Offset Error Match
Peak Harmonic or Spurious Noise
The difference in offset error between any two channels.
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 specifica-
tion 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.
Zero-Scale Error
The deviation of the first code transition (00000000) to
(00000001) from the ideal; that is, V
MID
− 1.25 V + 1 LSB (V
DD
=
5 V ± 10%), or V
MID
− 1.0 V + 1 LSB (V
DD
= 3 V ± 10%).
Full-Scale Error
Intermodulation Distortion
The deviation of the last code transition (11111110) to
(11111111) from the ideal; that is, V
MID
+ 1.25 V − 1 LSB (V
DD
=
5 V ± 10%), or V
MID
+ 1.0 V − 1 LSB (V
DD
= 3 V ± 10%).
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion
products at sum and difference frequencies of mfa ± nfb, where
m, n = 0, 1, 2, 3… . Intermodulation terms are those for which
neither m nor n is 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
AD7829-1 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 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.
Gain Error
The deviation of the last code transition (1111 . . . 110) to
(1111 . . . 111) from the ideal; that is, V
REF
− 1 LSB, after the
offset error has been adjusted out.
Gain Error Match
The difference in gain error between any two channels.
