AD7817/AD7818 Data Sheet
Rev. E | Page 10 of 20
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 + distortion)
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 (THD)
THD is the ratio of the rms sum of harmonics to the fundamental.
For the AD7817/AD7818, it is defined as:
1
2
6
2
5
2
4
2
3
2
2
log20
V
VVVVV
dBTHD
where:
V
1
is the rms amplitude of the fundamental.
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
determined by the largest harmonic in the spectrum; however,
for devices where the harmonics are buried in the noise floor, it
is a noise peak.
Intermodulation Distortion
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, 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 − ), while the third-order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).
The AD7817/AD7818 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 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.
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 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 four channels.
Relative Accuracy
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.
Gain Error
This is the deviation of the last code transition (1111 . . . 110) to
(1111 . . . 111) from the ideal, that is, VREF – 1 LSB, after the
offset error has been adjusted out.
Gain Error Match
This is the difference in gain error between any two channels.
Offset Error
This is the deviation of the first code transition (0000 . . . 000) to
(0000 . . . 001) from the ideal, that is, AGND + 1 LSB.
Offset Error Match
This is the difference in offset error between any two channels.
Track-and-Hold Acquisition Time
The 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 (the point at which
the track-and-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 AD7817 or the AD7818.
It means that the user must wait for the duration of the track-
and-hold acquisition 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 device operates to specification.
