AD7904/AD7914/AD7924
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The end-
points 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
Negative Gain Error Match
This is the difference in Negative Gain Error between any two
channels.
Channel-to-Channel Isolation
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
Channel-to-Channel Isolation is a measure of the level of crosstalk
between channels. It is measured by applying a full-scale 400 kHz
sine wave signal to all three nonselected input channels and deter-
mining how much that signal is attenuated in the selected channel
with a 50 kHz signal. The figure is given worst case across all
four channels for the AD7904/AD7914/AD7924.
PSR (Power Supply Rejection)
This 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 any two channels.
Gain Error
Variations in power supply will affect the full scale transition,
but not the converter’s linearity. Power supply rejection is the
maximum change in full-scale transition point due to a change
in power-supply voltage from the nominal value. See Typical
Performance Curves.
Track/Hold Acquisition Time
This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (i.e., REF
IN
– 1 LSB) after the
offset error has been adjusted out.
Gain Error Match
This is the difference in Gain error between any two channels.
Zero Code Error
The track/hold amplifier returns into track mode at the end
of conversion. Track/Hold acquisition time is the time required
for the output of the track/hold amplifier to reach its final
value, within
±
1 LSB, after the end of conversion.
Signal to (Noise + Distortion) Ratio
This applies when using the twos complement output coding
option, in particular to the 2
×
REF
IN
input range with –REF
IN
to +REF
IN
biased about the REF
IN
point. It is the deviation of
the midscale transition (all 0s to all 1s) from the ideal V
IN
volt-
age, i.e., REF
IN
– 1 LSB.
Zero Code Error Match
This is the difference in Zero Code Error between any two
channels.
Positive Gain Error
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, for a 10-bit converter
this is 62 dB, and for an 8-bit converter this is 50 dB.
Total Harmonic Distortion
This applies when using the twos complement output coding
option, in particular to the 2
×
REF
IN
input range with –REF
IN
to +REF
IN
biased about the REF
IN
point. It is the deviation of
the last code transition (011. . .110) to (011 . . . 111) from the
ideal (i.e., +REF
IN
– 1 LSB) after the Zero Code Error has been
adjusted out.
Positive Gain Error Match
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7904/AD7914/AD7924, it
is defined as:
This is the difference in Positive Gain Error between any two
channels.
Negative Gain Error
V
22
+
V
32
+
V
42
+
V
52
+
V
62
THD
(
dB
)
=
20 log
V
1
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.
This applies when using the twos complement output coding
option, in particular to the 2
×
REF
IN
input range with –REF
IN
to +REF
IN
biased about the REF
IN
point. It is the deviation of
the first code transition (100 . . . 000) to (100 . . . 001) from the
ideal (i.e., –REF
IN
+ 1 LSB) after the Zero Code Error has
been adjusted out.
REV. 0
–11–
AD [ ANALOG DEVICES ]
