Datasheet

AD7908/AD7918/AD7928
Rev. D | Page 12 of 32
TERMINOLOGY
Integral Nonlinearity Negative Gain Error Match
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 position 1 LSB
below the first code transition, and full scale, a position 1 LSB
above the last code transition.
This is the difference in negative gain error between any two
channels.
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 400 kHz sine wave signal to all seven nonselected input
channels and determining how much that signal is attenuated
in the selected channel with a 50 kHz signal. The figure is given
worst case across all eight channels for the AD7908/AD7918/
AD7928.
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 is the deviation of the first code transition (00 . . . 000) to
(00 . . . 001) from the ideal, that is, AGND + 1 LSB.
Power Supply Rejection (PSR)
Variations in power supply 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 the
Performance Curves section).
Offset Error Match
This is the difference in offset error between any two channels.
Gain Error
This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (that is, REF
IN
– 1 LSB) after the
offset error has been adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at 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 LSB, after the end of conversion.
Gain Error Match
This is the difference in gain error between any two channels.
Zero Code Error
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
voltage,
that is, REF
IN
− 1 LSB.
This is the measured ratio of signal-to-(noise + distortion) at
the output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals 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
quantization noise. The theoretical signal-to-(noise +
distortion) ratio for an ideal N-bit converter with a sine wave
input is given by
Zero Code Error Match
This is the difference in zero code error between any two
channels.
Positive Gain Error
Signal-to-(Noise + Distortion) = (6.02N + 1.76)dB
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
(that is, +REF
IN
− 1 LSB) after the zero code error has been
adjusted out.
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
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7908/AD7918/
AD7928, it is defined as:
Positive Gain Error Match
This is the difference in positive gain error between any two
channels.
()
1
2
6
2
5
2
4
2
3
2
2
log20
V
VVVVV
dBTHD
++++
=
Negative Gain Error
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
(that is, −REF
IN
+ 1 LSB) after the zero code error has been
adjusted out.
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.