AD7890
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 quan-
tization 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 12-bit converter, this is 74 dB.
Total Harmonic Distortion
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 1 kHz signal to any one of the other seven inputs and
determining how much that signal is attenuated in the chan-
nel of interest. The figure given is the worst case across all
eight 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
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7890, it is defined as:
THD
(dB )
=
20 log
2
V
2
2
+
V
3
2
+
V
4
2
+
V
5
2
+
V
6
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Positive Full-Scale Error (AD7890-10)
V
1
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (4
×
REF IN – 1 LSB) after the Bipolar
Zero Error has been adjusted out.
Positive Full-Scale Error (AD7890-4)
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
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (1.638
×
REF IN – 1 LSB) after the
Unipolar Offset Error has been adjusted out.
Positive Full-Scale Error (AD7890-2)
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 parts
where the harmonics are buried in the noise floor, it will be a
noise peak.
Intermodulation Distortion
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (REF IN – 1 LSB) after the Unipolar
Offset Error has been adjusted out.
Bipolar Zero Error (AD7890-10)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (AGND).
Unipolar Offset Error (AD7890-2, AD7890-4)
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, 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 – fb), while the third order terms
include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
The AD7890 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 differ-
ent 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 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
fundamental expressed in dBs.
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 0 V (AGND).
Negative Full-Scale Error (AD7890-10)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–4
×
REF IN + 1 LSB) after Bipolar
Zero Error has been adjusted out.
Track/Hold Acquisition Time
Track/Hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
±1/2
LSB,
after the end of conversion (the point at which the track/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 AD7890. It means that the user must wait for
the duration of the track/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 part oper-
ates to specification.
REV. A
–7–
AD [ ANALOG DEVICES ]
