REV. B
AD7865
–8–
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) = (6.02 N + 1.76) dB
Thus for a 14-bit converter, this is 86.04 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7865 it is defined as:
THD dB
VVVV
V
()
=
++++
20
2
2
3
2
4
2
5
2
6
2
1
log
V
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
and V
5
are the rms amplitudes of the second through the fifth
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 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
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 (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).
The AD7865 is tested using two input frequencies. 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 10 kHz sine wave signal to one channel and a 50 kHz
signal to another channel and measuring how much of that
signal is coupled onto the first channel. The figure given is the
worst case across all four channels of the AD7865.
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.
Positive Gain Error (AD7865-1, AD7865-3)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal 4 × V
REF
– 3/2 LSB (AD7865 at
± 10 V), 2 × V
REF
– 3/2 LSB (AD7865 at ± 5 V range) or
V
REF
– 3/2 LSB (AD7865 at ± 2.5 V range), after the Bipolar
Offset Error has been adjusted out.
Positive Gain Error (AD7865-2)
This is the deviation of the last code transition (111 . . . 110 to
111 . . . 111) from the ideal 2 × V
REF
– 3/2 LSB (AD7865 at
0 V to 5 V), V
REF
– 3/2 LSB (AD7865 at 0 V to 2.5 V) after
the Unipolar Offset Error has been adjusted out.
Unipolar Offset Error (AD7865-2)
This is the deviation of the first code transition (000 . . . 000 to
000 . . . 001) from the ideal AGND + 1/2 LSB.
Bipolar Zero Error (AD7865-1, AD7865-3)
This is the deviation of the midscale transition (all 0s to 1s)
from the ideal AGND – 1/2 LSB.
Negative Gain Error (AD7865-1, AD7865-3)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal –4 × V
REF
+ 1/2 LSB (AD7865 at
± 10 V), –2 × V
REF
+ 1/2 LSB (AD7865 at ± 5 V range) or
–V
REF
+ 1/2 LSB (AD7865 at ± 2.5 V range), after Bipolar Zero
Error has been adjusted out.
Track/Hold Acquisition Time
Track/Hold acquisition time is the time required for the out-
put 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 there is a step input change on the input voltage applied
to the selected V
INxA
/V
INxB
input of the AD7865. It means that
the user must wait for the duration of the track/hold acquisition
time after the end of conversion or after a step input change to
V
INxA
/V
INxB
before starting another conversion, to ensure that
the part operates to specification.
