AD7863
Rev. B | Page 8 of 24
TERMINOLOGY
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distortion) at the
output of the analog-to-digital converter. The signal is the rms
amplitude of the fundamental. Noise is the rms sum of all non-
fundamental 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
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 AD7863 it is defined as
()
1
5432
V
VVVV
dBTHD
222
log20
+++
=
2
where:
V
1
is the rms amplitude of the fundamental.
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
determined by the largest harmonic in the spectrum, but for
parts 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. Intermodulation terms are those for which
neither m nor n is equal to zero. For example, the second order
terms include (fa + fb) and (fa − fb), and the third order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).
The AD7863 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, and 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 decibels (dB).
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 50 kHz sine wave signal to all nonselected channels and
determining how much that signal is attenuated in the selected
channel. The figure given is the worst case across all 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.
Positive Gain Error (AD7863-10, ±10 V, AD7863-3, ±2.5 V)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal 4 × V
REF
− 1 LSB (AD7863-10, ±10 V
range) or V
REF
− 1 LSB (AD7863-3, ±2.5 V range), after the
bipolar offset error has been adjusted out.
Positive Gain Error (AD7863-2, 0 V to 2.5 V)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal V
REF
− 1 LSB, after the unipolar offset
error has been adjusted out.
Bipolar Zero Error (AD7863-10, ±10 V, AD7863-3, ±2.5 V)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (AGND).
Unipolar Offset Error (AD7863-2, 0 V to 2.5 V)
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal AGND + 1 LSB.
Negative Gain Error (AD7863-10, ±10 V, AD7863-3, ±2.5 V)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal −4 × V
REF
+ 1 LSB (AD7863-10, ±10 V
range) or –V
REF
+ 1 LSB (AD7863-3, ±2.5 V range), after bipolar
zero error has been adjusted out.
Track-and-Hold Acquisition Time
Track-and-hold acquisition time is the time required for the
output of the track/hold amplifier to reach its final value, with
±½ 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
AX/BX
input of the AD7863. 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
AX/BX
before starting another
conversion, to ensure that the part operates to specification.
