AD7858/AD7858L
TERMINOLOGY
1
Integral Nonlinearity
Total Harmonic Distortion
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 1/2 LSB
below the first code transition, and full scale, a point 1/2 LSB
above the last code transition.
Differential Nonlinearity
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7858/AD7858L, it is
defined as:
(V
2
+
V
3
+
V
4
+
V
5
+
V
6
)
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.
Peak Harmonic or Spurious Noise
2
2
2
2
2
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Total Unadjusted Error
This is the deviation of the actual code from the ideal code tak-
ing all errors into account (Gain,
Offset, Integral Nonlinearity, and
other errors)
at any point along the transfer function.
Unipolar Offset Error
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal AIN(+) voltage (AIN(–) + 1/2 LSB).
Positive Full-Scale Error
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 from the ideal
AIN(+) voltage (AIN(–) + Full Scale – 1.5 LSB) after the offset
error has been adjusted out.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of crosstalk between
the channels. It is measured by applying a full-scale 25 kHz
signal to the other seven channels and determining how much
that signal is attenuated in the channel of interest. The figure
given is the worst case for all channels.
Track/Hold Acquisition Time
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 distortion 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).
Testing is performed using the CCIF standard where two input
frequencies near the top end of the input bandwidth are used. In
this case, the second order terms are usually distanced in fre-
quency 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 sum
of the fundamentals expressed in dBs.
The track/hold amplifier returns into track mode and 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
±
1/2 LSB, after the end of conversion.
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 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 digitiza-
tion 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) =
(6.02
N
+1.76)
dB
Thus for a 12-bit converter, this is 74 dB.
1
AIN(+) refers to the positive input of the pseudo differential pair, and AIN(–)
refers to the negative analog input of the pseudo differential pair or to AGND
depending on the channel configuration.
–8–
REV. B
AD [ ANALOG DEVICES ]
