AD7492
Rev. A | Page 12 of 24
TERMINOLOGY
Integral Nonlinearity
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 point
1/2 LSB below the first code transition, and full scale, a point
1/2 LSB above the last code transition.
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.
Gain Error
The last transition should occur at the analog value 1 1/2 LSB
below the nominal full scale. The first transition is a 1/2 LSB
above the low end of the scale (zero in the case of AD7492). The
gain error is the deviation of the actual difference between the
first and last code transitions from the ideal difference between
the first and last code transitions with offset errors removed.
Track/Hold Acquisition Time
The track/hold amplifier returns into track mode after the end
of the conversion. Track/Hold acquisition time is the time
required for the output of the track/hold amplifier to reach its
final value, within ±0.5 LSB, after the end of conversion.
Signal-to-Noise and Distortion Ratio
This is the measured ratio of signal-to-noise and 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 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 and Distortion = (6.02 N + 1.76) dB
Thus for a 12-bit converter, this is 74 dB and for a 10-bit
converter is 62 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7492 it is defined as:
1
2
6
2
5
2
4
2
3
2
2
log20)(
V
VVVVV
dBTHD
++++
=
where:
V
1
is the rms amplitude of the fundamental.
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
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
ADCs 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, etc. Intermodulation distortion 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), while the
third order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and
(fa − 2fb).
The AD7492 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 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 sum of the fundamentals expressed in dBs.
Aperture Delay
In a sample/hold, the time required after the hold command for
the switch to open fully is the aperture delay. The sample is, in
effect, delayed by this interval, and the hold command would
have to be advanced by this amount for precise timing.
Aperture Jitter
Aperture jitter is the range of variation in the aperture delay. In
other words, it is the uncertainty about when the sample is
taken. Jitter is the result of noise that modulates the phase of the
hold command. This specification establishes the ultimate
timing error, hence the maximum sampling frequency for a
given resolution. This error increases as the input dV/dt
increases.
