AD7720
–7–REV. 0
TERMINOLOGY (IDEAL FIR FILTER USED WITH AD7720
[FIGURE 1])
Integral Nonlinearity
This is the maximum deviation of any code from a straight line
passing through the endpoints of the transfer function. The
endpoints of the transfer function are zero scale (not to be con-
fused with bipolar zero), a point 0.5 LSB below the first code
transition (100...00 to 100...01 in bipolar mode and
000...00 to 000...01 in unipolar mode) and full scale, a
point 0.5 LSB above the last code transition (011...10 to
011...11 in bipolar mode and 111...10 to 111...11 in
unipolar mode). The error is expressed in LSBs.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between two adjacent codes in the ADC.
Common-Mode Rejection Ratio
The ability of a device to reject the effect of a voltage applied to
both input terminals simultaneously—often through variation of
a ground level—is specified as a common-mode rejection ratio.
CMRR is the ratio of gain for the differential signal to the gain
for the common-mode signal.
Unipolar Offset Error
Unipolar offset error is the deviation of the first code transition
from the ideal VIN(+) voltage which is (VIN(–) + 0.5 LSB)
when operating in the unipolar mode.
Bipolar Offset Error
This is the deviation of the midscale transition (111...11
to 000...00) from the ideal VIN(+) voltage which is (VIN(–)
–0.5 LSB) when operating in the bipolar mode.
Gain Error
The first code transition should occur at an analog value 1/2
LSB above minus full scale. The last code transition should
occur for an analog value 3/2 LSB below the nominal full scale.
Gain error is the deviation of the actual difference between first
and last code transitions and the ideal difference between first
and last code transitions.
Signal-to-(Noise + Distortion)
Signal-to-(Noise + Distortion) is measured signal-to-noise at the
output of the ADC. The signal is the rms magnitude of the
fundamental. Noise plus distortion is the rms sum of all of the
nonfundamental signals and harmonics to half the output word
rate (f
MCLK
/128), excluding dc. Signal-to-(Noise + Distortion) is
dependent on the number of quantization levels used in the
digitization process; the more levels, the smaller the quantiza-
tion noise. The theoretical Signal-to-(Noise + Distortion) ratio
for a sine wave input is given by
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
where N is the number of bits.
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the rms sum
of harmonics to the rms value of the fundamental. For the
AD7720, THD is defined as
THD = 20 log
(V
2
2
+V
3
2
+V
4
2
+V
5
2
+V
6
2
)
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 harmonic.
Spurious Free Dynamic Range
Spurious free dynamic range is the difference, in dB, between
the peak spurious or harmonic component in the ADC output
spectrum (up to f
MCLK
/128 and excluding dc) and the rms value
of the fundamental. Normally, the value of this specification will
be determined by the largest harmonic in the output spectrum
of the FFT. For input signals whose second harmonics occur in
the stop band region of the digital filter, a spur in the noise floor
limits the spurious free dynamic range.
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 distortion terms are
those for which neither m or 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).
