REV. B
AD7722
–11–
TERMINOLOGY
Signal-to-Noise Plus Distortion Ratio (S/(N+D))
S/(N+D) is the measured signal-to-noise plus distortion ratio
at the output of the ADC. The signal is the rms magnitude of
the fundamental. Noise plus distortion is the rms sum of all
nonfundamental signals and harmonics to half the sampling rate
(f
CLKIN
/128), excluding dc. The ADC is evaluated by applying a
low noise, low distortion sine wave signal to the input pins. By
generating a fast Fourier transform (FFT) plot, the S/(N+D) data
can then be obtained from the output spectrum.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the harmonics to the rms
value of the fundamental. THD is defined as
THD = 20 log
SQRT 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
sixth harmonics. The THD is also derived from the FFT plot of
the ADC output spectrum.
Spurious-Free Dynamic Range (SFDR)
Defined as the difference in dB between the peak spurious or har-
monic component in the ADC output spectrum (up to f
CLKIN
/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 SFDR.
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, and so on. 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).
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
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 fundamental, expressed in dB.
Pass-Band Ripple
The frequency response variation of the AD7722 in the defined
pass-band frequency range.
Pass-Band Frequency
The frequency up to which the frequency response variation is
within the pass-band ripple specification.
Cutoff Frequency
The frequency below which the AD7722’s frequency response
will not have more than 3 dB of attenuation.
Stop-Band Frequency
The frequency above which the AD7722’s frequency response
will be within its stop-band attenuation.
Stop-Band Attenuation
The AD7722’s frequency response will not have less than 90 dB
of attenuation in the stated frequency band.
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 minus full scale, a point
0.5 LSB below the first code transition (100 . . . 00 to 100 . . .
01 in bipolar mode, 000 . . . 00 to 000 . . . 01 in unipolar mode)
and plus full scale, a point 0.5 LSB above the last code transition
(011 . . . 10 to 011 . . . 11 in bipolar mode, 111 . . . 10 to
111 . . . 11 in unipolar mode). The error is expressed in LSB.
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
(00 . . . 000 to 00 . . . 001) from the ideal differential voltage
(V
IN
(+) – V
IN
(–) + 0.5 LSB) when operating in the unipolar mode.
Bipolar Offset Error
This is the deviation of the midscale transition code
(111 . . . 11 to 000 . . . 00) from the ideal differential voltage
(V
IN
(+) – V
IN
(–) – 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 – full scale. The last transition should occur for an analog
value 1 1/2 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.
