AD7821
lem of antialiasing filter design, the sampling rate is usually set
much greater than the Nyquist criterion. T he maximum sam-
pling rate (fMAX) for the AD7821 in the WR-RD mode,
(tRD < tINT L) can be calculated as follows:
INTERMO D ULATIO N D ISTO RTIO N
For intermodulation distortion (IMD), an FFT plot consisting
of very low distortion sine waves at two frequencies is generated
by sampling an analog input applied to the ADC. Figure 9
shows a 2048 point plot for IMD.
1
f MAX
f MAX
=
=
tWR + tRD + tRI + tP
1
0.25E − 6 + 0.25E − 6 + 0.15E − 6 + 0.35E − 6
tWR = Write Pulse Width
tRD = Delay Time between WR and RD Pulses
tRI = RD to INT Delay
tP = Delay Time between Conversions
T his permits a maximum sampling rate for the AD7821 of
1 MHz, which is much greater than the Nyquist criterion for
sampling a 100 kHz analog input signal.
D IGITAL SIGNAL P RO CESSING AP P LICATIO NS
In Digital Signal Processing (DSP) application areas like voice
recognition, echo cancellation and adaptive filtering, the dy-
namic characteristics (Signal-to-Noise Ratio, Harmonic Distor-
tion, Intermodulation Distortion) of an ADC are critical. Since
the AD7821 is a very fast ADC with a built-in track-and-hold
function, it is specified dynamically as well as with standard dc
specifications (T otal Unadjusted Error, etc.).
Figure 9. FFT Plot for IMD
H ISTO GRAM P LO T
When a sine wave of specified frequency is applied to the VIN in-
put of the AD7821 and several thousand samples are taken, it is
possible to plot a histogram showing the frequency of occur-
rence of each of the 256 ADC codes. A perfect ADC produces a
probability density function described by the equation:
SIGNAL-TO -NO ISE RATIO AND D ISTO RTIO N
T he dynamic performance of the AD7821 is evaluated by apply-
ing a very low distortion sine wave signal to the analog input
(VIN) which is then sampled at a 512 kHz sampling rate. A Fast
Fourier T ransform (FFT ) plot is then generated from which
Signal-to-Noise Ratio (SNR) and harmonic distortion data are
obtained.
1
P(V ) =
π( A2 −V 2 )1/2
where A is the peak amplitude of the sine wave and P(V) is the
probability of occurrence at a voltage V.
If a particular step is wider than the ideal 1 LSB width, then the
code associated with that step will accumulate more counts than
for the code for an ideal step. Likewise, a step narrower than the
ideal width will have fewer counts. Missing codes are easily seen
because a missing code means zero counts for a particular code.
T he absence of large spikes in the plot indicates small differen-
tial nonlinearity.
Figure 8 shows a 2048 point FFT plot of the AD7821 with an
input signal of 100.25 kHz. T he SNR is 49.1 dB. It should be
noted that the harmonics are taken into account when calculat-
ing the SNR. T he theoretical relationship between SNR and
resolution (N) is expressed by the following equation:
SNR = (6.02 N + 1.76) dB . . . . . . . . . . . . . . . . . . . . . (1)
Figure 10 shows a histogram plot for the AD7821, which corre-
sponds very well with the ideal shape. T he plot indicates very
small differential nonlinearity and no missing codes for an input
frequency of 100.25 kHz.
Figure 8. AD7821 FFT Plot
EFFECTIVE NUMBER O F BITS
By working backwards from Equation (1) it is possible to get a
measure of ADC performance expressed in effective number of
bits (N). A plot of the effective number of bits versus input fre-
quency is given in the T ypical Performance Characteristics sec-
tion. T he effective number of bits typically falls between 7.7 and
7.9, corresponding to SNR figures of 48.1 and 49.7 dB.
Figure 10. AD7821 Histogram Plot
REV. A
–8–
ADI [ ADI ]
