AD7821

REV. A

–8–

lem of antialiasing filter design, the sampling rate is usually set

much greater than the Nyquist criterion. The maximum sam-

f

1

t

f

1

0.25E

− 6 + 0.25E − 6 + 0.15E − 6 + 0.35E − 6

This 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.

DIGITAL SIGNAL PROCESSING APPLICATIONS

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 (Total Unadjusted Error, etc.).

SIGNAL-TO-NOISE RATIO AND DISTORTION

The dynamic performance of the AD7821 is evaluated by apply-

ing a very low distortion sine wave signal to the analog input

Fourier Transform (FFT) plot is then generated from which

Signal-to-Noise Ratio (SNR) and harmonic distortion data are

obtained.

Figure 8 shows a 2048 point FFT plot of the AD7821 with an

input signal of 100.25 kHz. The SNR is 49.1 dB. It should be

noted that the harmonics are taken into account when calculat-

ing the SNR. The theoretical relationship between SNR and

resolution (N) is expressed by the following equation:

SNR = (6.02 N + 1.76) dB

. . . . . . . . . . . . . . . . . . . . . (1)

Figure 8. AD7821 FFT Plot

EFFECTIVE NUMBER OF 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 Typical Performance Characteristics sec-

tion. The effective number of bits typically falls between 7.7 and

7.9, corresponding to SNR figures of 48.1 and 49.7 dB.

INTERMODULATION DISTORTION

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.

Figure 9. FFT Plot for IMD

HISTOGRAM PLOT

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:

P(V )

=

1

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.

The absence of large spikes in the plot indicates small differen-

tial nonlinearity.

Figure 10 shows a histogram plot for the AD7821, which corre-

sponds very well with the ideal shape. The plot indicates very

small differential nonlinearity and no missing codes for an input

frequency of 100.25 kHz.

Figure 10. AD7821 Histogram Plot