AD7821

REV. B

–12–

APPLYING THE AD7821

The AD7821 is specified for a unipolar input range of 0 V to +5 V

below. See the Typical Performance Characteristics section for

operation with unspecified input voltage ranges.

UNIPOLAR OPERATION

Figure 18 gives the configuration and reference voltages required

for 0 V to +5 V operation. The nominal transfer characteristic

for this input range is shown in Figure 19. The output code is

Natural Binary with 1 LSB = (5/256) V = 19.5 mV.

Figure 18. Unipolar/Bipolar Operation

Figure 19. Nominal Transfer Characteristic for Unipolar

(0 V to +5 V) Operation

BIPOLAR OPERATION

Figure 18 gives the configuration and reference voltages required

for –2.5 V to +2.5 V operation. The nominal transfer characteris-

tic for this input range is shown in Figure 20. The output code is

Offset Binary with 1 LSB = ([+2.5 – (–2.5)]/256) V = 19.5 mV.

Figure 20. Nominal Transfer Characteristic for Bipolar

(–2.5 V to +2.5 V) Operation

16-CHANNEL TELECOM A/D CONVERTER

The fast sampling rate (1 MHz) and bipolar operation of the

AD7821 makes it useful in telecom applications for sampling a

number of input channels using a multiplexer. Figure 21 shows

a circuit for such an application.

The maximum signal frequency required for acceptable quality

in telecom applications is 3 kHz. The circuit given in Figure 21

permits each of the 16-input channels to be sampled at a rate of

16 kHz maximum. The sampling rate takes into account such

circuit also eases the problem of the antialiasing filter design by

sampling at a rate much greater than that required by the

Nyquist criterion.