AD7821
REV. B
–12–
APPLYING THE AD7821
The AD7821 is specified for a unipolar input range of 0 V to +5 V
and a bipolar input range of –2.5 V to +2.5 V. The V
REF
(–) and
V
REF
(+) voltages required for these input ranges are outlined
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
multiplexer parameters as t
ON
, settling time, and so on. The
circuit also eases the problem of the antialiasing filter design by
sampling at a rate much greater than that required by the
Nyquist criterion.
