AD7821
APPLYING THE AD7821
BIPOLAR OPERATION
The AD7821 is specified for a unipolar input range of 0 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 be-
low. See the Typical Performance Characteristics section for op-
eration with unspecified input voltage ranges.
UNIPOLAR OPERATION
Figure 18 gives the configuration and reference voltages
required for –2.5 V to +2.5 V operation. The nominal transfer
characteristic 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 18 gives the configuration and reference voltages re-
quired for 0 V to +5 V operation. The nominal transfer charac-
teristic 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 20. Nominal Transfer Characteristic for Bipolar
(–2.5 V to +2.5 V) Operation
16-CHANNEL TELECOM A/D CONVERTER
Figure 18. AD7821 Unipolar/Bipolar Operation
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 account of such
multiplexer parameters as t
ON
, settling time etc. 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.
Figure 19. Nominal Transfer Characteristic for Unipolar
(0 V to +5 V) Operation
Figure 21. 16-Channel Telecom A/D Converter System
REV. A
–11–
AD [ ANALOG DEVICES ]
