Datasheet

AD7392/AD7393
Rev. C | Page 15 of 20
BIPOLAR OUTPUT OPERATION
Although the AD7393 is designed for single-supply operation,
the output can be easily configured for bipolar operation. A
typical circuit is shown in
Figure 33. This circuit uses a clean,
regulated 5 V supply for power, which also provides the circuit’s
reference voltage. Since the AD7393 output span swings from
ground to very near 5 V, it is necessary to choose an external
amplifier with a common-mode input voltage range that extends
to its positive supply rail. The micropower consumption OP196
is designed just for this purpose and results in only 50 μA of
maximum current consumption. Connecting the two 470 kΩ
resistors results in a differential amplifier mode of operation
with a voltage gain of 2, which produces a circuit output span
of 10 V, that is, −5 V to +5 V. As the DAC is programmed from
zero-code 0x000 to midscale 0x200 to full scale 0x3FF, the circuit
output voltage, V
O
, is set at −5 V, 0 V, and +5 V (minus 1 LSB).
The output voltage, V
O
, is coded in offset binary according to
Equation 4.
51
512
×
⎥
⎦
⎤
⎢
⎣
⎡
−=
D
V
O
(4)
where
D is the decimal code loaded in the AD7393 DAC
register.
Note that the LSB step size is 10/1024 = 10 mV. This circuit
is optimized for micropower consumption including the 470 kΩ
gain setting resistors, which should have low temperature
coefficients to maintain accuracy and matching (preferably the
same resistor material, such as metal film).
If better stability is required, the power supply may be substi-
tuted with a precision reference voltage such as the low dropout
REF195, which can easily supply the circuit’s 162 μA of current,
and still provide additional power for the load connected to V
O
.
The micropower REF195 is guaranteed to source 10 mA output
drive current, but consumes only 50 μA internally.
If higher resolution is required, the AD7392 can be used with
two additional bits of data inserted into the software coding,
which results in a 2.5 mV LSB step size.
Table 8 shows examples
of nominal output voltages (V
O
) provided by the bipolar
operation circuit application.
01121-033
V
O
C
V
REF
GND
+5
V
OP196
+5V
–5V
BIPOLAR
OUTPUT
SWING
–5V
AD7393
NOTES
1. DIGITAL INTERFACE CIRCUITRY OMITTED FOR CLARITY
<50µA<100µA
I
SY
<162µ
A
<2µA
470kΩ 470kΩ
V
OUT
V
DD
Figure 33. Bipolar Output Operation
Table 8. Bipolar Code Table
DAC Register No.
Hexadecimal Decimal Analog Output Voltage (V)
0x3FF 1023 +4.9902
0x201 513 +0.0097
0x200 512 0.0000
0x1FF 511 −0.0097
0x000 0 −5.0000