Datasheet

AD694
–12–
REV. B
Figure 15. Low Cost Sensor Transmitter
The circuit shown, will convert a positive differential signal at
V
IN
to a 4–20 mA current. The circuit has common-mode range
of 3 V to 8 V. The low end of the common-mode range is limited
by the AD708’s ability to pull down on R
S
. A single supply am-
plifier could be used instead to extend the common-mode range
down to about 1.5 V.
As shown, the circuit handles positive differential signals (V
IN
positive). To handle bipolar differential signals (V
IN
is positive
or negative), the reference pin of the in amp (Point C) must be
offset positively from the 2 V reference. For example, discon-
necting Point C from the 2 V reference and connecting it to a
3 V source would result in a V
A
of 1 V (or half scale) for a zero
volt differential input from the sensor.