lent current VOUT/2 kΩ. This resistor directly affects the output
gain temperature coefficient, and must be of suitable stability for
the application. The external low power op amp, powered by
+VOSS and –VOSS, maintains its summing junction at output
common. All the current flowing through the 2 kΩ resistor flows
through the output Darlington pass devices. A Darlington con-
figuration is used to minimize loss of output current to the base.
Figure 18. Self-Powered Isolated Current Source
The low leakage diode is used to protect the base-emitter junc-
tion against reverse bias voltages. Using –VOSS as a current
return allows more than 10 V of compliance. Offset and gain
control may be done at the input of the AD210 or by varying
the 2 kΩ resistor and summing a small correction current
directly into the summing node. A nominal range of 1 mA–
5 mA is recommended since the current output cannot reach
zero due to reverse bias and leakage currents. If the AD210 is
powered from the input potential, this circuit provides a fully
isolated, wide bandwidth current output. This configuration is
limited to 5 mA output current.
Isolated V-to-I Converter
Illustrated in Figure 19, the AD210 is used to convert a 0 V to
+10 V input signal to an isolated 4–20 mA output current. The
AD210 isolates the 0 V to +10 V input signal and provides a
proportional voltage at the isolator’s output. The output circuit
converts the input voltage to a 4–20 mA output current, which
in turn is applied to the loop load RLOAD.
WITH 0V IN
Figure 19. Isolated Voltage-to-Current Loop Converter
Isolated Thermocouple Amplifier
The AD210 application shown in Figure 20 provides amplifica-
tion, isolation and cold-junction compensation for a standard J
type thermocouple. The AD590 temperature sensor accurately
monitors the input terminal (cold-junction). Ambient tempera-
ture changes from 0°C to +40°C sensed by the AD590, are can-
celled out at the cold junction. Total circuit gain equals 183;
100 and 1.83, from A1 and the AD210 respectively. Calibration
is performed by replacing the thermocouple junction with plain
thermocouple wire and a millivolt source set at 0.0000 V (0°C)
and adjusting RO for EOUT equal to 0.000 V. Set the millivolt
source to +0.02185 V (400°C) and adjust RG for VOUT equal to
+4.000 V. This application circuit will produce a nonlinearized
output of about +10 mV/°C for a 0°C to +400°C range.
Figure 20. Isolated Thermocouple Amplifier
Precision Floating Programmable Reference
The AD210, when combined with a digital-to-analog converter,
can be used to create a fully floating voltage output. Figure 21
shows one possible implementation.
The digital inputs of the AD7541 are TTL or CMOS compat-
ible. Both the AD7541 and AD581 voltage reference are pow-
ered by the isolated power supply + VISS. ICOM should be tied to
input digital common to provide a digital ground reference for
The AD7541 is a current output DAC and, as such, requires an
external output amplifier. The uncommitted input amplifier
internal to the AD210 may be used for this purpose. For best
results, its input offset voltage must be trimmed as shown.
The output voltage of the AD210 will go from 0 V to –10 V for
digital inputs of 0 and full scale, respectively. However, since
the output port is truly isolated, VOUT and OCOM may be freely
interchanged to get 0 V to +10 V.
This circuit provides a precision 0 V–10 V programmable refer-
ence with a ± 3500 V common-mode range.
0 - –10V
Figure 21. Precision Floating Programmable Reference