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AD8212
THEORY OF OPERATION
NORMAL OPERATION
(7 V TO 65 V SUPPLY (V+) RANGE)
BATTERY
1
R1
R SHUNT
AD8212
I LOAD
8
R2
In typical applications, the AD8212 measures a small
differential input voltage generated by a load current
flowing through a shunt resistor.
The operational amplifier (A1) is connected across the shunt
resistor (R SHUNT ) with its inverting input connected to the
battery/supply side, and the noninverting input connected
to the load side of the system. Amplifier A1 is powered via
an internal series regulator (depicted as a Zener diode in
Q1
A1
Figure 22). This regulator maintains a constant 5 V between
the battery/supply terminal of the AD8212 and COM (Pin 2),
which represents the lowest common point of the internal
BIAS
CIRCUIT
OUTPUT
CURRENT
COMPENSATION
circuitry.
A load current flowing through the external shunt resistor
produces a voltage at the input terminals of the AD8212.
Amplifier A1 responds by causing Transistor Q1 to conduct the
VOUT
R OUT
5
I OUT
2
3
6
necessary current through Resistor R1 to equalize the potential
at both the inverting and noninverting inputs of Amplifier A1.
The current through the emitter of Transistor Q1 (I OUT ) is
proportional to the input voltage (V SENSE ), and, therefore, the
load current (I LOAD ) through the shunt resistor (R SHUNT ). The
output current (I OUT ) is converted to a voltage by using an
external resistor, the value of which is dependent on the input
to output gain equation desired in the application.
The transfer function for the AD8212 is
I OUT = ( g m × V SENSE )
V SENSE = I LOAD × R SHUNT
V OUT = I OUT × R OUT
Figure 22. Typical Connection (7 V to 65 V Supply (Pin V+) Range)
When using the AD8212 as described, the battery/supply
voltage in the system must be between 7 V to 65 V. The 7 V
minimum supply range is necessary to turn on the internal
regulator (shown as a Zener diode in Figure 22). This regulated
voltage then remains a constant 5 V, regardless of the supply
(V+) voltage. The 65 V maximum limit in this mode of
operation is due to the breakdown voltage limitation of the
AD8212 process.
Typically, a 1% resistor can be used to convert the output
current to a voltage. Table 4 provides suggested R OUT values.
V OUT = (V SENSE × R OUT )/ 1000
where:
g m = 1000 μ A/V.
In normal voltage operation mode, the bias circuit is connected
to GND, as shown in Figure 22. In this mode, I BIAS is typically
185 μA throughout the 7 V to 65 V (V+) range.
Table 4. Suggested R OUT Values
Gain (V/V)
1
10
20
50
100
R OUT (kΩ)
1
10
20
49.9
100
Rev. B | Page 9 of 16
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