Data Sheet

AD8212

Rev. C | Page 9 of 13

THEORY OF OPERATION

NORMAL OPERATION

(7 V TO 65 V SUPPLY (V+) RANGE)

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

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

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

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

necessary current through Resistor R1 to equalize the potential

at both the inverting and noninverting inputs of Amplifier A1.

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

where:

In normal voltage operation mode, the bias circuit is connected

185 µA throughout the 7 V to 65 V (V+) range.

AD8212

BATTERY

OUTPUT

CURRENT

COMPENSATION

R1

R2

A1

Q1

VOUT

BIAS

CIRCUIT

8

6

3

2

5

1

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

Gain (V/V)

1

1

10

10

20

20

50

49.9

100

100