AD8207

Rev. 0 | Page 10 of 16

THEORY OF OPERATION

The AD8207 is a single-supply, zero drift, difference amplifier

that uses a unique architecture to accurately amplify small

differential current shunt voltages in the presence of rapidly

changing common-mode voltage.

In typical applications, the AD8207 is used to measure current

by amplifying the voltage across a shunt resistor connected to

its inputs.

The AD8207 includes a zero-drift amplifier, a precision resistor

network, a common-mode control amplifier, and a precision

reference (see Figure 25).

A set of precision-trimmed resistors make up the network

that attenuates the input common-mode voltage to within the

supply range of the amplifier, in this case with a ratio of 20/1.

This attenuation ensures that when the input pins are externally

at the common-mode extremes of −4 V and +65 V, the actual

voltage at the inputs of the main amplifier is still within the

supply range.

The input resistor network also attenuates normal (differential)

mode voltages. Therefore, the total internal gain of the AD8207

is set to 400 V/V to provide a total system gain of 20 V/V.

Total Gain (V/V) = 1/20 (V/V) × 400 (V/V) = 20 V/V

The AD8207 is designed to provide excellent common-mode

rejection, even with PWM common-mode inputs that can

change at very fast rates, for example, 1 V/ns. An internal

common-mode control amplifier is used to maintain the input

common mode of the main amplifier at 3.5 V (with 5 V supply),

and therefore eliminates the negative effects of such fast-

changing external common-mode variations.

The AD8207 features an input offset drift of less than

500 nV/°C. This performance is achieved through a novel

zero-drift architecture that does not compromise band-

width, which is typically rated at 150 kHz.

resistors to the positive input of the main amplifier, which

allows the output offset to be adjusted anywhere in the output

operating range. The gain is 1 V/V from the reference pins to

the output when the reference pins are used in parallel. When

the pins are used to divide the supply, the gain is 0.5 V/V.

The AD8207 offers breakthrough performance without

compromising any of the robust application needs typical

of solenoid or motor control. The part rejects PWM input

common-mode voltages, while the zero-drift architecture yields

the lowest offset and offset drift performance on the market.

ZERO-DRIFT

AMPLIFIER

AD8207

COMMON-MODE

CONTROL AMPLIFIER

3.5V/2.2V

REF

60kΩ

120kΩ

SHUNT

120kΩ

+IN

GND

–IN

OUT

60kΩ

9kΩ

50kΩ

6kΩ

6kΩ

100kΩ

100kΩ

100kΩ

100kΩ

100kΩ

Figure 25. Simplified Schematic