AD8216

Rev. 0 | Page 10 of 16

THEORY OF OPERATION

The AD8216 is a single-supply difference amplifier typically used

to accurately amplify a small differential current shunt voltage in

the presence of a rapidly changing common-mode voltage.

The AD8216 consists of an amplifier (A1), a resistor network,

a small voltage reference, and a bias circuit (not shown),

see Figure 23.

R

B

C

, which feature a combined series resistance of approximately

200 kΩ ± 20%. The purpose of these resistors is to attenuate the

input voltage to match the input voltage range of A1. This

balanced resistor network attenuates the common-mode signal

by a ratio of 1/18. A1 amplifier inputs are held within the power

supply range, even if Pin 1 and Pin 8 exceed the supply or fall

below common (ground). A reference voltage of 250 mV biases

the attenuator above ground, which allows the amplifier to

operate in the presence of negative common-mode voltages.

The input resistor network also attenuates normal (differential)

mode voltages. Therefore, Amplifier A1 features a gain of

54 V/V to provide a total system gain of 3V/V.

Total Gain (V/V) = 1/18 (V/V) × 54 (V/V) = 3 V/V

to the positive input of A1, 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 they are used to divide the supply,

the gain is 0.5 V/V.

B

C

precision to allow the CMRR to exceed 80 dB. This performance is

accomplished by laser trimming the resistor ratio matching to

better than 0.01%.

AD8216

+IN

–IN

250mV

GND

A1

Figure 23. Simplified Schematic