AD8218

Rev. A | Page 10 of 16

THEORY OF OPERATION

AMPLIFIER CORE

In typical applications, the AD8218 amplifies a small differential

input voltage generated by the load current flowing through

a shunt resistor. The AD8218 rejects high common-mode vol-

tages (up to 80 V) and provides a ground-referenced, buffered

output. Figure 27 shows a simplified schematic of the AD8218.

LOAD

SHUNT

4V

TO

80V

GND

5V

LDO

R2

R1

AD8218

R4

R3

+IN

–IN

OUT

ENB

REF

GND

Figure 27. Simplified Schematic

The AD8218 is configured as a difference amplifier. The

transfer function is

Resistors R4 and R1 are matched to within 0.01% and have

values of 1.5 MΩ and 75 kΩ, respectively, meaning an input-

to-output total gain of 20 V/V for the AD8218. The difference

AD8218 is

The AD8218 accurately amplifies the input differential signal,

rejecting high voltage common modes ranging from 4 V to 80 V.

The main amplifier uses a novel zero-drift architecture, providing

the end user with breakthrough temperature stability. The

offset drift is typically less than ±100 nV/°C. This performance

leads to optimal accuracy and dynamic range.

OUTPUT CLAMPING

After the input common-mode voltage in the application is

above 5.2 V, the internal LDO output of the AD8218 also

reaches its maximum value of 5.2 V, which is the maximum

output range of the AD8218. Because in typical applications

the output interfaces with a converter, clamping the AD8218

output voltage to 5.2 V ensures that the ADC input is not

damaged due to excessive overvoltage.