AD8217

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THEORY OF OPERATION

AMPLIFIER CORE

In typical applications, the AD8217 amplifies a small differential

input voltage generated by the load current flowing through

a shunt resistor. The AD8217 rejects high common-mode vol-

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

output that interfaces with an analog-to-digital converter (ADC).

Figure 24 shows a simplified schematic of the AD8217.

LDO

LOAD

R2

R1

AD8217

R4

R3

+IN

GND

–IN

SHUNT

OUT

4.5V

TO

80V

Figure 24. Simplified Schematic

The AD8217 is configured as a difference amplifier. The

transfer function is

OUT = (R4/R1) × (V1 − V2)

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 AD8217.

The AD8217 accurately amplifies the input differential signal,

rejecting high voltage common modes ranging from 4.5 V to 80 V.

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

the end user with an extremely stable part over temperature.

The offset drift is typically less than ±100 nV/°C. This perfor-

mance leads to optimal accuracy and dynamic range.

INTERNAL LDO

The AD8217 includes an internal LDO, which allows the device

to power directly from the common-mode voltage at the inputs.

No additional standalone supply is necessary, provided that the

common-mode voltage at the +IN pin is at least 4.5 V and up to

80 V. Once the common-mode voltage is above 5.6 V, the LDO

output reaches its maximum value, that is 5.6 V. This is also the

maximum output voltage range of the AD8217. Because the

AD8217 output typically interfaces with a converter, the 5.6 V

maximum output range ensures the ADC input is not damaged

due to excessive overvoltage.

The input bias current flowing through Pin +IN powers the

internal LDO and, therefore, doubles as the supply current

for the AD8217. This current varies depending on the input

common-mode voltage. See Figure 8 for additional information.