Datasheet

AD8211
Rev. A | Page 10 of 16
THEORY OF OPERATION
In typical applications, the AD8211 amplifies a small differential
input voltage generated by the load current flowing through
a shunt resistor. The AD8211 rejects high common-mode
voltages (up to 65 V) and provides a ground-referenced,
buffered output that interfaces with an analog-to-digital converter
(ADC). Figure 23 shows a simplified schematic of the AD8211.
06824-022
V+
V
OUT
= (I
SHUNT
× R
SHUNT
) × 20
G = +20
AD8211
A1
PROPRIETARY
OFFSET
CIRCUITRY
GND
R1
R
OUT
Q1
R
I
SHUNT
I
IN
R
SHUNT
Figure 23. Simplified Schematic
A load current flowing through the external shunt resistor
produces a voltage at the input terminals of the AD8211. The
input terminals are connected to Amplifier A1 by Resistor R
and Resistor R1. The inverting terminal, which has very high
input impedance is held to
(V
CM
) − (I
SHUNT
× R
SHUNT
)
because negligible current flows through Resistor R. Amplifier
A1 forces the noninverting input to the same potential. Therefore,
the current that flows through Resistor R1, is equal to
I
IN
= (I
SHUNT
× R
SHUNT
)/R1
This current (I
IN
) is converted back to a voltage via R
OUT
. The
output buffer amplifier has a gain of 20 V/V and offers excellent
accuracy as the internal gain setting resistors are precision trimmed
to within 0.01% matching. The resulting output voltage is equal to
V
OUT
= (I
SHUNT
× R
SHUNT
) × 20