Datasheet

AD8215 Data Sheet
Rev. A | Page 10 of 16
THEORY OF OPERATION
In typical applications, the AD8215 amplifies a small differential
input voltage generated by the load current flowing through a
shunt resistor. The AD8215 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
26 shows a simplified schematic of the AD8215.
07203-025
V+
OUT =
(I
SHUNT
× R
SHUNT
) × 20
G = +20
AD8215
A1
R1
R
OUT
I
IN
R
PROPRIETARY
OFFSET
CIRCUITRY
GND
I
SHUNT
R
SHUNT
Figure 26. Simplified Schematic
A load current flowing through the external shunt resistor
produces a voltage at the input terminals of the AD8215. The
input terminals are connected to A1 by R and R1. The inverting
terminal, which has very high input impedance, is held to
(V
CM
) − (I
SHUNT
× R
SHUNT
)
because negligible current flows through R. A1 forces the
noninverting input to the same potential. Therefore, the current
that flows through 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
OUT = (I
SHUNT
× R
SHUNT
) × 20