Datasheet

AD8218 Data Sheet
Rev. B | 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 28 shows a simplified schematic of the AD8218.
V
REF
LOAD
V
2
I
LOAD
V
1
SHUNT
4V
TO
80V
I
CHARGE
GND
5V
LDO
R2
R1
AD8218
R4
R3
+IN
–IN
OUT
ENB
REF
GND
V
S
C
F
09592-027
Figure 28. Simplified Schematic
The AD8218 is configured as a difference amplifier. The
transfer function is
OUT = ((R4/R1) × (V
1
− V
2
)) + V
REF
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
between V
1
and V
2
is the voltage across the shunt resistor, or
V
IN
. Therefore, the input-to-output transfer function of the
AD8218 is
OUT (V) = (20 × V
IN
) + V
REF
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.