Datasheet

AD8216 Data Sheet
Rev. B | Page 10 of 16
THEORY OF OPERATION
The AD8216 is a single-supply difference amplifier typically used
to accurately amplify a small differential current shunt voltage in
the presence of a rapidly changing common-mode voltage.
The AD8216 consists of an amplifier (A1), a resistor network,
a small voltage reference, and a bias circuit (not shown),
see Figure 23.
The set of input attenuators preceding A1 consist of R
A
, R
B
, and
R
C
, which feature a combined series resistance of approximately
200 kΩ ± 20%. The purpose of these resistors is to attenuate the
input voltage to match the input voltage range of A1. This
balanced resistor network attenuates the common-mode signal
by a ratio of 1/18. A1 amplifier inputs are held within the power
supply range, even if Pin 1 and Pin 8 exceed the supply or fall
below common (ground). A reference voltage of 250 mV biases
the attenuator above ground, which allows the amplifier to
operate in the presence of negative common-mode voltages.
The input resistor network also attenuates normal (differential)
mode voltages. Therefore, Amplifier A1 features a gain of
54 V/V to provide a total system gain of 3V/V.
Total Gain (V/V) = 1/18 (V/V) × 54 (V/V) = 3 V/V
The reference inputs, V
REF
1 and V
REF
2, are tied through resistors
to the positive input of A1, which allows the output offset to be
adjusted anywhere in the output operating range. The gain is
1 V/V from the reference pins to the output when the reference
pins are used in parallel. When they are used to divide the supply,
the gain is 0.5 V/V.
The ratios of R
A
, R
B
, R
C
, and R
F
are trimmed to a high level of
precision to allow the CMRR to exceed 80 dB. This performance is
accomplished by laser trimming the resistor ratio matching to
better than 0.01%.
AD8216
+IN
–
IN
250mV
GND
A1
R
A
R
A
R
B
R
B
R
F
R
C
R
C
V
OUT
R
REF
R
REF
V
REF
1
V
REF
2
07062-028
Figure 23. Simplified Schematic