Data Sheet AD8208
Rev. C | Page 11 of 20
THEORY OF OPERATION
The AD8208 is a single-supply difference amplifier typically used
to amplify a small differential voltage in the presence of rapidly
changing, high common-mode voltages.
The AD8208 consists of two amplifiers (A1 and A2), a resistor
network, a small voltage reference, and a bias circuit (not shown);
see Figure 24.
The set of input attenuators preceding A1 consists of R
A
, R
B
, and
R
C
, which feature a combined series resistance of approximately
400 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 1/14. The A1 amplifier inputs are held within the power supply
range, even as Pin 1 and Pin 8 exceed the supply or fall below the
common (ground). A reference voltage of 350 mV biases the
attenuator above ground, allowing Amplifier A1 to operate in
the presence of negative common-mode voltages.
The input resistor network also attenuates normal (differential)
mode voltages. Therefore, A1 features a gain of 140 V/V to provide
a total system gain, from ±IN to the output of A1, equal to 10
V / V, as shown in the following equation:
Gain (A1) = 1/14 (V/V) × 140(V/V) = 10 V/V
A precision trimmed, 100 kΩ resistor is placed in series with the
output of Amplifier A1. The user has access to this resistor via
an external pin (A1). A low-pass filter can be easily implemented
by connecting A1 to A2 and placing a capacitor to ground (see
Figure 33).
The value of R
F1
and R
F2
is 10 kΩ, providing a gain of 2 V/V for
Amplifier A2. When connecting Pin A1 and Pin A2 together, the
AD8208 provides a total system gain equal to
Total Gain of (A1 + A2) (V/V) = 10 (V/V) × 2 (V/V) = 20 V/V
at the output of A2 (the OUT pin).
The ratios of R
A
, R
B
, R
C
, and R
F
are trimmed to a high level of
precision, allowing a typical CMRR value that exceeds 80 dB. This
performance is accomplished by laser trimming the resistor ratio
matching to better than 0.01%.
OUT
GND
V
S
R
C
R
F
–IN
350mV
+
–
R
B
R
B
R
A
R
A
R
C
R
F
R
G
R
FILTER
R
M
A1
A2
+IN
A1
A2
+
–
R
F1
R
F2
08714-023
Figure 24. Simplified Schematic
