AD8209 データシートの表示（PDF） - Analog Devices
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AD8209 Datasheet PDF : 16 Pages
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Data Sheet
THEORY OF OPERATION
The AD8209 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 AD8209 consists of two amplifiers (A1 and A2), 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 RA, RB, and
RC, 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 97 V/V to provide
a total system gain, from ±IN to the output of A1, equal to 7 V/V,
as shown in the following equation:
Gain (A1) = 1/14 (V/V) × 97 (V/V) = 7 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
AD8209
by connecting A1 to A2 and placing a capacitor to ground (see
Figure 32).
The value of RF1 and RF2 is 10 kΩ, providing a gain of 2 V/V for
Amplifier A2. When connecting Pin A1 and Pin A2 together, the
AD8209 provides a total system gain equal to
Total Gain of (A1 + A2) (V/V) = 7 (V/V) × 2 (V/V) = 14 V/V
at the output of A2 (the OUT pin).
The ratios of RA, RB, RC, and RF 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%.
–IN
+IN
VS
A1 A2
RA
RA
RB
RF RC
RB
RG
RC
+
A1
–
RFILTER
RF
+
A2
–
RM
OUT
RF1
RF2
350mV
GND
Figure 23. Simplified Schematic
Rev. C | Page 11 of 16
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