AD8209A

Data Sheet

Rev. A | Page 10 of 15

THEORY OF OPERATION

The AD8209A 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 AD8209A consists of two amplifiers (A1 and A2), a resistor

network, a small voltage reference, and a bias circuit (not shown);

see Figure 23.

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 by connecting A1 to

A2 and placing a capacitor to ground (see Figure 32).

Amplifier A2. When connecting Pin A1 and Pin A2 together, the

AD8209A 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).

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%.

Figure 23. Simplified Schematic

OUT

GND

–IN

350mV

+

–

A1

A2

+IN

A1

A2

+

–