AD8202

Rev. D | Page 12 of 20

THEORY OF OPERATION

The AD8202 consists of a preamp and buffer arranged as shown

in Figure 40. Like-named resistors have equal values.

The preamp uses a dynamic bridge (subtractor) circuit.

Pin 8. When equal amplitude signals are asserted at Input 1 and

Input 8, and the output of A1 is equal to the common potential

(that is, 0), the two attenuators form a balanced-bridge network.

When the bridge is balanced, the differential input voltage at

A1, and thus its output, is 0.

Any common-mode voltage applied to both inputs keeps the

bridge balanced and the A1 output at 0. Because the resistor

networks are carefully matched, the common-mode signal

rejection approaches this ideal state.

However, if the signals applied to the inputs differ, the result is a

difference at the input to A1. A1 responds by adjusting its output

input until it matches the voltage at its noninverting input.

By attenuating voltages at Pin 1 and Pin 8, the amplifier inputs

are held within the power supply range, even if Pin 1 and Pin 8

input levels exceed the supply or fall below common (ground).

The input network also attenuates normal (differential) mode

forcing large output signals to balance relatively small differen-

tial inputs. The resistor ratios establish the preamp gain at 10.

Because the differential input signal is attenuated and then

amplified to yield an overall gain of 10, Amplifier A1 operates

at a higher noise gain, multiplying deficiencies such as input

offset voltage and noise with respect to Pin 1 and Pin 8.

A1

A3

(TRIMMED)

100k

Ω

–IN

+IN

COM

A2

AD8202

5

4

3

1

2

8

Figure 40. Simplified Schematic

To minimize these errors while extending the common-mode

range, a dedicated feedback loop is used to reduce the range of

common-mode voltage applied to A1 for a given overall range at

the inputs. By offsetting the voltage range applied to the com-

pensator, the input common-mode range is also offset to include

voltages more negative than the power supply.

Amplifier A3 detects the common-mode signal applied to A1

the common-mode voltage range at the A1 inputs. By adjusting

the common voltage of these resistors, the common-mode input

range is extended while, at the same time, the normal mode

signal attenuation is reduced, leading to better performance

referred to input.

The output of the dynamic bridge taken from A1 is connected

to Pin 3 by way of a 100 kΩ series resistor, provided for low-

pass filtering and gain adjustment. The resistors in the input

networks of the preamp and the buffer feedback resistors are

ratio-trimmed for high accuracy.

The output of the preamp drives a gain-of-2 buffer amplifier,

The 2-stage system architecture of the AD8202 enables the user

to incorporate a low-pass filter prior to the output buffer. By

separating the gain into two stages, a full-scale, rail-to-rail

signal from the preamp can be filtered at Pin 3, and a half-scale

signal, resulting from filtering, can be restored to full scale by

the output buffer amp. The source resistance seen by the

inverting input of A2 is approximately 100 kΩ to minimize the

effects of the input bias current of A2. However, this current is

quite small, and errors resulting from applications that mismatch

the resistance are correspondingly small.

The A2 input bias current has a typical value of 40 nA, however,

this can increase under certain conditions. For example, if the

because the maximum specified voltage for correct operation is

current increases (see Figure 41 for more information).