AD8276/AD8277

Rev. A | Page 14 of 20

THEORY OF OPERATION

CIRCUIT INFORMATION

Each channel of the AD8276/AD8277 consists of a low power, low

noise op amp and four laser-trimmed on-chip resistors. These

resistors can be externally connected to make a variety of amplifier

configurations, including difference, noninverting, and inverting

configurations. Taking advantage of the integrated resistors of

the AD8276/AD8277 provides the designer with several benefits

over a discrete design, including smaller size, lower cost, and

better ac and dc performance.

2

5

3

1

6

7

4

40kΩ

40kΩ

40kΩ

–VS

+VS

IN–

IN+

SENSE

OUT

REF

AD8276

40kΩ

Figure 38. Functional Block Diagram

DC Performance

Much of the dc performance of op amp circuits depends on the

accuracy of the surrounding resistors. Using superposition to

analyze a typical difference amplifier circuit, as is shown in

Figure 39, the output voltage is found to be

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛ +

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

=

−

+

R3

R4

V

R3

R4

R2

R1

R2

V

V

IN

IN

OUT

1

This equation demonstrates that the gain accuracy and common-

mode rejection ratio of the AD8276/AD8277 is determined

primarily by the matching of resistor ratios. Even a 0.1% mismatch

in one resistor degrades the CMRR to 66 dB for a G = 1 difference

amplifier.

The difference amplifier output voltage equation can be reduced to

()

−

=

N

I

IN

OUT

V

V

R3

R4

V

as long as the following ratio of the resistors is tightly matched:

R3

R4

R1

R2 =

The resistors on the AD8276/AD8277 are laser trimmed to match

accurately. As a result, the AD8276/AD8277 provide superior

performance over a discrete solution, enabling better CMRR,

gain accuracy, and gain drift, even over a wide temperature range.

AC Performance

Component sizes and trace lengths are much smaller in an IC

than on a PCB, so the corresponding parasitic elements are also

smaller. This results in better ac performance of the AD8276/

AD8277. For example, the positive and negative input terminals

of the AD8276/AD8277 op amps are intentionally not pinned

out. By not connecting these nodes to the traces on the PCB, the

capacitance remains low, resulting in improved loop stability

and excellent common-mode rejection over frequency.

DRIVING THE AD8276/AD8277

Care should be taken to drive the AD8276/AD8277 with a low

impedance source: for example, another amplifier. Source

resistance of even a few kilohms (kΩ) can unbalance the resistor

ratios and, therefore, significantly degrade the gain accuracy and

common-mode rejection of the AD8276/AD8277. Because all

configurations present several kilohms of input resistance, the

AD8276/AD8277 do not require a high current drive from the

source and so are easy to drive.

INPUT VOLTAGE RANGE

The AD8276/AD8277 are able to measure input voltages beyond

the supply rails. The internal resistors divide down the voltage

before it reaches the internal op amp and provide protection to

the op amp inputs. Figure 39 shows an example of how the

voltage division works in a difference amplifier configuration.

For the AD8276/AD8277 to measure correctly, the input

voltages at the input nodes of the internal op amp must stay

below 1.5 V of the positive supply rail and can exceed the

negative supply rail by 0.1 V. Refer to the Power Supplies section

for more details.

R4

R3

R1

R2

R2

R1 + R2

R2

R1 + R2

Figure 39. Voltage Division in the Difference Amplifier Configuration

The AD8276/AD8277 have integrated ESD diodes at the inputs

that provide overvoltage protection. This feature simplifies

system design by eliminating the need for additional external

protection circuitry, and enables a more robust system.

The voltages at any of the inputs of the parts can safely range

supplies, input voltages can go as high as ±30 V. Care should be

to avoid risking damage to the parts.