AD8278/AD8279

Rev. C | Page 16 of 24

THEORY OF OPERATION

CIRCUIT INFORMATION

Each channel of the AD8278 and AD8279 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 AD8278 and AD8279 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

20kΩ

40kΩ

–VS

+VS

–IN

+IN

SENSE

OUT

REF

20kΩ

AD8278

Figure 48. 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 49, 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 AD8278 and AD8279 is determined

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

mismatch in one resistor degrades the CMRR to 69 dB for a

G = 2 difference amplifier.

The difference amplifier output voltage equation can be reduced to

()

−

=

IN

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 AD8278 and AD8279 are laser trimmed to

match accurately. As a result, the AD8278 and AD8279 provide

superior performance over a discrete solution, enabling better

CMRR, gain accuracy, and gain drift, even over a wide tempera-

ture range.

AC Performance

Component sizes and trace lengths are much smaller in an IC

than on a PCB; therefore, the corresponding parasitic elements

are also smaller. This results in better ac performance of the

AD8278 and AD8279. For example, the positive and negative

input terminals of the AD8278 and AD8279 op amps are

intentionally not pinned out. By not connecting these nodes

to the traces on the PCB, their capacitance remains low and

balanced, resulting in improved loop stability and excellent

common-mode rejection over frequency.

DRIVING THE AD8278 AND AD8279

Care should be taken to drive the AD8278 and AD8279 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 AD8278 and AD8279. Because all

configurations present several kilohms (kΩ) of input resistance,

the AD8278 and AD8279 do not require a high current drive

from the source and are easy to drive.

INPUT VOLTAGE RANGE

The AD8278 and AD8279 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 49 shows an example

of how the voltage division works in a difference amplifier

configuration. For the AD8278 and AD8279 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 49. Voltage Division in the Difference Amplifier Configuration

The AD8278 and AD8279 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 damaging the parts.