AD8278ARMZ-R7 datasheet(16/24 Pages) AD

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AD8278

Rev. 0 | Page 16 of 24

THEORY OF OPERATION

CIRCUIT INFORMATION

The AD8278 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 confi-

gurations, including difference, noninverting, and inverting

configurations. Taking advantage of the integrated resistors

of the AD8278 provides the designer with several benefits

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

better ac and dc performance.

40kΩ

20kΩ

40kΩ

–VS

+VS

–IN

+IN

SENSE

OUT

REF

20kΩ

AD8278

Figure 45. 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 46, the output voltage is found to be

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛ +

⎟

⎠

⎞

⎜

⎝

⎛

−

OUT

This equation demonstrates that the gain accuracy and common-

mode rejection ratio of the AD8278 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

()

−

+ −

OUT

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

R2 =

The resistors on the AD8278 are laser trimmed to match accurately.

As a result, the AD8278 provides 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 AD8278.

For example, the positive and negative input terminals of the

AD8278 op amp 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

Care should be taken to drive the AD8278 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. Because all configurations present

several kilohms (kΩ) of input resistance, the AD8278 does not

require a high current drive from the source and so is easy to

drive.

INPUT VOLTAGE RANGE

The AD8278 is 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 46 shows an example of how the voltage

division works in a difference amplifier configuration. For the

AD8278 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.

VIN+

VIN–

R1 + R2

(VIN+)

R1 + R2

(VIN+)

Figure 46. Voltage Division in the Difference Amplifier Configuration

The AD8278 has 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

from +VS − 40 V up to −VS + 40 V. For example, on ±10 V

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

taken to not exceed the +VS − 40 V to −VS + 40 V input limits

to avoid risking damage to the parts.

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