AD8271

Rev. 0 | Page 15 of 20

THEORY OF OPERATION

1

7

6

10k

Ω

AD8271

10k

Ω

10k

Ω

10k

Ω

20k

Ω

20k

Ω

10k

Ω

P4

P3

P2

P1

OUT

N1

N2

N3

10

9

8

2

3

4

5

Figure 43. Functional Block Diagram

CIRCUIT INFORMATION

The AD8271 consists of a high precision, low distortion op amp

and seven trimmed resistors. These resistors can be connected

to create a wide variety of amplifier configurations, including

difference, noninverting, and inverting configurations. The

resistors on the chip can be connected in parallel for a wider range

of options. Using the on-chip resistors of the AD8271 provides

the designer with several advantages over a discrete design.

DC Performance

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

accuracy of the surrounding resistors. The resistors on the AD8271

are laid out to be tightly matched. The resistors of each part are

laser trimmed and tested for their matching accuracy. Because

of this trimming and testing, the AD8271 can guarantee high

accuracy for specifications, such as gain drift, common-mode

rejection, and gain error.

AC Performance

Because feature size is much smaller in an integrated circuit than

on a printed circuit board (PCB), the corresponding parasitics are

also smaller. The smaller feature size helps the ac performance of

the AD8271. For example, the positive and negative input terminals

of the AD8271 op amp are not pinned out intentionally. By not

connecting these nodes to the traces on the PCB, the capacitance

remains low, resulting in both improved loop stability and

common-mode rejection over frequency.

Production Costs

Because one part, rather than several discrete components, is

placed on the PCB, the board can be built more quickly and

efficiently.

Size

The AD8271 fits an op amp and seven resistors in one MSOP

package.

DRIVING THE AD8271

The AD8271 is easy to drive, with all configurations presenting

at least several kilohms (kΩ) of input resistance. The AD8271

should be driven with a low impedance source: for example,

another amplifier. The gain accuracy and common-mode rejection

of the AD8271 depend on the matching of its resistors. Even

source resistance of a few ohms can have a substantial effect on

these specifications.

POWER SUPPLIES

A stable dc voltage should be used to power the AD8271. Noise

on the supply pins can adversely affect performance. A bypass

capacitor of 0.1 μF should be placed between each supply pin

and ground, as close as possible to each supply pin. A tantalum

capacitor of 10 μF should also be used between each supply and

ground. It can be farther away from the supply pins and, typically,

it can be shared by other precision integrated circuits.

The AD8271 is specified at ±15 V and ±5 V, but it can be used with

The difference between the two supplies must be kept below 36 V.

INPUT VOLTAGE RANGE

The AD8271 has a true rail-to-rail input range for the majority

of applications. Because most AD8271 configurations divide down

the voltage before they reach the internal op amp, the op amp sees

only a fraction of the input voltage. Figure 44 shows an example

of how the voltage division works in the difference amplifier

configuration.

R4

R3

R1

R2

R2

R1 + R2

R2

R1 + R2

Figure 44. Voltage Division in the Difference Amplifier Configuration

The internal op amp voltage range may be relevant in the

following applications, and calculating the voltage at the

internal op amp is advised.

•

Difference amplifier configurations using supply voltages

of less than ±4.5 V

•

Difference amplifier configurations with a reference

voltage near the rail

•

Single-ended amplifier configurations

For correct operation, the input voltages at the internal op amp

must stay within 1.5 V of either supply rail.

Voltages beyond the supply rails should not be applied to the

part. The part contains ESD diodes at the input pins, which

conduct if voltages beyond the rails are applied. Currents greater

than 5 mA may damage these diodes and the part. For a similar

part that can operate with voltages beyond the rails, see the

AD8274 data sheet.