AD8270

Rev. 0 | Page 13 of 20

THEORY OF OPERATION

1

–IN1A

2

–IN2A

3

+IN2A

4

+IN1A

11 –IN2B

12 –IN1B

10 +IN2B

9+IN1B

AD8270

_

+

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

20kΩ

20kΩ

20kΩ

20kΩ

_

+

Figure 40. Functional Block Diagram

CIRCUIT INFORMATION

The AD8270 has two channels, each consisting of a high precision,

low distortion op amp and seven trimmed resistors. These resis-

tors can be connected to make a wide variety of amplifier

configurations: difference, noninverting, inverting, and more.

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

range of options. Using the on-chip resistors of the AD8270

provides the designer 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 AD8270

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 AD8270 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 PCB board, the corresponding parasitics are smaller, as well.

The smaller feature size helps the ac performance of the AD8270.

For example, the positive and negative input terminals of the

AD8270 op amp are not pinned out intentionally. By not

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

capacitance remains low, resulting in both improved loop

stability and common-mode rejection over frequency.

Production Costs

Because one part, rather than several, is placed on the PCB

board, the board can be built more quickly.

Size

The AD8270 fits two op amps and 14 resistors in a 4 mm ×

4 mm package.

DRIVING THE AD8270

The AD8270 is easy to drive, with all configurations presenting

at least several kilohms (kΩ) of input resistance. The AD8270

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

another amplifier. The gain accuracy and common-mode rejection

of the AD8270 depend on the matching of its resistors. Even

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

these specifications.

PACKAGE CONSIDERATIONS

The AD8270 is packaged in a 4 mm × 4 mm LFCSP. Beware of

blindly copying the footprint from another 4 mm × 4 mm LFCSP

part; it may not have the same thermal pad size and leads. Refer

to the Outline Dimensions section to verify that the PCB symbol

has the correct dimensions.

The 4 mm × 4 mm LFCSP of the AD8270 comes with a thermal

pad is not necessary for electrical performance; the pad can be

left unconnected or can be connected to the negative supply rail.

Connecting the pad to the negative supply rail is recommended

in high vibration applications or when good heat dissipation is

required (for example, with high ambient temperatures or when

driving heavy loads). For best heat dissipation performance, the

negative supply rail should be a plane in the board. See the

Absolute Maximum Ratings section for thermal coefficients

with and without the pad soldered.

Space between the leads and thermal pad should be as wide as

possible to minimize the risk of contaminants affecting perform-

ance. A thorough washing of the board is recommended after the

soldering process, especially if high accuracy performance is

required at high temperatures.

POWER SUPPLIES

A stable dc voltage should be used to power the AD8270. 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 AD8270 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.