AD8270ACPZ-R7 datasheet(13/20 Pages) AD

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Data Sheet

AD8270

Rev. A | Page 13 of 20

THEORY OF OPERATION

AD8270

10kΩ

20kΩ

–IN1A

–IN2A

+IN2A

+IN1A

–IN1B

–IN2B

+IN2B

+IN1B

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 resistors 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 device 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 printed circuit board (PCB), 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, 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, 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

device; 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. This pad is connected internally to −VS. Connecting to this

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

unbalanced supplies, as well. For example, −VS = 0 V, +VS = 20 V.

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

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