AD8274ARZ-R7 datasheet(12/16 Pages) AAVID

12 / 16 page

AD8274

Rev. C | Page 12 of 16

THEORY OF OPERATION

12kΩ

6kΩ

12kΩ

–VS

+VS

6kΩ

Figure 38. Functional Block Diagram

CIRCUIT INFORMATION

The AD8274 consists of a high precision, low distortion op amp

and four trimmed resistors. These resistors can be connected to

make a wide variety of amplifier configurations, including

difference, noninverting, and inverting configurations. Using

the on-chip resistors of the AD8274 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 AD8274

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 AD8274 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 AD8274. For example, the positive and negative input terminals

of the AD8274 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 AD8274 fits a precision op amp and four resistors in one

8-lead MSOP or SOIC package.

DRIVING THE AD8274

The AD8274 is easy to drive, with all configurations presenting

at least several kilohms (kΩ) of input resistance. The AD8274

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

another amplifier. The gain accuracy and common-mode rejection

of the AD8274 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 AD8274. 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 AD8274 is specified at ±15 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.

INPUT VOLTAGE RANGE

The AD8274 can measure voltages beyond the rails. For the G = ½

and G = 2 difference amplifier configurations, see the input voltage

range in Table 2 for specifications.

The AD8274 is able to measure beyond the rail because the

internal resistors divide down the voltage before it reaches the

internal op amp. Figure 39 shows an example of how the voltage

division works in the difference amplifier configuration. For the

AD8274 to measure correctly, the input voltages at the internal

op amp must stay within 1.5 V of either supply rail.

R1 + R2

(VIN+)

R1 + R2

(VIN+)

Figure 39. Voltage Division in the Difference Amplifier Configuration

For best long-term reliability of the part, voltages at any of the

part’s inputs (Pin 1, Pin 2, Pin 3, or Pin 5) should stay within

+VS – 40 V to −VS + 40 V. For example, on ±10 V supplies,

input voltages should not exceed ±30 V.

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