AD8227

Rev. 0 | Page 20 of 2

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REFERENCE TERMINAL

The output voltage of the AD8227 is developed with respect to

the potential on the reference terminal. This is useful when the

output signal needs to be offset to a precise midsupply level. For

example, a voltage source can be tied to the REF pin to level-

shift the output so that the AD8227 can drive a single-supply

ADC. The REF pin is protected with ESD diodes and should

For best performance, source impedance to the REF terminal

should be kept below 2 Ω. As shown in Figure 58, the reference

terminal, REF, is at one end of a 50 kΩ resistor. Additional imped-

ance at the REF terminal adds to this 50 kΩ resistor and results

in amplification of the signal connected to the positive input.

follows:

Only the positive signal path is amplified; the negative path

is unaffected. This uneven amplification degrades CMRR.

INCORRECT

V

CORRECT

AD8227

OP1177

–

+

V

REF

AD8227

REF

Figure 59. Driving the Reference Pin

INPUT VOLTAGE RANGE

Most instrumentation amplifiers have a very limited output

voltage swing when the common-mode voltage is near the

upper or lower limit of the part’s input range. The AD8227 has

very little of this limitation. See Figure 9 through Figure 16 for

the input common-mode range vs. output voltage of the part.

LAYOUT

To ensure optimum performance of the AD8227 at the PCB

level, care must be taken in the design of the board layout. The

pins of the AD8227 are arranged in a logical manner to aid in

this task.

8

7

6

1

2

3

5

4

–IN

REF

+IN

TOP VIEW

(Not to Scale)

AD8227

Figure 60. Pinout Diagram

Common-Mode Rejection Ratio over Frequency

Poor layout can cause some of the common-mode signals to be

converted to differential signals before reaching the in-amp.

Such conversions occur when one input path has a frequency

response that is different from the other. To keep CMRR over

frequency high, the input source impedance and capacitance of

each path should be closely matched. Additional source resis-

tance in the input path (for example, for input protection) should

be placed close to the in-amp inputs, which minimizes the

interaction of the source resistance with parasitic capacitance

from the PCB traces.

Parasitic capacitance at the gain setting pins can also affect CMRR

over frequency. If the board design has a component at the gain

setting pins (for example, a switch or jumper), the component

should be chosen so that the parasitic capacitance is as small as

possible.

Power Supplies

A stable dc voltage should be used to power the instrumentation

amplifier. Noise on the supply pins can adversely affect perfor-

mance. See the PSRR performance curves in Figure 23 and

Figure 24 for more information.

A 0.1 μF capacitor should be placed as close as possible to each

supply pin. As shown in Figure 61, a 10 μF tantalum capacitor

can be used farther away from the part. In most cases, it can be

shared by other precision integrated circuits.

AD8227

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

Figure 61. Supply Decoupling, REF, and Output Referred to Local Ground

References

The output voltage of the AD8227 is developed with respect to

the potential on the reference terminal. Care should be taken to

tie REF to the appropriate local ground.