AD8228

Rev. 0 | Page 17 of 24

REFERENCE TERMINAL

The output voltage of the AD8228 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 AD8228 can drive a single-supply ADC. The

REF pin is protected with ESD diodes and should not exceed

For best performance, source impedance to the REF terminal

should be kept below 1 Ω. As shown in Figure 41, the reference

terminal, REF, is at one end of a 10 kΩ resistor. Additional imped-

ance at the REF terminal adds to this 10 kΩ resistor and results

in amplification of the signal connected to the positive input.

(

)

REF

REF

R

R

+

+

×

kΩ

20

kΩ

10

2

Only the positive signal path is amplified; the negative path is

unaffected. This uneven amplification degrades the CMRR of

the amplifier.

INCORRECT

V

CORRECT

AD8228

OP1177

+

–

V

REF

AD8228

REF

Figure 43. Driving the Reference

LAYOUT

The AD8228 is a high precision device. To ensure optimum

performance at the PCB level, care must be taken in the design

of the board layout. The AD8228 pins are arranged in a logical

manner to aid in this task.

8

7

6

5

1

2

3

4

–IN

G1

G2

REF

+IN

TOP VIEW

(Not to Scale)

AD8228

Figure 44. Pinout Diagram

Common-Mode Rejection Ratio over Frequency

The AD8228 has a higher CMRR over frequency than typical

in-amps, which gives it greater immunity to disturbances such

as line noise and its associated harmonics. The AD8228 pinout

was designed so that the board designer can take full advantage

of this performance with a well-implemented layout.

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

converted to a differential signal before it reaches the in-amp.

Such conversions occur when one input path has a frequency

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

frequency high, input source impedance and capacitance of each

path should be closely matched. Additional source resistance in

the input path (for example, for input protection) should be placed

close to the in-amp inputs, which minimizes their interaction

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 part

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 perform-

ance. See the PSRR performance curves in Figure 17 and Figure 18

for more information.

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

supply pin. As shown in Figure 45, 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.

AD8228

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

Figure 45. Supply Decoupling, REF, and Output Referred to Local Ground