AD8231

Rev. 0 | Page 15 of 20

INCORRECT

AD8231

IN-AMP

CORRECT

AD8231

IN-AMP

AD8231

OP AMP

+

–

Figure 31. Driving the Reference Pin

LAYOUT

The AD8231 is a high precision device. To ensure optimum

performance at the PC board level, care must be taken in the

design of the board layout. The AD8231 pinout is arranged in

a logical manner to aid in this task.

Power Supplies

The AD8231 should be decoupled with a 0.1 μF bypass capacitor

between the two supplies. This capacitor should be placed as close

as possible to Pin 11 and Pin 12, either directly next to the pins or

beneath the pins on the backside of the board. The AD8231’s auto-

zero architecture requires a low ac impedance between the supplies.

Long trace lengths to the bypass capacitor increase this impedance,

which results in a larger input offset voltage.

A stable dc voltage should be used to power the instrumentation

amplifier. Noise on the supply pins can adversely affect

performance.

Package Considerations

The AD8231 comes 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. Space between the

leads and thermal pad should be kept as wide as possible for the

best bias current performance.

Thermal Pad

The AD8231 4 mm × 4 mm LFCSP comes with a thermal pad.

left unconnected or connected to the negative supply rail. For

high vibration applications, a landing is recommended.

Because the AD8231 dissipates little power, heat dissipation is

rarely an issue. If improved heat dissipation is desired (for

example, when ambient temperatures are near 125°C or when

driving heavy loads), connect the thermal pad to the negative

supply rail. For the best heat dissipation performance, the

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

Thermal Resistance section for thermal coefficients with and

without the pad soldered.

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8231 must have a return path

to common. When the source, such as a thermocouple, cannot

provide a return current path, one should be created, as shown

in Figure 32.

THERMOCOUPLE

REF

AD8231

CAPACITIVELY COUPLED

REF

C

C

AD8231

TRANSFORMER

REF

AD8231

INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

AD8231

1

THERMOCOUPLE

REF

10MΩ

AD8231

TRANSFORMER

REF

AD8231

CORRECT

RF INTERFERENCE

RF rectification is often a problem when amplifiers are used in

applications where there are strong RF signals. The disturbance

can appear as a small dc offset voltage. High frequency signals

can be filtered with a low-pass, RC network placed at the input

of the instrumentation amplifier, as shown in Figure 33. The

filter limits the input signal bandwidth according to the

following relationship:

)

(2

2

1

C

D

Diff

C

C

R

FilterFreq

+

π

=

C

CM

RC

FilterFreq

π

=

2

1