AD8231ACPZ-R7 datasheet(19/24 Pages) AD

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Data Sheet

AD8231

Rev. E | Page 19 of 24

LAYOUT

The AD8231 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 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 auto-zero

architecture of the AD8231 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 cannot 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.

This pad is connected internally to −VS. The pad can either be

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 51.

THERMOCOUPLE

+VS

REF

–VS

AD8231

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8231

TRANSFORMER

+VS

REF

–VS

AD8231

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8231

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10MΩ

AD8231

TRANSFORMER

+VS

REF

–VS

AD8231

CORRECT

Figure 51. Creating an IBIAS Path

INPUT PROTECTION

All terminals of the AD8231 are protected against ESD. In

addition, the input structure allows for dc overload conditions

a diode drop above the positive supply and a diode drop below

the negative supply. Voltages beyond these limits cause the ESD

diodes to conduct and current to flow. If overvoltage events are

anticipated, an external resistor should be used in series with

each of the inputs to limit the current to below 10 mA. Currents

up to 100 mA can be sustained for a few seconds.

Note that if either input is brought below the negative supply

to the point where the ESD diode turns on, the AD8231 output

can phase-reverse.

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