AD8235

Rev. 0 | Page 16 of 20

INCORRECT

Board Thickness

Typical board thicknesses used in the industry range from

0.4 mm to 1.6 mm and are most applicable for the AD8235. The

thickness selected depends on the required robustness of the

populated system assembly. The thinner board results in smaller

shear stress range, creep shear strain range, and creep strain

energy density range in the solder joints under the thermal

loading. Therefore, the thinner build-up board leads to longer

thermal fatigue life of solder joints [John H. Lau and S.W. Ricky

Grounding

The output voltage of the AD8235 is developed with respect to

the potential on the reference terminal, REF. To ensure the most

accurate output, the trace from the REF pin should either be

connected to the AD8235 local ground (see Figure 39) or

connected to a voltage that is referenced to the AD8235 local

ground (Figure 36).

REFERENCE TERMINAL

The reference terminal, REF, is at one end of a 210 kΩ resistor

(see Figure 35). The output of the instrumentation amplifier

is referenced to the voltage on the REF terminal; this is useful

when the output signal needs to be offset to voltages other than

common. For example, a voltage source can be tied to the REF

pin to level-shift the output so that the AD8235 can interface

with an ADC. The allowable reference voltage range is a function

of the gain, common-mode input, and supply voltages. The REF

For best performance, especially in cases where the output is not

measured with respect to the REF terminal, source impedance to

the REF terminal should be kept low because parasitic resistance

can adversely affect CMRR and gain accuracy. Figure 38

demonstrates how an op amp is configured to provide a low

source impedance to the REF terminal when a midscale

reference voltage is desired.

Thickness on the Solder Joint Reliability of a Wafer Level Chip Scale Package

(WLCSP),” IEEE Transactions on Components and Packaging Technologies,

Vol.25, No.1, March 2002, pages 3-14.

V

CORRECT

AD8235

REF

OP AMP

+

–

V

AD8235

REF

Figure 38. Driving the REF Pin

POWER SUPPLY REGULATION AND BYPASSING

The AD8235 has high power supply rejection ration (PSRR).

However, for optimal performance, a stable dc voltage should be

used to power the instrumentation amplifier. Noise on the supply

pins can adversely affect performance. As in all linear circuits,

bypass capacitors must be used to decouple the amplifier.

A 0.1 μF capacitor should be placed close to each supply pin.

A 10 μF tantalum capacitor can be used farther away from the

part (see Figure 39). In most cases, it can be shared by other

precision integrated circuits.

AD8235

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

SDN

Figure 39. Supply Decoupling, REF, and Output Referred to Ground