AD8237ARMZ-R7 datasheet(24/29 Pages) AD

24 / 29 page

Data Sheet

AD8237

Rev. 0 | Page 23 of 28

Resistance at the reference pin does affect the gain of the AD8237;

however, if this resistance is constant, the gain setting resistors can be

adjusted to compensate. For example, the AD8237 can be driven

with a voltage divider, as shown in Figure 72.

AD8237

+IN

–IN

REF

VOUT

G = 1 +

R2 + R3

||R4

Figure 72. Using Voltage Divider to Set Reference Voltage

LAYOUT

Common-Mode Rejection Ratio over Frequency

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

converted to a differential signal before reaching the in-amp. This

conversion can occur when the path to the positive input pin has

a different frequency response than the path to the negative input

pin. For best CMRR vs. frequency performance, closely match the

impedance of each path. Place additional source resistance in the

input path (for example, for input protection) close to the in-amp

inputs to minimize interaction between the resistors and the parasitic

capacitance from the printed circuit board (PCB) traces.

Power Supplies

Use a stable dc voltage to power the instrumentation amplifier.

Noise on the supply pins can adversely affect performance. For

more information, see the PSRR performance curves in Figure 17

through Figure 20.

Place a 0.1 µF capacitor as close as possible to each supply pin.

As shown in Figure 73, a 10 µF tantalum capacitor can be used farther

away from the part. This capacitor, which is intended to be effective at

low frequencies, can usually be shared by other precision integrated

circuits. Keep the traces between these integrated circuits short to

minimize interaction of the trace parasitic inductance with the shared

capacitor. If a single supply is used, decoupling capacitors at −VS

can be omitted.

AD8237

+VS

+IN

–IN

0.1µF

10µF

0.1µF

10µF

–VS

VOUT

REF

Figure 73. Supply Decoupling, REF, and Output Referred to Local Ground

Reference

The output voltage of the AD8237 is developed with respect to

the potential on the reference terminal. Take care to tie REF to

the appropriate local ground.

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8237 must have a return path to

ground. When the source, such as a thermocouple, cannot provide

a return current path, create one, as shown in Figure 74.

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