AD8295BCPZ-R7 datasheet(21/28 Pages) AD

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AD8295

Rev. 0 | Page 21 of 2

Power Supplies

A stable dc voltage should be used to power the instrumentation

amplifier. Noise on the supply pins can adversely affect perfor-

mance. See the PSRR performance curves in Figure 14 and

Figure 15 for more information.

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

supply pin. An additional capacitor, a 10 μF tantalum for the

lower frequencies, can be used farther away from the IC. In

most cases, the 10 μF bypass capacitor can be shared by other

integrated circuits on the same PCB.

AD8295

IN-AMP

+VS

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

–VS

VOUT

Figure 55. Supply Decoupling, REF, and Output Referred to Local Ground

INPUT PROTECTION

All terminals of the AD8295 are protected against ESD

by diodes at the inputs. If voltages beyond the supplies are

anticipated, resistors should be placed in series with the inputs

to limit the current. Resistors should be chosen so that current

does not exceed 6 mA into the internal ESD diodes in the over-

load condition. These resistors can be the same as those used

for RFI protection. (See the RF Interference section for more

information.)

For applications where the AD8295 encounters extreme

overload voltages, as in cardiac defibrillators, external series

resistors and low leakage diode clamps, such as BAV199Ls,

FJH1100s, or SP720s can be used.

INPUT BIAS CURRENT RETURN PATH

The input bias currents of the AD8295 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 56. Otherwise, the input currents charge up the input

capacitance until the in-amp is turned off or saturated.

THERMOCOUPLE

+VS

REF

–VS

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8295

IN-AMP

AD8295

IN-AMP

TRANSFORMER

+VS

REF

–VS

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8295

IN-AMP

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10MΩ

TRANSFORMER

+VS

REF

–VS

CORRECT

AD8295

IN-AMP

AD8295

IN-AMP

AD8295

IN-AMP

Figure 56. Creating an Input Bias Current Return Path

RF INTERFERENCE

RF interference is often a problem when amplifiers are used in

applications where there are strong RF signals. The precision

circuits in the AD8295 can rectify the RF signals so that they

appear as a dc offset voltage error. To avoid this rectification,

place a low-pass filter before the input. Figure 57 shows such a

network in front of the instrumentation amplifier. The filter

limits both the differential and common-mode bandwidth, as

shown in the following equations:

)

(

)

(

FILTER

Diff

FILTER

)

(

where CD ≥ 10CC.

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