AD8553 datasheet(24/28 Pages) AD

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AD8426

Rev. 0 | Page 24 of 28

INPUT BIAS CURRENT RETURN PATH

The other AD8426 terminals should be kept within the supplies.

All terminals of the AD8426 are protected against ESD.

The input bias current of the AD8426 must have a return path

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

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

in Figure 66.

For applications where the AD8426 encounters voltages beyond

the allowed limits, external current limiting resistors and low

leakage diode clamps such as the BAV199L, the FJH1100, or the

SP720 should be used.

THERMOCOUPLE

+VS

REF

–VS

AD8426

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8426

TRANSFORMER

+VS

REF

–VS

AD8426

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8426

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10MΩ

AD8426

TRANSFORMER

+VS

REF

–VS

AD8426

CORRECT

RADIO FREQUENCY INTERFERENCE (RFI)

RF interference is often a problem when amplifiers are used in

applications where there are strong RF signals. The precision

circuits in the AD8426 can rectify the RF signals so that they

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

place a low-pass RC filter at the input of the instrumentation

amplifier (see Figure 67). The filter limits both the differential

and common-mode bandwidth, as shown in the following

equations:

)

(

DIFF

uency

FilterFreq

uency

FilterFreq

where CD ≥ 10 CC.

AD8426

+VS

+IN

–IN

0.1µF

10µF

0.1µF

REF

OUT

–VS

10nF

1nF

4.02kΩ

Figure 67. RFI Suppression

Figure 66. Creating an Input Bias Current Return Path

CD affects the differential signal, and CC affects the common-

mode signal. Values of R and CC should be chosen to minimize

RFI. Any mismatch between the R × CC at the positive input

and the R × CC at the negative input degrades the CMRR of the

AD8426. By using a value of CD one order of magnitude larger

than CC, the effect of the mismatch is reduced, and performance

is improved.

INPUT PROTECTION

The AD8426 has very robust inputs and typically does not

need additional input protection. Input voltages can be up to

40 V from the opposite supply rail. For example, with a +5 V

positive supply and a −8 V negative supply, the part can safely

withstand voltages from −35 V to +32 V. Unlike some other

instrumentation amplifiers, the part can handle large differen-

tial input voltages even when the part is in high gain. Figure 16,

Figure 17, Figure 19, and Figure 20 show the behavior of the

part under overvoltage conditions.

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