AD8227ARMZ-R71 datasheet(22/25 Pages) AD

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AD8227

Rev. 0 | Page 21 of 2

INPUT BIAS CURRENT RETURN PATH

The other AD8227 terminals should be kept within the supplies.

All terminals of the AD8227 are protected against ESD.

The input bias current of the AD8227 must have a return path to

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

a return current path, one should be created, as shown in Figure 62.

For applications where the AD8227 encounters voltages beyond

the allowed limits, external current limiting resistors and low

leakage diode clamps such as the BAV199L, the FJH1100s, or

the SP720 should be used.

THERMOCOUPLE

+VS

REF

–VS

AD8227

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8227

TRANSFORMER

+VS

REF

–VS

AD8227

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8227

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10MΩ

AD8227

TRANSFORMER

+VS

REF

–VS

AD8227

CORRECT

RADIO FREQUENCY INTERFERENCE (RFI)

RF rectification is often a problem when amplifiers are used in

applications that have strong RF signals. The disturbance can

appear as a small dc offset voltage. High frequency signals can

be filtered with a low-pass RC network placed at the input of

the instrumentation amplifier, as shown in Figure 63. The filter

limits the input signal bandwidth, according to the following

relationship:

Figure 62. Creating an Input Bias Current Return Path

INPUT PROTECTION

The AD8227 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 differential input voltages

even when the part is in high gain. Figure 17 through Figure 20

show the behavior of the part under overvoltage conditions.

)

(

DIFF

uency

FilterFreq

uency

FilterFreq

where CD ≥ 10 CC.

AD8227

+VS

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–VS

10nF

1nF

4.02kΩ

Figure 63. RFI Suppression

CD affects the differential signal and CC affects the common-

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

RFI. A mismatch between R × CC at the positive input and

R × CC at the negative input degrades the CMRR of the AD8227.

By using a value of CD one magnitude larger than CC, the effect

of the mismatch is reduced, and performance is improved.

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