AD8221BRZ-R7 datasheet(19/24 Pages) AD

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AD8221

Rev. C | Page 19 of 24

REFERENCE TERMINAL

As shown in Figure 43, the reference terminal, REF, is at one

end of a 10 kΩ resistor. 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 a precise

midsupply level. For example, a voltage source can be tied to the

REF pin to level-shift the output so that the AD8221 can interface

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

of the gain, input, and supply voltage. The REF pin should not

exceed either +VS or –VS by more than 0.5 V.

For best performance, source impedance to the REF terminal

should be kept low, because parasitic resistance can adversely

affect CMRR and gain accuracy.

POWER SUPPLY REGULATION AND BYPASSING

A stable dc voltage should be used to power the instrumentation

amplifier. Noise on the supply pins can adversely affect

performance. Bypass capacitors should be used to decouple

the amplifier.

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

As shown in Figure 47, a 10 µF tantalum capacitor can be used

further away from the part. In most cases, it can be shared by

other precision integrated circuits.

AD8221

+VS

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

–VS

VOUT

Figure 47. Supply Decoupling, REF, and Output Referred to Local Ground

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8221 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 48.

+VS

REF

TRANSFORMER

THERMOCOUPLE

–VS

AD8221

+VS

REF

–VS

AD8221

CAPACITOR COUPLED

+VS

REF

–VS

AD8221

fHIGH-PASS =

πRC

Figure 48. Creating an IBIAS Path

INPUT PROTECTION

All terminals of the AD8221 are protected against ESD, 1 kV

Human Body Model. In addition, the input structure allows for

dc overload conditions below the negative supply, −VS. The

internal 400 Ω resistors limit current in the event of a negative

fault condition. However, in the case of a dc overload voltage

above the positive supply, +VS, a large current flows directly

through the ESD diode to the positive rail. Therefore, an external

resistor should be used in series with the input to limit current

for voltages above +Vs. In either scenario, the AD8221 can

safely handle a continuous 6 mA current, I = VIN/REXT for

positive overvoltage and I = VIN/(400 Ω + REXT) for negative

overvoltage.

For applications where the AD8221 encounters extreme

overload voltages, as in cardiac defibrillators, external series

resistors, and low leakage diode clamps, such as BAV199Ls,

FJH1100s, or SP720s should be used.

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