AD8221ARM datasheet(15/20 Pages) AD

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AD8221

Rev. A | Page 15 of 20

REFERENCE TERMINAL

As shown in Figure 42, the reference terminal, REF, is at one end

of a 10 kΩ resistor. The instrumentation amplifier’s output 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, since parasitic resistance can adversely affect

CMRR and gain accuracy.

POWER SUPPLY REGULATION AND BYPASSING

A stable dc voltage should be used to power the instrumenta-

tion amplifier. Noise on the supply pins may 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 may be used

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

other precision integrated circuits.

Figure 46

AD8221

+VS

+IN

–IN

LOAD

REF

0.1

µF

0.1

µF

–VS

VOUT

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

INPUT BIAS CURRENT RETURN PATH

The AD8221’s input bias current 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 ESD1. 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 would flow 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.

1 1 kV—Human Body Model.

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