AD8220ARMZ-R7 datasheet(21/28 Pages) AD

21 / 28 page

AD8220

Rev. 0 | Page 21 of 28

REFERENCE TERMINAL

The reference terminal, REF, is at one end of a 20 kΩ resistor

(see Figure 55). 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 voltages other than

common. For example, a voltage source can be tied to the REF

pin to level-shift the output so that the AD8220 can interface

with an ADC. The allowable reference voltage range is a

function of the gain, common-mode input, and supply voltages.

The REF pin should not exceed either +VS or −VS by more than

0.5 V.

For best performance, especially in cases where the output is

not measured with respect to the REF terminal, 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

The AD8220 has high PSRR. However, for optimal

performance, a stable dc voltage should be used to power the

instrumentation amplifier. Noise on the supply pins can

adversely affect performance. As in all linear circuits, bypass

capacitors must be used to decouple the amplifier.

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

10 µF tantalum capacitor can be used further away from the

part (see Figure 59). In most cases, it can be shared by other

precision integrated circuits.

AD8220

+VS

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

–VS

VOUT

Figure 59. Supply Decoupling, REF and Output Referred to Ground

INPUT BIAS CURRENT RETURN PATH

The AD8220 input bias current is extremely small at less than

10 pA. Nonetheless, the input bias current must have a return

path to common. When the source, such as a transformer,

cannot provide a return current path, one should be created

(see Figure 60).

+VS

REF

TRANSFORMER

–VS

AD8220

AC-COUPLED

+VS

REF

–VS

AD8220

fHIGH-PASS = 2πRC

Figure 60. Creating an IBIAS Path

INPUT PROTECTION

All terminals of the AD8220 are protected against ESD.4 In

addition, the input structure allows for dc overload conditions a

diode drop above the positive supply and a diode drop below

the negative supply. Voltages beyond a diode drop of the

supplies cause the ESD diodes to conduct and enable current to

flow through the diode. Therefore, an external resistor should

be used in series with each of the inputs to limit current for

voltages above +Vs. In either scenario, the AD8220 safely

handles a continuous 6 mA current at room temperature.

For applications where the AD8220 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.

4 ESD protection is guaranteed to 4 kV (human body model).

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