AD8251ARMZ-R7 datasheet(19/25 Pages) AD

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AD8251

Rev. B | Page 18 of 24

POWER SUPPLY REGULATION AND BYPASSING

The AD8251 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 per-

formance. As in all linear circuits, bypass capacitors must be

used to decouple the amplifier.

Place a 0.1 μF capacitor close to each supply pin. A 10 μF tantalum

capacitor can be used farther away from the part (see Figure 55)

and, in most cases, it can be shared by other precision integrated

circuits.

AD8251

+VS

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

–VS

DGND

OUT

DGND

Figure 55. Supply Decoupling, REF, and Output Referred to Ground

INPUT BIAS CURRENT RETURN PATH

The AD8251 input bias current must have a return path to its

local analog ground. When the source, such as a thermocouple,

cannot provide a return current path, one should be created

(see Figure 56).

THERMOCOUPLE

+VS

REF

–VS

AD8251

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8251

TRANSFORMER

+VS

REF

–VS

AD8251

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8251

fHIGH-PASS =

πRC

THERMOCOUPLE

+VS

REF

–VS

10M

Ω

AD8251

TRANSFORMER

+VS

REF

–VS

AD8251

CORRECT

Figure 56. Creating an IBIAS Return Path

INPUT PROTECTION

All terminals of the AD8251 are protected against ESD. Note

that 2.2 kΩ series resistors precede the ESD diodes as shown in

Figure 51. The resistors limit current into the diodes and allow

for dc overload conditions 13 V above the positive supply and

13 V below the negative supply. An external resistor should be

used in series with each input to limit current for voltages

greater than 13 V beyond either supply rail. In either scenario,

the AD8251 safely handles a continuous 6 mA current at room

temperature. For applications where the AD8251 encounters

extreme overload voltages, external series resistors and low

leakage diode clamps, such as BAV199Ls, FJH1100s, or SP720s,

should be used.

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