AD8235 datasheet(17/20 Pages) AD

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AD8235

Rev. 0 | Page 17 of 20

fHIGH-PASS = 2πRC

TRANSFORMER

AC-COUPLED

REF

+VS

–VS

AD8235

REF

–VS

+VS

AD8235

TRANSFORMER

AC-COUPLED

REF

+VS

–VS

AD8235

REF

–VS

+VS

AD8235

Figure 40. Creating an IBIAS Path

INPUT BIAS CURRENT RETURN PATH

The AD8235 input bias current is extremely small at less than

50 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 40).

INPUT PROTECTION

All terminals of the AD8235 are protected against ESD. 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 AD8235 safely handles a continuous 6 mA

current at room temperature.

For applications where the AD8235 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.

RF INTERFERENCE

RF rectification is often a problem in applications where there are

large RF signals. The problem appears as a small dc offset voltage.

The AD8235, by its nature, has a 3.1 pF gate capacitance, CG, at

each input. Matched series resistors form a natural low-pass filter

that reduces rectification at high frequency (see Figure 41). The

relationship between external, matched series resistors and the

internal gate capacitance is expressed as

DIFF

FilterFreq

AD8235

VOUT

–VS

REF

–VS

+IN

–IN

0.1µF

10µF

0.1µF

10µF

SDN

Figure 41. RFI Filtering Without External Capacitors

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