AD8236ARMZ-R71 datasheet(17/21 Pages) AD

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AD8236

Rev. 0 | Page 16 of 20

fHIGH-PASS = 2πRC

TRANSFORMER

AC-COUPLED

REF

–VS

+VS

AD8236

REF

–VS

+VS

AD8236

TRANSFORMER

AC-COUPLED

REF

–VS

+VS

AD8236

REF

–VS

+VS

AD8236

Figure 38. Creating an IBIAS Path

INPUT BIAS CURRENT RETURN PATH

The AD8236 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 38).

INPUT PROTECTION

All terminals of the AD8236 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 AD8236 safely

handles a continuous 6 mA current at room temperature.

For applications where the AD8236 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 AD8236, 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 39). The

relationship between external, matched series resistors and the

internal gate capacitance is expressed as

DIFF

FilterFreq

AD8236

VOUT

REF

–VS

+VS

–VS

+IN

–IN

0.1µF

10µF

0.1µF

10µF

Figure 39. RFI Filtering Without External Capacitors

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