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In biopotential amplifiers like ECG amplifier, I often see protection resistors (around 100kohm) between electrode and the input of instrumentation amplifier or ECG amplifiers like ADS1298. I understand that the resistor are supposed to limit the current flowing into the patient. But the input impedance of the amplifiers are already in Gohm range. How does adding the resistors make difference?
Here is an example:

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Note that current flowing in via Rx3 might find another path to return than the electrodes connected via Rx1 and Rx2. In that case, the input impedance of the AD620A is ignored and what matters is the output impedance of the OP97 which is (I assume) very low, and Rx3 sets a minimum for it. –  RBerteig Jul 31 at 18:21

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up vote 11 down vote accepted

The input impedance is high only when the circuit is operating correctly. If a fault occurs inside the equipment, you can no longer say for sure that that assumption is still true. By explicitly placing resistors in series with the leads, you can enforce a lower bound on the input impedance, and you can easily analyze the limited set of faults that might bypass those resistors.

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IEC60601 states that in a single fault condition, patient current must be limited to 50uA. –  Matt Young Jul 31 at 15:19
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In the circuit as shown the single failure of a short in Rx3 would probably exceed that current. For that reason such resistors are often composed of two resistors in series. –  Wouter van Ooijen Jul 31 at 15:28
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+1. You might also mention which particular faults would cause a good instrumentation amp to leak a lot of current through its normally high-impedance input pins: ones that "turn on" the internal protection diodes. –  davidcary Jul 31 at 15:50
    
+1 Exactly. Protection is required as close as possible to the patient to reduce the chance of injury for as wide a range of faults (and misconfigurations) as possible. Medical equipment has many lines of defense. Intraoperative software has a similar philosophy; failure analysis and mitigation is a job for an entire department. –  Jason C Jul 31 at 18:33
    
@davidcary: Indeed. Even something as simple as switching off the power could cause those diodes to start conducting. –  Dave Tweed Jul 31 at 18:44

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