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During the design of an ECG using an instrumentation and a differential amplifier, we can across this question:

  • Comparing the one and three op-amp designs from a safety point of view: of the two designs, which provides a current path from the electrodes to the ground (and therefore are potentially dangerous)?

I know that is something related that in bioelectronics, current is dangerous. Especially, because nerves are current sources due to an exchange of ions, so allowing current to flow to ground could be dangerous without protection.

However, I am not sure how this is related 1 and 3 op-amp designs. Some of this topic was discussed in the following link (but nothing regarding safety): https://electronics.stackexchange.com/questions/91499/three-opamp-instrumentation-amplifier-vs-single-opamp-implementation#=

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  • \$\begingroup\$ There is almost always a 1M-10M resistor from patient body to INA input. Additionally, in a proper commercially available ECG, the (5V) power supply to the INA and digitizer (and RLD) is isolated (typically 1cm air gap between primary and secondary windings, both windings at around 5V). \$\endgroup\$ – Indraneel Nov 8 '18 at 4:56
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I am not sure of what topology you are referring to or if this is an “ideal” type of situation, but I believe this might be irrelevant.

In reality, both topologies are equivalent from a safety point of view. You have to explicitly introduce isolation barriers when it comes to medical applications.

Although op-amp and instrumentation amps have very high input impedances, these impedances are only valid if maximum ratings are not exceeded. You will always have a conduction path through their protection circuitry.

It might be possible to find instrumentation amplifiers that can tolerate relatively high common-mode voltages above their supply rails, but that just means higher maximum ratings.

In medical applications you have to design for the possibility of circuit failure (and in some cases the presence of defibrillation voltages), this includes a worst-case failure of the amplifier itself.

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  • \$\begingroup\$ Inverting amps are not necessarily high impedance \$\endgroup\$ – Scott Seidman Nov 8 '18 at 1:09
  • \$\begingroup\$ @ScottSeidman true. But from the point of view of this question these would actually be safer. As the presence of a large enough external resistor would count as an additional mean of patient protection. \$\endgroup\$ – Edgar Brown Nov 8 '18 at 1:13
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    \$\begingroup\$ As would a resistor on the input of an inamp, which is standard practice. \$\endgroup\$ – Scott Seidman Nov 8 '18 at 1:18
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Note that a 3-amp instrumentation amp will have the inputs connected directly to the op amps. What would happen if one of the op amps failed short to an internal ground connection? Would the same thing happen to a 1-amp diff amp?

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