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Why are there resistors in an in-amp circuit?
I stumbled upon a in-amp circuit for current measurement that had similar, mysterious input resistors (1.3k on both inputs). Apparently the rationale behind the resistors is to limit fault currents in case the CM goes beyond rails, e.g. when disconnecting a sensor with long leads.
Why does an input resistor have zero voltage?
The inverter’s gain behavior, due to the principles of infinite op amp gain, zero input offset, and zero bias current, gives rise to an effective node of zero voltage at the (−) input. The input and feedback currents sum at this point, which logically results in the term summing point.
Why are shunt resistors used in Fet opamps?
Especially with very high input impedances like FET opamps there seems to be no need for them. This may be a mistake in the circuit diagram. Possibly, the intent was that the 100K resistors be shunt resistors to the input, rather than in series. Shunt resistors would serve the purpose of lowering the input impedance to 100K.
How is the gain of an input resistor insensitive?
As with the follower gain stage, the gain is ratio dependent, and is relatively insensitive to the exact R F and R G values. The inverter’s gain behavior, due to the principles of infinite op amp gain, zero input offset, and zero bias current, gives rise to an effective node of zero voltage at the (−) input.
What is the purpose of a resistor on the non inverting terminal?
You could cancel out the common current drawn by both inputs by driving them with the same impedance, leaving the additional voltage offset due to the lower offset current. In your example, R3 is there to present the same impedance to the + input as is driving the – input, which is R1//R2.
What is the purpose of a shunt resistor?
Shunt resistors would serve the purpose of lowering the input impedance to 100K. (Astronomical input impedance is not always desirable: for one thing, it is susceptible to noise.) The second purpose would be to provide a DC return if there is a coupling capacitor just before the input.
What is the purpose of the R1 resistor?
R1 is there to counter-act any small DC offset since the integral of any DC offset tends to infinity. These offsets can come from source errors (your source may not be exactly a sign wave) or they can be intrinsic to the Op-Amp (known as input offset voltage).