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I have a summing amplifier set up like so, and I need to come up with a test to find the input impedance, I know because its a 741 IC that it will be in the region of 250k - 10 mil ohms, but don't know how to test it and explain the test results

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Anything wrong with the age-old trick for measuring the input impedance of a "black box"? Create a voltage divider of a resistor and the input to be measured, selecting a type of resistor or variable resistor that will behave as a pure resistance at the frequency involved. One then substitutes resistors, or adjusts the variable resistor, until the signal amplitudes across the resistor and the input are the same. The DC resistance of the resistor should then be the same as the input's impedance at that frequency, no? (If a variable resistor is used, it would be removed from the circuit and measured with an ohmmeter.) Of course, this is a simplification. A simple calculation would avail with a ratio of amplitudes other than 1:1.

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