You got it! The input impedance can´t be the same! They are different. That´s what I´m saying all the time.So how can each single input impedance can be the same but the voltage drop and current waves are different?
For the lazy ones (sorry should not have said that) here a classic diff amp sim, which is what the OP seems to have intended.
So in closing we have a case where AIN+ Z = AIN- Z, and we have case(s) where its not.
Using different - unsuitable - test setups
while I refer to the general circuit, independent of Opamp. The OPAMp does not matter at all.
ignoring simulation results
Connect one input (AIN+) to GND and use an OhmMeter w.r.t. GND at the other input (AIN-)
Scared to show the results here? Why?
You're just making a fool of yourself with statements like that. I never said you could omit an OPAMP.OMG Klaus, I did not know these circuits one could omit entirely the active element. Wow
what a cost saving measure.....Please write Analog Devices and TI that OpAmps and their
finite G and poles etc do not affect impedances, as they have found in their ap notes. IEEE
as well. Oh and my sim shows OpAmp impact on Z's, known now for decades.
Now you try to move to wideband. Why?I do not use OhmMeters for wideband verification. I use a VNA.
Now you try to move to wideband. Why?
Everyone can see that you are always looking for new special cases to avoid the obvious: Individual: one 4k8, the other 6k.
One can not see "individual" results when one signal source is connected to both inputs.
One does not need to be a specialist, one does not need special knowledge, one does not need special equipment.
Every one can prove it.
The probe selections include "More Probe Functions" then "Impedance", one clicks on@danadakk
I have a small question related to post #44 plots. Can you show how the measurements "Impedance @ R1" and "Impedance @ R3" are defined in SIMetrix? With the shown floating source configuration, voltages at AIN+ and AIN- have different magnitude (referenced to ground) due to asymmetrical input, as explained by KlaussST. I'd expect to see this in an impedance measurement.
Here is the common mode case (as opposed to my earlier diff sims) :@danadakk
I have a small question related to post #44 plots. Can you show how the measurements "Impedance @ R1" and "Impedance @ R3" are defined in SIMetrix? With the shown floating source configuration, voltages at AIN+ and AIN- have different magnitude (referenced to ground) due to asymmetrical input, as explained by KlaussST. I'd expect to see this in an impedance measurement.
It´s the 4kOhms Zin at AIN- of the first simulation.You say something is unexpected in the simulation results. What is it?
That still contradicts theory. Since the negative feedback changes the current with voltage to null the differential input according to the gain and signal level, but the input impedances are identical.You got it! The input impedance can´t be the same! They are different. That´s what I´m saying all the time.
Klaus
Yes but you still do not. both the voltage and currents are different but the input impedance is constant and only equal to the series input R.You got it! The input impedance can´t be the same! They are different. That´s what I´m saying all the time.
Klaus
both the voltage and currents are different but the input impedance is constant and only equal to the series input R.
You can't calculate impedance in frequency domain for a superposition of orthogonal signal components. It can be only calculated for each component separately.Now how does the tool treat this mixture signal when calcuating impedance. The applied voltage is 10kHz only, but the current is 10kHz and 1kHz.
Apparently we are having different definitions of amplifier input impedance. For me it's Voltage at input terminal/Current into input terminal. Doesn't matter where the current flows.The 2nd and 3rd plots do not compute input impedance since the feedback current is shared giving a false sense of V/I(R1)= Zin..
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