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Re: how to measure input impedance of an analog active circu
To achieve the input impedance,the variable resistance technique would work for small signals ,but it won't be very convenient for input impedance with imaginary parts (complex input impedance) .You can try another method by placing a small resistance (R1) in series with the input of your active circuit, then by measuring the voltage across on it on the CRO(V1) => Iin=V1/R1 and by measuring the input voltage at the active circuit (Vin) => Rin=Vin/Iin .If you have a dual channel CRO, you can get the relative phase between the current and the voltage and thus can get the complex input impedance .It should be noted that this method is valid also for small signals only and the input frequency should be at the frequency of interest as Zin is function of frequency .
As for the CMRR, you should measure the common mode gain of the circuit i.e applying same input at both inputs of the circuit and Acm=vout/vin ,then, apply a differential signal to get differential gain .The CMRR will be 20*log(Adiff/Acm) .
As usual I'd like to ask what frequency your measurement is concerned. Is it single ended or differential?
DC test is simple.
For high frequency, a modern 4-port network analyzer shall serve your need well if you are not testing DC and extremely high Zin. Convert the Sdd11 to Zdd11 and use CMRR=Sdd21/Sdc21 if your ckt uses four port differential signaling.
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