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Impedance Selection

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vinothe83

New Member
Hi,

I want to measure R,L,C using impedance measurement method.
Here i attached an sample circuit. The device under test is mentioned as unknown. It can be R or C or L. With this circuit, i can find the impedance(The input resistance of the inverting amp). Based on the input frequecy, i will calculate the L or C. The Op07 is not supporting the high frequency (like gain10 in 20Khz). So i am not able to measure the pF values. If i select high gain bandwidth ICs(like opa627), it is working only when i give resistance in the Unknown place. when connect C or L, it is giving noisy output.

How to choose an correct opamp for this application? or any filter circuit is required?
 

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MikeMl

Well-Known Member
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I dont understand how this circuit helps you determine what the unknown component is? Assuming that the opamp is working well at the test frequency, then the inverting input is at a virtual ground, so the impedance there is Zero! The current that flows in the selected feedback resistor is equal to the current that flows through the unknown component. All the circuit does is produce an output voltage which is proportional to the current?
 

vinothe83

New Member
Hi,
I am explaining my test configuration & problem below. Go through it and give your suggestions.

With this inverting amp circuit. I am giving AC input. based on the AC input, the reactance of the L or C will act as a input resistance(Rin). So that i will get a Vout.

The inverting amp Vout = (Rf/ Rin)*Vin.
So Rin = (Rf/Vout)*Vin.

If it is capacitance C= 1/(2*π*F(input frequency)*Rin(Xc)).
If it is Inductance L= (2*π*F(input frequency)*Rin(Xl)).

The procedure of the measurement was explained in the above. With this i measured the capacitance from the range of 100nF to 10mF. But when i want to measure below 100nF, The input signal frequency & Feedback resistance must be high (like 19.5KHz & 10KΩ).
The configuration will be
Input voltage = (100mVpp)70mVrms/19.5KHz
Input impedance = 815Ω (10nf capacitor as Rin, based on frequency The Rin calculated).
Feedback R = 10KΩ
Output voltage = (1.227Vpp)867mVrms (as per theory calculation)
But the real output is 2Vpp Triangle wave. It shoild be 1.227Vpp Sine.

By
Vinoth
 

MikeMl

Well-Known Member
Most Helpful Member
Haven't gotten to your question about opamps yet, but to do the math to figure out what the unknown is, you will have to consider the phase angle between the current in the opamp's feedback resistor and the applied voltage. The expression you derived for Rin works for resistors, but not for inductors or capacitors. To work for reactive unknowns, the math must involve phasors (complex number X+jY arithmetic).

You could do measurement of reactance, but your method will have to involve measuring both the magnitude of the voltage at the output of the opamp, as well as the phase angle of that voltage. A PIC with an ADC could do it.
 

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