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Hello,
In my circuit, Rf=2 kohm,Cf=1pF, R1=69ohm, Cs=57pF per cable and op amp is LM7171. Dual supply is used and voltage is 15V.
I measured the Vin, Vout and phase, so I know the gain for different frequencies from 0.2kHz to 10MHz. I want to calculate Rx and Cx. Based on the above equation, I can solve the equations in Matlab and find the Rx and Cx. ( I'm not so familiar with electrical things.)
My questions are,
1) Is the 'Gain' equation correct?
2) If the equation is correct, why am I getting negative values for Cx? What is the meaning or reason of negative value?
Thank you.
Burak Erdogan
Hello Burak,
To measure the impedance of your sensor, there are probably better ways to do this (using Impedance Analyzers and similar).
I'd terminate your "cable" on the left hand side of your sensor to avoid dealing with the cable capacitance to the left. You would also want to minimize Cs to the right (by bringing the sensor as close as possible to the LM7171).
I cannot read your hand written expression to say if it is valid or not. The mathematical expression, even if it is correct, would have many limitations / boundaries where it is valid as we approximate many things (e.g. inverting input being ground, etc.). So, any mathematical expression eventually breaks down if you extend its operation beyond valid assumptions.
Assuming you get rid of Cs (as mentioned above), here is what I get (assuming Zin >> 50ohm / 2):
RF= 2k
CF= 1pF
R1= 6.9ohm?
s= jw
Regards,
Hooman
