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AD5933 feedback resistance changed to capacitance?

Question asked by richeekdey on Dec 18, 2017
Latest reply on Dec 18, 2017 by Snorlax

Is it possible to have a capacitor in place of RFB in the AD5933 evaluation board setup?

I know that RFB is part of a current to voltage amplifier. RFB must be selected to keep the signal below VDD. Replacing it by a capacitor will change the function of the amplifier, so there is no assurance that the circuit will work as expected. This will also remove the DC part of the signal, and the IC probably needs it since it specify a maximum for the measurable impedance.

However,  the only thing the chip’s input stage does, is measure a voltage.

The issue (actually shortcoming) is that the generator has a variable DC, depending on the amplitude range.  Replacing the Rfb with a cap, and knowing that the load is also capacitive, the DC issue completely drops away and the only thing that counts is the amplification (range) depending on the Zcapfb, which itself depends obviously on the f.

 

Only to convert the reading in impedance there is the issue that the chip is callibrated for a resistive Rfb.

However, again introducing the f, this can be solved in the aposteriori calcs.

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