Rfb Calculation

Hi,
I am working with AD5933 Impedance converter and related to this I want to know how to select the RFB values for Resistor,capacitor and Randle cell circuit.I have gone through the datasheet but I didn't get a clear understanding of selection of RFB value for any circuit.Thank you!

Kind Regards,
Shweta

  • The RFB resistor value has to be selected so that the voltage waveform at the RFB pin of the AD5933 is a well-formed sine wave throughout your calibration and measurement procedures within the frequency range you are scanning. If you have access to an oscilloscope it is worth occasionally checking that there is no distortion in this waveform.and no clipping at the power supply rails. A second criterion is to keep peak-to-peak RFB voltage at about 2V so that the dynamic range of the ADC is utilized reasonably well.

    In practice this means that if you select excitation voltage at 2V p-p and PGA gain of 1, you should select RFB as the lowest expected impedance of the measuring range you are trying to cover. For example, if you expect that your unknown impedance under test is within range of 100k to 10M you should select RFB = 100k and use calibration resistor of 100K or so as well. Then you can reasonably expect that the circuit will be able to measure any unknown impedance equal or higher than 100K.