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# AD5933- Calculating Calibration Resistance Values

Hello,

I am using the AD5933 Impedance Analyzer and the corresponding evaluation board EVAL-AD5933EBZ for my senior design project. We were able to follow the example measurement for a 15 pF capacitor and 200kOhm calibration resistor in AN-1053 Example Measurement document. However we are having some problems calibrating for other capacitor values:

1) How do we determine the appropriate calibration resistance for a capacitor in the nano- or micro- farad range? When we tried to take measurements for these capacitor values using a 200kOhm calibration resistor, we got ascending curves rather than expected capacitor behavior.

2) What is the usefulness of the RC parallel and RC series calibration impedance settings? The users guide seemed to indicate that only a resistor should be used for calibration because is not a complex impedance.

3) If we want to measure the impedance of an RC parallel or RC series circuit, how do we determine the appropriate calibration resistor for the set up based on the capacitor and resistor values we are testing?

4) Is there a known constant offset between the measured impedance values and the expected values determined by circuit theory? We saw some kind of offset in some of our measurements. If so, how do we account for this?

Parents
• 1) You calculate the equivalent impedance of your capacitor at the upper frequency of your frequency range, replace the RFB with the resistor of value reasonably close to this equivalent impedance and use the another resistor of the same value as the calibration resistor. With the capacitors in uF range at high frequencies one should be mindful of equivalent impedance to be too low to be easily measureable, so additional external circuits might be needed.

2) I am not affiliated with ADI, so cannot comment on the usefulness of the RC parallel and RC series calibration impedance settings. It does make sense to use only a resistor for calibration because, as you stated, it is not a complex impedance.

3) You calculate the equivalent impedance of your RC parallel or RC series circuit at the upper frequency of your frequency range, replace the RFB with the resistor of value reasonably close to this equivalent impedance and use the another resistor of the same value as the calibration resistor.

4) There supposed to be no offset between the measured impedance values and the expected values determined by circuit theory if calibration is done according to 1 and 3 and system phase is taken into account as described in the section MEASURING THE PHASE ACROSS AN IMPEDANCE, pp.19-21 of the datasheet.

• 1) You calculate the equivalent impedance of your capacitor at the upper frequency of your frequency range, replace the RFB with the resistor of value reasonably close to this equivalent impedance and use the another resistor of the same value as the calibration resistor. With the capacitors in uF range at high frequencies one should be mindful of equivalent impedance to be too low to be easily measureable, so additional external circuits might be needed.

2) I am not affiliated with ADI, so cannot comment on the usefulness of the RC parallel and RC series calibration impedance settings. It does make sense to use only a resistor for calibration because, as you stated, it is not a complex impedance.

3) You calculate the equivalent impedance of your RC parallel or RC series circuit at the upper frequency of your frequency range, replace the RFB with the resistor of value reasonably close to this equivalent impedance and use the another resistor of the same value as the calibration resistor.

4) There supposed to be no offset between the measured impedance values and the expected values determined by circuit theory if calibration is done according to 1 and 3 and system phase is taken into account as described in the section MEASURING THE PHASE ACROSS AN IMPEDANCE, pp.19-21 of the datasheet.

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