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AD5933- Calculating Calibration Resistance Values


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? 

Thank you for your help.