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# Measure higher Impedance with higher frequency

Category: Datasheet/Specs

Hi to everyone.
I have two questions.
I am using AD 5933 evaluation board to measure the impedance of my design
1).   I want to measure the impedance of my design which is around 26MΩ. When I calibrated the AD 5933 by using a 15MΩ resistor, I measured the impedance of a pure resistor of 17.78 MΩ. Although this is beyond the range of this device ( Mx limit 10MΩ)
I don't have any resistor of rating 25MΩ to calibrate the device, Can anybody tell me, if I calibrate the device with a 25MΩ resistor and then measure the impedance of the device that is around 26MΩ without swiping the frequency, as frequency sweeping gave the wrong results as compared to the constant frequency. usage. Will I get the correct result?

2) Can I use the constant frequency of around 1MHz for my impedance measurement ( accurate result limit: 100KHz). I will not sweep the frequency, but use the constant value, would it give the correct results or not?
Kindly experienced person guide me.
Best Regards

• 1. In addition to using an adequate calibration resistor, it is important that the RFB resistor is in the same order of magnitude. If you use the 15 Mohm resistor for RFB, the signal at the ADC would be 60% of the full scale theoretically. You will still need another 15 Mohm resistor for calibration. However parasitic capacitance can spoil the measurement.

2. The maximum frequency of the AD5933 is 100 kHz, not 1 MHz. For higher frequency and accuracy, refer to EVAL-ADMX2001.

• You are right Sir
1)   It means I can measure the Z even more than 25MΩ  If calibrate the device correctly ( e,.g could measure 26MHΩ  if I used 25MHz RFB resistor  and  25MHz Z resistor for calibration, and after calibration, I can achieve the desired result)

2) I can experiment with low frequency, it's fine for me
I have one more question, as I got the results after the experiment, I came to know that sweeping frequency disturbs the results as compared to the use of constant frequency, am I right and my analysis is correct?
e.g. I measure the impedance of a 12MΩ resistor in two ways
a) sweep the frequency from 1KHz to 100KHz.
b) usage of constant frequency, suppose I use 10KHz independently, 50KHz independently,  and 100KHz independently,
the result I got is better in the case of using independent frequency.
Can you tell me, my result and analysis are correct or not? and the device responds better in the case of using independent frequency as compared to sweeping frequency.

• You are right Sir
1)   It means I can measure the Z even more than 25MΩ  If calibrate the device correctly ( e,.g could measure 26MHΩ  if I used a 25MHz RFB resistor  and  25MHz Z resistor for calibration, and after calibration, I can achieve the desired result)

2) I can experiment with low frequency, it's fine for me
I have one more question, as I got the results after the experiment, I came to know that sweeping frequency disturbs the results as compared to the use of constant frequency, am I right and my analysis is correct?
e.g. I measure the impedance of a 12MΩ resistor in two ways
a) sweep the frequency from 1KHz to 100KHz.
b) usage of constant frequency, suppose I use 10KHz independently, 50KHz independently,  and 100KHz independently,
the result I got is better in the case of using independent frequency.
Can you tell me, my result and analysis are correct or not? and the device responds better in the case of using independent frequency as compared to sweeping frequency.