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Measure higher Impedance with higher frequency

Category: Datasheet/Specs
Product Number: AD 5933

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.
Thanks in advance
Best Regards

Thread Notes

  • 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.
    Will wait of your reply

  • 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.
    Will wait for your reply

  • 1. It will be challenging to measure 26 Mohm. You should expect a phase shift due to parasitic capacitance. If you are testing resistors, you may get away with the module. For reactive impedances, the phase error will be high.

    2. The GUI has two calibration modes: single point or multi-point. Multi-point calibration is more appropriate when the measurement is affected by parasitic impedance. This calibration performs a frequency sweep and obtains a different calibration coefficient for every frequency.