EVAL-AD5933 Basic Impedance Measurements

Hi,


I got today my EVAL-AD5933EBZ board and have some issues with the first measurements. I followed the instructions in the guide of the EVAL board, but getting some strange results. Maybe I misunderstood some parts in the guide.


After installing the software (downloaded from the internet), I checked the configuration of the jumpers. Following configuration is active on the board:


LK1 = open, LK2 = open, LK3 = closed, LK4 = open, LK5 = closed, LK6 = A


To perform the first frequency sweep of 30 kHz to 32 kHz across a 200 kOhm resistor, I put in the sockets of RFB (left of R2 and above R4) a 200 kOhm resistor as feedback resistor. Additionally I connected the "unknown" 200 kOhm resistor to the Z sockets left of the place for C2.


I configured the parameters in the software and programmed the device registers. Then I performed a gain factor calibration by pressing "Calculate Gain Factor" and got 5.11035456561638e-10, which is similar to the value of the screenshot in the manual.


When starting a sweep I got following impedance curve:


When now replacing the "unknown" 200 kOhm by a 100 kOhm resistor, I get following results. Yes, RFB is still 200 kOhm.

The results is nearly the half of the first measurement, which I expect. But in both measurement I got a wrong exponent. Did I do something wrong during calibration? Is it software issue with Windows 7?

Thanks in advance.

-Christoph

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  • You seem to do everything right, but with your settings setting you cannot measure resistors smaller than Rfb installed on the board, which is likely 200k. Your calibration is probably OK as your Rcal=Rfb. I would try measuring larger resistors, say, 220k, 250k, 300k, etc.and see how those values come out. 

  • Thanks for your help. I tried higher values (220k, 330k, 470k) and got the same issue. Results were 2.2e-005, 3.3e-005, 4.7e-005. I guess it's a bug of the plot, because when you remove the minus sign at the exponent you get the correct value (e.g. 2.2e005 = 220k).

    Whatsoever causes this issue...The absolute impedance plot does also not have the correct size compared to the screenshots in the manual.

    Edit: Okay I solved the issue. It was a localization issue. I was working under German localization settings (decimal separator is comma instead of a point) and this screwed up the data. I switched to English localization and this solved the issue.

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  • Thanks for your help. I tried higher values (220k, 330k, 470k) and got the same issue. Results were 2.2e-005, 3.3e-005, 4.7e-005. I guess it's a bug of the plot, because when you remove the minus sign at the exponent you get the correct value (e.g. 2.2e005 = 220k).

    Whatsoever causes this issue...The absolute impedance plot does also not have the correct size compared to the screenshots in the manual.

    Edit: Okay I solved the issue. It was a localization issue. I was working under German localization settings (decimal separator is comma instead of a point) and this screwed up the data. I switched to English localization and this solved the issue.

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