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Measure conductivity below 25us with EVAL-CN0349 (AD5934)

Question asked by Glanthor on Nov 17, 2017
Latest reply on Nov 26, 2017 by R.L

Dear All,

 

I have a problem measuring conductivity with my EVAL-CN0349 (AD5934) below 25us (impedances above 40kOhm).

My setup is the following:

EVAL-CN0349(AD5934) board. I wrote a custom software which is working well with the default calibration and feedback resisors in the range which is stated in the documentation of the eval board (up to 40kOhm). I did not try the demo software provided by you, and I have no possibilities for that right now.

I read the forum regarding this matter and found some discussions like the following:

 

https://ez.analog.com/thread/85549
https://ez.analog.com/thread/11767

 

After reading these threads, I changed some calibration and feedback resistors. I soldered a 200kOhm and a 2.2MOhm calibration and feedback resistor also. After this I tried to measure some "unknown" resistors like: 100kOhm, 220kOhm, 440kOhm, 1.5MOhm but the measurements were not accurate, but the order of magnitude were good. 

 

#I used the following parameters:

start frequency: 30kHz

external 16Mhz clock

pga gain: 1

frequency inrement: 1Hz

number of increments: 10

settling time: 50

 

I use gain factor calibration which is mentioned in the datasheet of the AD5934, but I have not implemented the multipoint calibration yet. I also dont use calibration for the frequency dependency, but I also dont use too big frequency increment. I tried to play with the parameters I mentioned above, but it did not increase accuracy. 

The only thing which is interesting that the gain factors (thus the magnitude) of the calibration resistors is not really changing during the measurements(which is good), but the results for the unknown impedances are not consistent(I mean if i run a measurement with the same parameters the results differentiate).

 

Later maybe I will provide some output data. Also if you need any more deatail I will try to provide that.

Now I know it is really hard to help in special cases like this, but please if you can give me some advice what should I check, implement nad try to be able to measure impedances in the 40kOhm - 2MOhm range with this evaluation board.

 

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