How do I measure an unknown impedance with the AD5933?

I just got the EVAL-AD5933EBZ board. I have been trying to calibrate it correctly to measure resistance. I tried calibrating with a 150kohm resistance and measuring 200kohm with it. This works and I have attached an image of the results. Then I also tried calibrating with 27kohms and measuring 33kohms but the values I am getting are nowhere close to 33kohm. I am getting a measurement of 135 ohms. This can be seen in the second image.

This is what I do to use the device:

First I insert the calibrating resistor into the "CAL" resistor slot on the board and type the value of the resistor into the "Resistor value R1" section of the program. Then I program device registers and calculate gain factor.

Then I insert the "unknown" resistance into the "Z" resistor slot on the board and start sweep.

Am I doing this wrong? Also, I tried different sweep parameters to try and see if I could get a better measurement that way, but that did not really make a difference.

If someone could please give a step by step procedure as to how they use the AD5933EBZ evaluation board that would be of great help.

Thanks

attachments.zip
  • 0
    •  Analog Employees 
    on Feb 23, 2012 10:53 AM over 9 years ago

    Hi harshita,

    Before focus in your problem with 33k, I'd like to remark that the 200k resistance doesn't seem flat.

    Could you repeat the measurement but using the multipoint measurement calculation?

    Regards,

    Miguel

  • 0
    •  Analog Employees 
    on Feb 23, 2012 9:31 PM over 9 years ago

    Hi harshita,

    Maybe I misunderstood the sequence you did, but you need both RCAL and Z in the eval board to do a calibration.  Calibration has to be done with a known Z impedance before you change Z to something "unknown".

  • 0
    •  Analog Employees 
    on Feb 23, 2012 9:42 PM over 9 years ago

    Hi,

    Just to clarify something. There is an error in the new Eval board schematic where the RFB resistor is actually labelled as CAL. During measurement routine use the calibration resistor in position Z to calculate gain factor calibrate system etc Then replace with unknowon impedance and press sweep on the GUI to calculate unknwon Z value. As Miguel mentioned be consious of the RFB value so that the gain through the signal chain is optimised so as to make the best use of the internal ADC.

    Regards,

    Liam

  • Hello Everyone,

    Thanks for your responses. So this means that when I am calibrating I need to put two resistances of the same value in the Rfb ("CAL") slot and the Z slot?

    I read somewhere that I should get a gain factor of 1 with that if my "PGA control Gain is x1).

    Also, after I am done calibrating do I leave the resistor in Rfb on the board or do I take it out?

    Another thing was, I thought I was meant to except my impedance values to show a decreasing plot because it's being measured at different frequencies. Or is this not supposed to happen with just a resistor?

  • 0
    •  Analog Employees 
    on Feb 23, 2012 10:39 PM over 9 years ago

    Leave the RFB value in - the only component you replace is the Calibration Resistor with the Unknown Impedance (at position Z).

    Impedance is constant with Resistor only.

    Regards,

    Liam