Post Go back to editing

I need a reference circuit for 4_wire the impedance measurement using AD5934, range 50 to 250 ohm.

Category: Hardware
Product Number: AD5934

I saw the reference design file CN0349 and CN0217, I think both are 2-wire implementation of the AD5P3x family IC. Can I get the circuit reference for the 4-wire implementation, with impedance range of 50 to 250 ohm. 

Help will be highly appreciated. 

  • In the link, it is mentioned that setting Rcal = 20 can set the impedance range from 1 to 60. Can you please explain this, I did not get this. How can Rcal resistor set the range.

  • Looks good, congrats!

    Thanks, your help was the main factor.

    I will keep peak-to-peak voltage under the rated value.

    Frequency is making troubles. Should I post my questions related to frequency and impedance measurement here or should I open a new issue?

  • I would just keep discussing it in this thread - forum moderators usually move questions to different groups/threads as they see fit, so the questions endup in the right place anyways.  

  • Ok, sounds great.

    I follow the CN0349 design in which 1Mhz crystal is used. But now I noticed that, I can get around 7800Hz frequency (maximum) using 1Mhz crystal. Though, I can get higher frequency using sweep() function, but that one is also limited, I think max is 15KHz. For this part I already order 16Mhz crystal, so I will change the crystal. 

    As you said.

    "The oscilloscope indicates signal frequency at  ~ 2.68 KHz, what is the clock frequency coming to the AD5934? Signal frequency might be potentially too low for the "DFT" to give accurate readings".

    I did calibrate my system with 100 Ohm resistor, with 100 Ohm resistor Vin was almost 1.09 V peak-to-peak. But when I measure 120 Ohm resistor I get impedance value 91. I check Vin for 120 Ohm, which was almost 1.2 V peak-to-peak.

    Below is the print values for measuring 120 Ohm resistor.

    Red 120 Ohm mean the actual value. 

    My question is,

    Is this because of low frequency or there can be some other issue?

  • Before getting into the weeds with the frequency, when you say Vin voltage, do you mean RFB voltage? From the screenshot we know the raw Re and Im  values, but what were  the raw Re and Im values when you connected the calibration resistor of 100 Ohms? Also, is the software that calculates the impedance something you wrote?  

  • Let me add the pictures of the waveform.


    120 ohm

    AD9534 frequency is 5khz and sweep increment I set to 4 Hz. The acquired signal amplitude does changes but I get 91 Ohm when I place 120 Ohm resistor after calibration.  

    After calibration with 100 ohm, when I do measure 100 ohm I get 100 ohm.

  • Yes, I am checking the signal on RFB resistor. 

    For 100 Ω the Re is 3113 and Im is 4171.

    I wrote the software for this part. 

  • For 100 Ω the Re is 3113 and Im is 4171.

    Output log shown in pic below.

  • The suspicion is that your code thinks that the output is inversely proportional to the impedance under test - which is correct when the AD5934 does not have the 4-wire analog circuits between Vout and Vin. With this 4-wire circuit the output is directly proportional to the impedance under test.

    In abstract sense the circuit is converting the impedance into integers in the Re and im registers that constitute complex output Re + j*Im. The conversion factor  between physical Ohms and counts in those registers can be viewed as complex gain G. This complex gain can be calibrated with a known resistor: resistor impedance has only real component and zero imaginary component, so Zcal = 100 + j*0 Ohm. The correspondent output is Outcal = Re + j*Im = 3113 + j*4171 = G * (100 + j*0) Ohm, so G = (3113 + j*4171 ) / ((100 + j*0) Ohm). The "unknown" impedance Zx produces output,  which is proportional to Zx: Outx = 3400 + j*4574 = G * Zx, so Zx = Outx / G = (3400 + j*4574)*(100 + j*0) Ohm / (3113 + j*4171)  109.5 + j*0.21 Ohm. As expected from a resistor, the imaginary component is nearly zero. The real component is 109.5 Ohms, which is a bit far from the expected 120 Ohm, but it could still be within manufacturing tolerance of both the resistors used for calibration and as the "unknown."

  • For this part I already order 16Mhz crystal, so I will change the crystal. 

    The undocumented issue with the "DFT" implemented in the AD5933/4 is that it only approximates the results given by the textbook DFT. Generally speaking, the synthesized  frequency with respect to clock frequency should be relatively high for the "DFT" on the chip to produce reasonably accurate results. To measure at lower frequencies it is often useful to run the chip at lower clock frequency.   

  • Is this because of low frequency or there can be some other issue?

    Since you are running your chip on 1MHz clock the frequency should not cause this, but it could if clock frequency were 16MHz.

Reply Children
No Data