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LDC1101: detail contact of Rp setting

Part Number: LDC1101

Hello, I'm hyuni.

I measured a sensor about LC-Tank

this was measured to L,C on resonance frequency.

that means a sensor characteristic(L,C) is measured by resonance frequency. not passive element value like 100pF small chip 

I checked this calculation Rp=L/Rs/C 

the calculation is right on resonance frequency.

so, 

I would liked to know RPMIN setting to detail

because RPMIN setting has Rp=pi*V/4/I_DRIVE

and then I checked my sensor characteristic(L,C) on resonance frequency.

I can calculate that RP but that calculation is not correct with real RP.

How do explain you guys case above?

  • Hi Hyuni,

    How are you measuring the real Rp? Is it based on the equation Rp=L/Rs/C? Is your issue that the two equations to calculate Rp are not agreeing, or that the second equation does not agree with Rp measurements made in the lab?

    Regards,
  • Yes, two equations to calculate Rp are not agreed.
    In LHR mode, To get Vamp must be known Rp, but, when measured Rp of my sensor calculated to Rp=pi*V/4/I_DRIVE, The amplitude of the Vamp in the oscilloscope does not match.

    (additionally)

    Rp of safety range L,C, frequecy is not associated with measuring distance.

    Rp of my sensor is not included to Rp range in LHR mode.

    but distance is detected...

  • Hi Hyuni,

    I've copy/pasted the text of your other E2E question about this topic:

    ----------------------------------------------------------------------------------------------

    Hi , I'm Hyeon Yeong

    example)

    Rp of my sensor is 3.51kOhm and so I'm coded to 3KOhm(RPMIN SETTING,sensor drive: 150uA)

    and then 

    I calculated wiht this 

    Rp=pi*Vamp/4/I_DRIVE

    I got a Vamp that is 0.668V

    but when I check the sensor signal Vamp is 0.3V

    why are they different?

    ----------------------------------------------------------------------------------------------

    Here is my response:

    Are you using a wire-wound coil or a PCB coil for your sensor? And how did you find Rs for your original Rp calculation? The equations are accurate, so it seems most likely that there is an error in your original Rp calculation. 

    Regards,

  • I use a wire-wound coil, not PCB. I found Rs and Rp on LCR meter. measuring frequency of LCR meter is resonance frequency of LC-Tank

    and I checked Vamp(0.3V) by an oscilloscope 

    but calculated Vamp is 0.668V 

    What I want to say is that the real Vamp and the calculated Vamp are different.

  • Hi Hyuni,

    The way you measured Rs and Rp should work, as long as the LCR meter is appropriately calibrated. Could you try recalibrating it to double-check your Rs and Rp measurement? In your original post, it looks like you used the Rp and Rs from your LCR measurement and found them to be consistent with this equation: Rp = L/(Rs*C). Is this correct?

    Just to verify, Vamp on your oscilloscope is the peak voltage of your sensor oscillation referenced to ground, correct? I apologize for the basic questions, but I just want to make sure that we are measuring data the same way.

    Regards,
  • Hi Kristin Jones,

    The way I used the Pr and Rs from my LCR measurement and you found them to be consistent with that equation.
    this is correct. I checked.!!
    and

    I checked Vamp to ground. it's correct.

    definitely the signals that are measured from LCR meter and LDC chip are differnt.
    because LCR siganl is sinewave and LDC signal is half-sinewave.

    but Rp=L/(Rs*C) this equation is correct in two measured system.

    therefore I think the signals are not important, they just are differnt measuring methods.

    so, your questioning has no problem. ㅠㅠ
  • Hi Hyuni,

    Thanks for the additional information. What Rp does your LDC1101 measure when the sensor is oscillating without a target present?

    Regards,
  • Hi Kristin Jones93,

    Rp without a target present measures only LC-Tank
  • Hi Hyuni,

    I've been advised by other members of my team that LCR meters are often less accurate at high frequencies, and we would recommend using an impedance analyzer to measure the value of Rp. I suspect that the problem with your setup is an incorrect Rp measurement. If the impedance analyzer confirms the LCR meter's Rp measurement, you could be seeing this discrepancy due to a lossy capacitor (like an X7R). 

    Regards,