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Webench/LDC1614: Design Doubts/waveform/drift/Rp

Michael Blouw
Prodigy 40 points
Part Number: LDC1614

Tool/software: Webench

I am trying to measure weight using LDC1614 and the coils suggested by TI (Dout=13.9mm, W=S=0.152mm, 19 turns, 2 layer) with a capacitor of 150pf.

Due to WEBENCH, F=4.5MHz and Rp is around 10k, in which CHx_IDRIVE[15:11]=b01111 (based on datasheet). For this value of DriveCurrent I got strange waveforms in outputs of the coils.

Since the measured frequency was beyond the value WEBENCH gave (F=7.5MHz), I decided to find Rp using the procedure suggested in datasheet(auto calibration) and I found it around 1k and then waveforms seemed to be correct.

To have the highest accuracy I already set RCOUNT=0×FFFF so with Fref=40MHz I need tCONVERSION=26mS which second waveform fulfills the condition.

 

Now the questions are:

  1. Is the way I am setting values correct?
  2. Why this much difference in Rp (Measured and WEBENCH)?
  3. If the second way I am going is correct: when I measure an actual weight, it is not working properly. First it stabilizes to a point then keeps adding up/decreasing down in every one/two measuring cycle in case I put a weight or take up a weight, respectively(ex: I put 100grms after a few cycles it reaches to say 110grms then keeps adding up by 1grm in every one/two cycle. And when I take 50grms of it up, it stabilizes around say 55grms and then starts decreasing down by 1grm in every one/two cycles). However when CHx_IDRIVE[15:11]=b01111 (strange waveforms in coils output), with the same coding, after stabilization, it just fluctuates around a certain point. What can be the problem? Is it LDC related?

(4 coils are used in continues mode and Data is read using DRDY bit).

Thanks in advance

Michael

  • 0
    TI__Mastermind 41310 points

    Michael,

    I have moved your post to the Inductive Sensing forum.

  • 0
    Prodigy 40 points
    Dear all
    I just wanted to follow up on the question I asked above. I would be very grateful for some inputs.
    Michael
  • 0 in reply to Michael Blouw
    TI__Genius 13650 points
    Hello Michael,
    I apologize about the delay; this post was missed.
    This apps note explains how to set the IDRIVE with a scope. I don't recommend using the auto-amplitude feature. Please follow the procedure from the apps note which should just a few minutes, and let us know if you still have issues: www.ti.com/.../snoa950.pdf

    I don't know how your system is set up mechanically, but it sounds like there is some mechanical hysteresis present in your system, or that your mechanics settles very slowly. Is it possible that the target does not return to the same position in your setup if weight is added rather than subtracted?
  • 0 in reply to Ben Kasemsadeh
    Prodigy 40 points

    Hello Ben

    Thanks for the reply. It helped a lot.

    I am trying to measure weight from 0 to 8kg with the accuracy of 1gram. The distance between the target and coils are 2mm and when I put 8kg (8000 grams) the distance is 1mm. To measure every grams it seems I will need to detect 0.125 micro-meter difference(1mm/8000). Now:

    1. Is it possible to detect this distance with LDC1614?

    2. How much inductance changes with 0.125um change in distance?

    3. The plot below is my relative error when NO weight is put (CH0, CH3) and when I put away target (CH1, CH2). It seems when there is no target almost no error. But when I have the target but no weight I have high relative errors. I am wondering if some LSBs in 28bit data are changing a lot and probably they follow a certain pattern. Do you think truncating some LSB bits will help?

    Thanks in advance

  • 0 in reply to Michael Blouw
    Prodigy 40 points

    Dear Ben

    I just wanted to follow up on the question I asked above. I would be very grateful for some inputs.

    Michael