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FDC2214 EVM Problems regarding the onboard-L ?

Adrian Benker
Prodigy 100 points

Hello everyone,

I'll just repost a question I asked earlier in an already answered thread:


Using the EVM I found that it shows approx. 50nF with nothing connected and approx. 140 nF with my testing equipment (wiring, oscilloscope etc.) connected, but still without the sensors. This all makes sense and the corresponding frequencies in the range of 3 MHz shown on the oscilloscope are as expected.

However, my sensors are of variable capacitance in the range of .66 to 7.2 nF. This is where the problem starts. If I connect a 33nF film capacitor to the EVM, the frequency shoots up to 10 MHz instead of significantly decreasing. The oscilloscope and the GUI both show the same frequency, so the frequency measurement is correct.

Can someone explain what happens here?

I suspect it has to do with the energy stored in the capacitance (E=1/2 * C * U^2) and in the inductance (E=1/2*L*I^2). Since the oscillation is in effect the energy swinging back and forth between the two, the energy stored at peak must be the same in both L and C.

I can measure the maximum voltage of the oscillation (in the range of 1.2 V to 1.8 V, as per data sheet) and thus know about the energy stored in the capacitor. What about the current? I am unclear of how the current oscillating between L and C is related to the DRIVE_CURRENT set in the GUI. Can someone enlighten me?

Summary: I suspect that if my sensor has a large capacitance, I also need a large inductor. Is there some guidance from TI how to set the value of L for a given range of C, defined by the physics of my sensor?

Thank you in advance for helping me out.

  • 0
    TI__Expert 6480 points

    Hi Adrian,

    I have a few follow-up questions below:

    1. Could you clarify the units that you are using? 50nF for the EVM does not seem correct - did you mean pF? Also, the 33nF capacitor you connected does not lie in the range that you have specified (.66 to 7.2 nF).

    2. How are your sensors being connected? Are they connected single-endedly or differentially? 

    3. Could you show me scope shots of the waveforms? Waveforms of both the base case and the variable capacitance case would be helpful.

    Thanks,

    Rachel

  • 0 in reply to Rachel Liao
    Prodigy 100 points
    Hello Rachel,

    thank you for your response.

    First of all, you are right, I measured 50pF for the EVM. I simply got the units mixed up.

    I used a 33 nF capacitor because it was the closest to the sensor capacitance, that was laying nearby.

    I connected the sensors in both, the differential mode and the single ended mode with similar results.
    These are the pictures I took in the single ended mode:

    www.fotos-hochladen.net/.../scope0s9yt648kof.png
    www.fotos-hochladen.net/.../scope1w5pdjamiy2.png

    (Please do not get destracted by the frequencies shown on the right side of the pictures. Some of the are wrong.)

    Channel 3 (yellow) was not connected to any sensor, capacitance or anything, while sensor 2 (blue) was connected to my stretch sensor. When I connected the stretch sensor to the EVM the amplitude became significantly smaller (like in the picture) and when I stretched the sensor (which increases the capacitance) the amplitude decreased even further. Also the signal doesnt look very much like a proper signal, when I "Zoom in". Thats probably what stops the oscilloscope and the EVM measuring the right frequency.

    As I said: I am using the onboard-L and I think it might not be large enough?

    Thank you,

    Adrian
  • 0 in reply to Adrian Benker
    TI__Expert 6480 points

    Hi Adrian,

    Thanks for the additional information!

    You are correct, you may need to increase your inductance since your sensor capacitance is only 33 nF. Another option would be to increase the drive current (IDRIVE) to see if that helps as well.

    Thanks,

    Rachel

  • 0 in reply to Rachel Liao
    Prodigy 100 points

    Hi Rachel,

    thank you again for your response.

    The actual sensor has an even lower capacitance between 0.66 and 7.2 nF. Sorry for confusing you. Do you have any experience on how to find out the perfekt value for the inductance or do you think "trial and error" will be the best method?

    I am surprised about the fact that you suggest a LARGER inductance to measure a relatively LOW capacity? Is that correct?

    Thank you,

    Adrian

  • 0 in reply to Adrian Benker
    TI__Expert 6480 points
    Hi Adrian,

    We usually use this equation to characterize the system:

    Rp = (1/Rs)*(L/C) where Rp should be around at least 1 kOhm

    Can you give us more information on your application? The 0.66 to 7.2 nF range for the sensor capacitance seems very large.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 100 points
    Hi Rachel,
    Rp turned out to be large enough for my application.
    Unfortunately I found out that I will probably have to use the high power mode on channel 0.
    I also found out, that my system is somehow unstable. My test installation would only work when I connect both, the ethernet LAN Cable to the laptop I run the GUI on AND the oscilloscope. Otherwise the wave form looks strange and the chip does not give me the right result.
    I will try to power the system via battery now.
    Thanks,
    Adrian
  • 0 in reply to Rachel Liao
    Prodigy 60 points
    Could you explain the terms of this equation?
  • 0 in reply to Eduardo Gomez Suarez
    TI__Expert 6480 points
    Hi Eduardo,

    Rp and Rs are the equivalent parallel and series resistance of the LC tank. L is the inductor while C is the capacitor. You can take a look at the following application notes for further information: www.ti.com/.../snoa930.pdf and www.ti.com/.../snoa936.pdf

    Thanks,
    Rachel