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LDC1612EVM: Measure derivation and detecting hand

CarmeloF
Prodigy 50 points
Part Number: LDC1612EVM

Good Morning,

I'm testing the LDC1612EVM, the inductive sensor will be used for linear posiition sensing.

I connected  thePCB  coil B to Ch0 and used a parallel capacitor of 100pF.

Using EVM GUI, I enbled only Ch0 e set IDrive to have a Vp of 1.6V. The measured raw value is always increasing or decreasing with or without a target, furthermore if i bring my hand or plastic bottle near the coil the measure change.

I try to use different capacitor value between 100 and 330pF and I obtain the same results.

Subsequently I made the same tests using the coil Ch1 with the coil present on the EVM board and not disconnect and ai see that the measure always derive even if more slowly and a measure change when my hand a non- conductive object is nearby the coil.

Are there recommendations to power EVM Board (relative to the ground), I use a powered USB Hub to connect my Laptop to EVM Board. If i use my  stationaty computer The EVM GUI display always disconnected.

Thanks in advance

  • 0
    TI__Mastermind 40295 points

    Carmelo,

    It is hard to say exactly what  is going on.
    Can you probe the inductor's pins with a high impedance probe and an oscilloscope?
    If you can do this, it could confirm the inductor's waveform is the correct amplitude and frequency.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    Good Morning John,

    Thanks for your answer. Yesterday in the afternoon, using an oscilloscope and a probe between a ground and inductor's pin (I displayed a semi wave), i saw that the frequency and the amplitude of the waveform was costant even when i bring my hand near the CH1 coil (native coil of LDC1612EVM) but the raw value fell even more of 10000 points. The coil is on a wood table withou target. What may be the cause of this behavior?

    Thanks,

    Carmelo

  • 0 in reply to CarmeloF
    TI__Mastermind 40295 points

    Carmelo,

    The LDC device can measure a small change in sensor frequency, so any change may be too small to see on an oscope.
    Can you include a snapshot of the measured waveform without any conducting target nearby?
    Is the waveform level consistent with your IDRIVE settings?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    Good morning John,

    sorry fof the late reply, I was out for work. Using Ch1 coil of LDC1612EVM, configuration is the following:

    • Diveder code: 2 to have a Fref to 20MHz (I'm using only a coil and than Fref have to be < 35MHz)
    • Idrive code: 19 corresponding to 264uA

    Using this configuration, without a targer, I have (measures obtained with an oscilloscope):

    • Vp = about 1.71V
    • Frequency: about 2.981 MHz

    Yesterday, i started to log Data, using EVM GUI and I obtained these graph:

    I started to log yesteraday at 12.30 pm and stopped this morning at 8.30 am.

    You can see that the measure derives until a peak (i switched off the oscilloscope). After the peak, a few hours later the measures seem to sbabilize and after in the early morning they went back to change.

    If I bring my hand near the coil, the measure changes but i don't see change in the oscilloscope waveform.

    Thanks in advance for the support.

    Carmelo

  • 0 in reply to CarmeloF
    TI__Mastermind 40295 points

    Carmelo,

    Thank you for the additional data.

    The variations shown in your plots are pretty small compared to the nominal data values.
    For example, the top plot has an approximate max value of 39965250 and a min value of 39964000, with a difference of 1250.
    We can't easily calculate the mean value from the plot, but approximating the mean by the mid-point data value between max and min gives 39964625. 
    The ratio of the difference (=1250) to the mid-point is  31.2ppm.The same calculations based on the other two plots give similar results.
    These small variations are in line with what we see in our lab, so it looks like your device is behaving normally.

    I hope this helps.

    Please let me know if you have any questions.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    Thanks a lot John for your answer.

    Another question, why if I bring my hand to the coil the measure changes?

    As you can see in the picture, when i bring my finger (no ring, no wristwatch or others metallic things), about 1 cm, near the coil, there is a change in the measure, more than 15000 raw points. I approached my finger and removed it after few second.

    Same behavior for inductance and frequency graphs.

    Thanks,

    Carmelo

  • +1 in reply to CarmeloF
    TI__Mastermind 40295 points

    Carmelo,

    Your hand is acting like a lossy ground plane near the sensor.
    It isn't as conductive as a metal object, but it is enough that it affects affect the sensor's magnetic field, thereby changing its inductance and resulting in the data transitions you see.

    Regards,
    John