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LDC1612: Glitches

Henri Bouchard
Prodigy 20 points
Part Number: LDC1612

TI E2E Community,

I am using LDC1612 to measure the inductance of the coils of a variable reluctance differential pressure transducer.

The inductance of the coils is roughly 18 mH, and the nominal capacitance of the capacitor that I soldered on is 560 pF which results in a sensor frequency of roughly 50 kHz.

This is how it is set up:

The problem I am having is that there are large glitches that occur in the measurement. Such as the following (250 Hz sampling rate):

When I probe the INA or INB pins, I see the following:

It looks like what I think it should look like except that there seems to be an "extra" oscillation that occurs occasionally at the end of the measurement (In the image above, it is the slightly grayed out one).

The deglitch filter is set on the lowest frequency.

My questions are:

1. What could be the cause of the glitches?

2. Is the extra oscillation that occurs at the end of the measurement a concern?

Thank you

Henri Bouchard

  • 0
    TI__Mastermind 23810 points
    The extra oscillation at the end should not be a concern. Depending on how your sensor frequency lines up with the internal reference frequency, it's possible for there to be a difference in the number of oscillations measured. Of course, this could also contribute to a shift in output code, although I would not expect it to be as bit as what you see.

    Have you tried decreasing the sampling rate?

    Also, have you measured the RP of your sensor?
  • 0 in reply to Clancy Soehren
    Prodigy 20 points

    I am switching back to the EVM GUI from the MSP432 for ease.

    To show that the same problem appears with the EVM (same configuration as original post):

    (I do not know why the output codes are in the 268000000 range in the "Data Streaming" graph. In the "Configuration" interface of the GUI, the Raw Code is around 344000)

    Regarding the sampling rate, when I double the settlecount and rcount values to 500 and 774, respectively, then I see this:

    When I set rcount and settlecount to their maximum values (0xFFFF), I see this:

    For all the plots above, I set the graph to add samples every 4 ms.

    Regarding Rp, the LCR meter tells me:

    f = 100 kHz, Rp = 19.506 kΩ

    f = 10 kHz, Rp = 5.4953 kΩ

    f = 1 kHz, Rp = 558.9 Ω

  • 0 in reply to Henri Bouchard
    TI__Mastermind 23810 points
    It looks as if the higher RCOUNT setting causes the glitch to completely go away.

    Have you thought about reducing the parallel capacitor to increase the frequency? It looks as if there is already some parasitic capacitance in your sensor since the measured frequency is less than 50kHz.