• State
  • Locked Locked
  • Replies 7 replies
  • Subscribers 31 subscribers
  • Views 571 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Oscillating at different frequency than resonance?

Joakim
Prodigy 40 points
Other Parts Discussed in Thread: LDC1000EVM

We have a coil at 100uH. When we connect different capacitors 30-1800 nF, the coil does not oscillate at the resonance frequency once we go above ~400kHz. The coil oscillates at ~400kHz even if the resonance frequency is 1 MHz. Anything below 400kHz works fine. Any ideas what could cause this?

Off topic: The amplitude configuration 1V or 4V applies to what signal? The CF capacitor, or coil?

  • 0
    TI__Expert 6550 points

    Hello Joakim,

    I assume you are LDC1000EVM.

    1. For the coil to oscillate, 2 requirements must be satisfied: the frequency range and the Rp range. Did you check what is the Rp of your sensor and if it falls to the range?

    2. The amplitude applies to the sensor oscillation (meaning the coil and the || cap).

    Best Regards,

    Natallia Holubeva

  • 0
    TI__Genius 9300 points

    Something doesn't click - even at 30nF frequency is 91kHz

  • 0 in reply to Natallia Holubeva1
    Prodigy 40 points

    I see that i wrote nF, while i meant to write pF. The highest we have is ~2nF.

    We are using our own hardware, but it is the LDC1000. Rp is set to 0x00-0x3F, the frequency is set to minimum ~260 kHz for all || cap variations (dont have the code here, but think freq value was 0x8B). We get "valid" measurements from the sensor with these settings, except the frequency on the coil is incorrect.

  • 0 in reply to Joakim
    Prodigy 40 points

    I should add that the amplitude on CF and sensor outputs are very low, ~100mV. We are already using 20pF CF in the higher frequencies. At some point we are going to try lowering the CF capacitor at ~500kHz to see if we can get a proper amplitude and frequency output...

    The system does not have a good Q factor for higher frequencies, and it appears the chip is having problems. But does it explain why its oscillating at a different frequency than resonance?

  • 0 in reply to Joakim
    TI__Genius 9300 points

    Hello,


    Do you use EVM GUI to collect the data? If yes, do you adjust Sensor capacitor field accordingly when you vary the cap?

    100mV is a good start, let's worry about that later, after we get readings in check.

  • 0 in reply to Evgeny Fomin
    Prodigy 40 points

    We dont use the EVM GUI to collect data, but a simple serial port.

    The configuration is the following:

    0x01 - RPMAX = 0x00

    0x02 - RPMIN = 0x3F

    0x03 - FREQ = 0x8B (262 kHz)

    0x04 - CONFIG = 0x14 (4V, 768 resolution)

    0x05 - CLK = 0x02 (enable external 8MHz crystal)

    0x0B - PWR = 0x01

    We then read registers 0x21-0x25 in a single operation, and store like this:

    proxData = data[0] | (data[1] << 8);
    freqData = data[2] | (data[3] << 8) | (data[4] << 16);

    When we measure the coil and || cap with an active probe, we don't see the resonance frequency.

  • 0 in reply to Joakim
    TI__Genius 9300 points

    Hello,

    For higher frequencies, please try

    0x01 - RPMAX = 0x12

    0x02 - RPMIN = 0x3A to 3F

    If possible, please post a screenshot from INA pin - this will help us debug further!