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LDC1101: Can I measure a 0.763mH 0.2 Ohm coil with the LDC1101?

Craig Beiferman
Intellectual 270 points
Part Number: LDC1101

Is there a minimum or maximum inductance the LDC1101 can measure?

  • 0
    Intellectual 270 points
    Whoops, correction, its 9.2 Ohms.
  • 0 in reply to Craig Beiferman
    TI__Genius 17440 points
    Hi Craig,

    The LDC1101 does not have a minimum or maximum inductance, but it does have minimum and maximum sensor frequencies and Rp values that it can drive. The minimum sensor frequency is 500kHz and the maximum sensor frequency is 10MHz, as specified in the electrical characteristics table in the datasheet. The minimum and maximum Rp values can be found in section 8.6.2 of the datasheet, though I highly recommend reading the Applications Section (9.1) before setting this register.

    I assume that your coil has a series resistance of 9.2 ohms. You can convert this to Rp using equation 2 in the datasheet, or measure it in the lab using the methods in this application note: www.ti.com/.../snoa936.pdf. Note that both Rp and the sensor frequency depend on both L and C, so you can measure a very wide range of L values by selecting an appropriate capacitor.

    Best Regards,
  • 0 in reply to Kristin Jones93
    Intellectual 270 points

    Thanks, my coil can measure between 2 positions (extended and retracted) (I have a sliding magnet inside my coil)
    I used the network analyzer method and measured at both 500KHz and 10MHz, and then measured at the resonance frequency.


    Retracted measurements:
    F=500kHz Rs=254 Xl=1510 Rp=8970
    F=10MHz Rs=64 Xl = -395 Rp=2430
    Fresonance=1.21MHz Rs=17.3K Rs=-5.48K Rp=1735



    Extended measurements:
    F=500kHz Rs=214 Xl=1860 Rp=16166
    F=10MHz Rs=65.9 Xl = -417 Rp=2638
    Fresonance=1.12MHz Rs=23.5K Rs=-3.66K Rp=570


    Based on these measurements, what would you recommend for Cp? Rpmin, and Rpmax?

    Thanks,
    Craig Beiferman

  • 0 in reply to Craig Beiferman
    TI__Genius 17440 points
    Hi Craig,

    Do you want to use Rp+L mode or LHR (high resolution L) mode? We generally recommend LHR mode for applications that do not need Rp sensing (which is most). I ask because LHR mode does not need Rpmax.

    Could you also clarify if you measured just the coil using the network analyzer or if you had a capacitor in parallel?

    Best Regards,
  • 0 in reply to Kristin Jones93
    Intellectual 270 points

    I only want LHR mode, and I measured the coil with no parallel capacitor. (Is this correct?)

  • 0 in reply to Craig Beiferman
    TI__Genius 17440 points

    Hi Craig,

    If you measured the coil without a parallel capacitor, then the resonant frequency will be the coil's self resonance frequency (SRF). If you are unfamiliar with this term, section 2.5 of LDC Sensor Design explains it in more detail. This frequency is fairly unstable, so we recommend keeping the sensor frequency below 75% of the SRF. This is the major limitation for selecting a capacitance value. You can use the inductive sensing calculator tool to quickly calculate acceptable capacitance values for a target sensor frequency.  

    In LHR mode, Rpmax is not used and Rpmin is used to determine the sensor drive current. The parallel capacitor will change the sensor's Rp value. An equation in section 1.2 of LDC Sensor Design shows this relationship. Once you have selected a capacitor, connect it in parallel with your coil and remeasure Rp at the sensor's resonant frequency using the network analyzer. Use the minimum Rp (which looks like it will be the extended position) and Table 36 in the LDC1101 datasheet to set Rpmin.

    Best Regards,