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Using a spring with very small inductance as the coil

Joe Tiseo
Prodigy 50 points
Other Parts Discussed in Thread: LDC1000EVM

I am trying to incorporate the LDC1000 into a new design, however I have been running into some issues. The LDC1000EVM works fine, however I wanted to replace the coil with a spring and thats when I started to have issues. I do not see any oscillations at INA or INB.

My capacitor is a 100pf ceramic, and these are the characteristics of my spring:

Radius = 0.118 inch

Number of Turns = 15

Height = 0.393 inch

Calculated Inductance = 0.621uH

Measured Inductance (LCR Meter) = 0.761uH

Measured Rs (LCR Meter) = 0.981ohm

Measured Inductance (compressed) = 0.685uH

Measured Rs (LCR Meter/compressed) = 0.893ohm

Calculated RpMAX = 7.182kohm

Calculated RpMin = 1.347kohm

Based on this setup, what would I need to change in the LDC1000EVM GUI to get this working? or do I actually need to modify the LC tank further?  If I add series inductance (22uH), I can get see oscillations at INA and INB.  However, the change in inductance is extremely small from the spring I have.  How small of an inductance could I use to still see oscillations at INA and INB?

Any help would be greatly appreciated. 

  • 0
    TI__Prodigy 715 points

    Hello Joe,

    Based upon the values above, it appears you are violating the Fsensor_max frequency of 5MHz, as listed in the datasheet.

    Doing some quick calculations using F = 1/[2*pi*SQRT(LC)], the minimum value of the overall inductance would have to be ~10.13uH, but this would put you right on the edge of 5MHz. You could increase the capacitance from there to give you enough overhead for the frequency change of spring compression (but make sure you stay above RPmin!).

    Thank you for your interest in the LDC1000!

    Best Regards,

    Daniel Lanzone

  • 0 in reply to Daniel Lanzone
    TI__Prodigy 715 points

    Hi Joe,

    Just to clarify this a bit more, start with adding a series inductor on the order of about 10uH. If this gives you enough sensitivity, we are done! If not, proceed to decrease the series inductor value while increasing your capacitor value such that you stay within the frequency requirements. Eventually, you will get to the point where you can no longer decrease the inductance value and increase the capacitor value without going below RPmin.

    Thanks,

    Daniel Lanzone

  • 0 in reply to Daniel Lanzone
    Prodigy 50 points

    Thanks you for the guidance.  I will look into this and try to determine my optimal LC circuit.

    - Joe