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LDC1101EVM: LDC1101EVM Series resistance and performance considerations

Brian Nguyen
Prodigy 30 points
Part Number: LDC1101EVM

Hi, 

I have two questions:

1. I'm trying to build a model of the circuit to calculate parallel resistance (Rp). How do I calculate the series resistance of this circuit? 

2. I've read and implemented all the recommendations on the User Guide, but I'd like to push the device's existing performance even more. Specifically, I'm interested in increasing sensor Q. I'm considering increasing the voltage and/or current of this device by adding a gain stage in between the coil and the EVM. I'm also considering maxing the USB a coaxial cable. Can you list some recommendations on how to make the device more sensitive? 

  • 0
    TI__Mastermind 34125 points

    Paul,

    You have the equations that will help calculate the resistance due to the coil:

    Q = Rp·√(C/L) = (1/Rs)·√(L/C)

    Rp = L/(Rs·C)

    The starting point is the expression for the complex conductance:

    1/Z = 1/Rp + 1/jωL + jωC

    Work the expression into an equivalent impedance with real and imaginary parts:

    Z = R + jY = Re(Z) + j·Im(Z)

    where Rs will be the real part.

    The best way to make increase the sensitivity is to make the coil as large as possible and keep the sensor frequency low.
    Note that the ability of the LDC to drive the coil will limit the max size you can use.
    Our Inductive Sensing Design Calculator Tool can help with this by allowing you to see how the device will respond to a given target distance.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 30 points

    Hi John, 

    Thanks for your response. I'm still curious on how to calculate the parallel resistance (Rp). I understand that if I can calculate Rs, I can get Rp (and vice-versa). But currently, my model assumes neither Rp or Rs are given. How do I go about calculating Rs given only L, C, and the driving frequency f? 

    Best,

    Paul

  • 0 in reply to Brian Nguyen
    TI__Mastermind 34125 points

    Paul,

    You can't estimate Rp or Rs from only L and C.

    Do you know the Q of the circuit?
    If you do, the you can use that to compute Rs or Rp if you also know L & C.

    If you don't have a direct number, you can estimate Q from:

    Q = f_res/Δf

    where f_res is the resonant frequency and Δf is the -3dB bandwidth of the circuit.

    Does his help in any way?

    John