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Minimum sensor capacitance LDC 1000

Bjornar
Prodigy 200 points
Other Parts Discussed in Thread: LDC1000EVM

Hi

We have made a coil based on the old webbench tool.

I am not able to reproduce the results in the new tool. Why is this?

Our coil is 51mm, has 2 layers of each 115 turns.

In the first design the webbenchtool suggested using a 22pF capacitance, but the new design guidelines indicate that this is below minimum:

www.ti.com/.../snoa930.pdf

Also the new tool has a lower limit of 50pF.

I must say that the 22pF cap works very well in our setup, so I just wanted to recreate that setup in the tool but the tool.

The old tool does not even give me altervatives for the LDC1000, only for the 1612 and the 1614.

http://webench.ti.com/webench5/ldc/index.cgi?d=4.5&res=999&td=3&tm=2&resolutionBasis=l

Any thoughts on this?

  • 0
    TI__Genius 14270 points
    Hello Bjornar,

    Use of smaller sensor capacitors can produce parasitic capacitive sensitivities. For example, if you have a 1pF parasitic capacitive added onto a 22pF sensor, the sensor frequency will shift and this shift will not be discernible from a shift caused by target motion.

    Regards,
    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 200 points

    Ok, but the guideline says:

    "3.2.1 Recommended Capacitor Values The range of 300pF to 2nF is typically near the optimum unless a specific sensor frequency is needed."

    This means from the lower 300pF and up to 2nF (2000pF), but the cap on the LDC1000EVM board is a 100pF cap.

    What am I not getting here?

  • 0 in reply to Bjornar
    TI__Genius 14270 points
    Hello Bjornar,

    LDCs can drive sensors with smaller capacitor values without a problem, as long as the oscillation frequency is within the range supported by the LDC. However, from a system standpoint, it is generally advantageous to use a larger capacitor.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 200 points
    Ok, but is there a good way to test different caps? How do I check the oscillation frequency vs range in my case?
  • 0 in reply to Chris Oberhauser66
    Prodigy 200 points
    Hi Chris
    Is there a good way to check wether oscillation frequency is within range? I would likt to test out different higher caps but I need to know more about which reactions I can expect in my system.
    what do you mean by "system standpoint" Are you thinking about the parasittic capacitance?
  • 0 in reply to Bjornar
    TI__Intellectual 1690 points

    Hi Bjornar,

    The best way to check the oscillation frequency is to use a scope and probe across the capacitor and make sure the oscillation frequency is within the device range in case of presenee/absence of target. In case of LDC1000, the sensor oscillation frequency ranges from 5kHz to 5Mhz.

    "System standpoint" as in the requirement of the resolution of the system which is needed in the application.

    -Best Regards,

     Suhas R C