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Original question:

LDCCOILEVM: the formula for the appropriate capacitor value

LDCCOILEVM: Changing the capacitance manually

Manish_Thakur
Genius 5085 points
Part Number: LDCCOILEVM
Other Parts Discussed in Thread: LDC1101EVM

Hi experts,

In the Sensor design guide I found this "For mostapplication,a sensorcapacitanceof 300 pF to 2 nF is generallyrecommendedunlessa specificsensorfrequencyis needed.For sometouch-on-metalapplicationsin whichthe sensoris placedverycloseto the conductivetarget,smallersensorcapacitancesdownto 47 pF may be optimum." What happens when we change the capacitor from the existing board will it have any impact. And evem if we change what is the formula? How we can calculate it? Please make me clear about this topic I shall be grateful for that.

EDIT 1 [ In the GUI there a option to change the capacitor value by default it is 390pF. Can we change it? what will happen if we change that value what will be the impact on my application if we change that value]

  • 0
    TI__Expert 7599 points

    Hi Manish,

    Changing the capacitor of the sensor can impact the resonant frequency. First you would have to know what is the frequency by first calculating the inductance on the PCB using the coils, to get the desired frequency of operation C needs to be calculated using this formula.

    Fo=1/(2π×√LC)

    C=1/(4*π^2*L*Fo^2).

    Once you have identified the capacitance, please go ahead and use the LDC calculator http://www.ti.com/lit/zip/slyc137 to check if the design is within what is expected. Every device is designed and capacitor and inductor values needs to be tweaked to get the maximum appropriate response. The LDC calculator does have tools that can work for different LDC devices as well. 

  • 0 in reply to Arjun_Prakash
    Genius 5085 points

    Hi Arjun,

    In Sensor design document I found this "Also use larger sensor capacitor values to minimize the effects of parasitic capacitances". So my question is do we need to change the coil which is there in LDC1101EVM? Please answer this

  • 0 in reply to Manish_Thakur
    TI__Expert 7599 points

    Hi Manish,

    The capacitance change is only needed to change as per the application needs. Typically as the capacitor reduces we have seen the application gets sensitive to capacitive effects. So our recommendation is to use a higher cap as possible. In general ~300pF is what we recommend. 

    The EVM comes populated with a 390pF of capacitor. We do have a design guide and an LDC Calculator tool that can be used for the design for your specific application.