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CAPTIVATE-FR2676: Typical ranges of the capacitance ratio between the electrode C_touch and the C_electrode ?

Part Number: CAPTIVATE-FR2676

Hello,

Can I ask typical ranges of the *capacitance ratio* between the electrode C_touch (touch capacitance) and the C_electrode (electrode capacitance) ?

We assume a Self-capacitance electrode for a finger.

  

I have written which I could find:

(1) The linked page shows a bar graph. The left bar sounded to me that a good center value would be 25%.

http://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/CapTIvate_Design_Center/latest/exports/docs/users_guide/html/CapTIvate_Technology_Guide_html/markdown/ch_basics.html#self-capacitance-parasitics

 

(2) The linked page tells the C_touch would be 1-10pF.

http://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/CapTIvate_Design_Center/latest/exports/docs/users_guide/html/CapTIvate_Technology_Guide_html/markdown/ch_basics.html#self-capacitance

 

(3) The linked page tells C_electrode example is 3-4pF.

http://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/CapTIvate_Design_Center/latest/exports/docs/users_guide/html/CapTIvate_Technology_Guide_html/markdown/ch_design_guide.html#id27

 

(4) I tried to calculate the ratio:

  • From (2): C_touch would be 1-10pF
  • From (3): C_electrode example is 3-4pF

From the above, a good *capacitance ratio* might be from (1/4=25%) to (10/3=330%), but I was afraid the 25% - 330% would be a good or typical range.

 

  • Hello Hideaki,

    "Typical" is used very loosely.  The values used in the documentation comes from CapTIvate EVM or other boards we have design over time and experience working with various customers.  Your values will totally depend on your sensor design and the baseline parasitic capacitances in your system.

    Keep in mind that CapTIvate is a relative measurement system, which is why the important value is the "change in capacitance" and not so much the electrode or baseline capacitance.  For example, if you consider a person has 1pF coupling to ground and your baseline capacitance is 10pF, then when touched, the change in capacitance is 10% and that is considered very good.  Obviously smaller or larger baseline capacitances can impact the "change in capacitance", but this handled by the Captivate's calibration method that effectively reduces the size of the baseline capacitance in order to increase the "change in capacitance".  This is what the conversion_gain and conversion_count settings in the GUI are for. 

    How small of a change is detectable?  Well, very small but you want to be careful to not be too sensitive or have a low threshold, else you become sensitive to false detects caused by noise.

    We have established recommended minimum thresholds that will help mitigate any potential of false detects caused by the device itself, so you always want to set your thresholds > the min.

    Your system will potentially introduce additional noise so you will possibly have to have an even higher min threshold.

    I would strongly recommend that you read Sensitivity, SNR and design Margins to get a better understanding.

    In the example above, if you use our GEN2 device, the min threshold is 0.3%, which means you can reliably detect a 0.3% change in a 10pF sensor or 0.03pF.  But remember, if a person is ~1pF then you can see with this threshold you can very easily detect a touch.  Because of this you will want to increase the touch threshold probably somewhere in the 1-2% range to provide an adequate noise margin.

    Does this help?

  • Hello Dennis,

    Thank you for your response. Very kind.

    I could talk to my customer again.

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