TIDM-1021: About the size of the hole of the RX electrode button

Hello,

TIDM-1021 uses mutual capacitance mode for the 12 buttons. As you mentioned, increasing the hole for the LED will reduce the overall area of the electrode. However, since you're probably maintaining the distance between the RX and TX electrodes for each button, the effect on the touch shouldn't be reduced as much.

You can find an example of this scenario in the Performance Mutual Capacitance section in the CapTIvate Technology Guide. The figure below demonstrates how the sensitivity is fairly flat as the electrode area changes.

The flatter behavior of the curve is specific to Mutual capacitance and caused by the fact that the field is generated between the TX and RX electrode which have a fixed distance. This leads to a flat performance behavior across the different electrode sizes. The sensitivity for mutual mode is mainly depends on the overlay material, overlay thickness and the space between TX and RX. The minimum recommended threshold is based on the SNR and design margin analysis, please set the threshold accordingly based on your system design.

Now, I do want to point out that increasing the size of the hole in the RX electrode would decrease how much the nearly-solid electrode shields the sensor from ground coupling on other layers. Keep this in mind.

Regards,

James

Hi James,

Thank you for your reply.

I understand RX electrode area doesn't change sensitivity so much.

>Now, I do want to point out that increasing the size of the hole in the RX electrode would decrease how much the nearly-solid electrode shields the sensor from >ground coupling on other layers. Keep this in mind.

Does the above your advice mean that it has less impact on sensitivity but is more likely to be falsely detected in water?

If I send my garber data to you with private message, is it possible to review my design by TI?

Regards,

U-SK

Hi U-SK,

Making the LED holes larger will impact the sensitivity of the button.  The less copper sensor area there is to interact with a person's finger, the more you will have to increase the sensor's sensitivity to compensate for this.  One solution, if you have room is to space the keys further apart and make them bigger.  Otherwise, make the hole only big enough for the purpose of the LED and adjust the sensitivity to achieve a good touch margin (delta conversion count).

Also, it is recommended to place a 1nF cap across the LED if you plan on using PWM to control the brightness.  You can read about this is the design guideline chapter, LED backlighting section of the Captivate Technology Guide.

I wouldn't be too concerned about operation in water. Using the mutual technique demonstrated in TIDM-1021, water will actually improve a button's sensitivity due to the improved coupling it creates between the TX and RX electrode, making it more sensitive to a finger touch.  This is described in the user guide section 2.2.3.2.  I would actually be more concerned about sensitivity in dry conditions with less copper for the electrode.