Depending on the application and its environment, the use of a driven shield may cause excessive radio frequency emissions. This is caused by high-speed switching of large-area electrodes and can lead to products failing to meet the required RFI standards.
High emissions are particularly prevalent not at the switching frequency of the touch sensors but higher frequencies dependent on the MCU core speed and the I/O pin slew rate.
Radiation Mitigation
The following describes design techniques that will reduce radiation from the shield.
- Add or increase the series
resistor to the shield electrode:
- By increasing the series resistance, the time constant of the RC shield is increased, and the amount of energy available at high frequencies is reduced
Note: The resistor package has a parasitic capacitance which at RF frequencies may be lower impedance than the resistor itself.
- Reduce the area of the driven
shield.
- Instead of using a full flood, consider utilizing patches of shield electrodes behind each touch sensor, extending beyond the edge of each sensor. This layout is shown in Figure 1.
- Connect the patches at a single physical point and connected to the resistor in a ‘star‘ formation. This layout is shown in Figure 1.
Figure 1. Minimum Driven Shield
Area
