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40 Document No. 001-64846 Rev. *I Getting Started with CapSense

Figure 3-10. Impact of Slew Rate on Emissions

The CapSense sensing methods use a switched capacitor front end to interact with the sensors. Selecting a low-

frequency for the switched-capacitor clock helps you to reduce the radiated noise from the CapSense sensor.

3.3.3 Conducted Immunity and Emissions

The noise current generated by high-frequency switching circuits entering the system through the power and

communication lines is called conducted noise.

3.3.3.1 Board Level Solutions

Proper use of decoupling capacitors as recommended by the datasheet can limit the problem with conducted

emissions. For further protection, a passive filter can be used. Passive filter effectively limits not just the conducted

noise emitted but also the noise entering the system. Thus, it improves the conducted noise immunity of the system.

A pi-filter is a simple bidirectional low-pass filter. The two main types of pi-filters are the series inductor and the series

resistor. The series inductor pi-filter has two shunt capacitors and one series inductor configured similar to the Greek

letter π, as shown in Figure 3-11. The noise is filtered by all three elements (L1, C1, and C2) in both directions. The

bidirectional nature of the filter is important. Not only does it prevent the supply noise from affecting sensitive parts, it

can also prevent the switching noise of the part from coupling back onto the power planes.

Figure 3-11. Series Inductor Pi-filter

The values of the components are selected based on the frequency that needs to be attenuated.

3.3.3.2 Power Supply Solutions

The following guidelines help you to prevent conducted noise from entering your CapSense design:

 Provide GND and V

planes that reduce current loops

 If the CapSense controller PCB is connected to the power supply by a cable, minimize the cable length and

consider using a shielded cable

 To reduce high-frequency noise, place a ferrite bead around power supply or communication lines

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