User guide

36 Document No. 001-64846 Rev. *A Getting Started with CapSense®
3.3.3 Conducted Immunity and Emissions
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. This 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 like the Greek letter
π, as shown in Figure 3-10.The noise is filtered by all three elements (L1, C1 and C2) in both directions. The
bidirectional nature of the filter is very important. Not only does it prevent the supply noise from affecting sensitive
parts, it can also prevent the switching noise of the part itself from coupling back onto the power planes.
Figure 3-10. 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 will help prevent conducted noise from entering your CapSense design:
 Provide GND and V
DD
 If the CapSense controller PCB is connected to the power supply by a cable, minimize the cable length and
consider using a shielded cable.
planes that reduce current loops.
 Place a ferrite bead around power supply or communication lines to help reduce high frequency noise.