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44 Document No. 001-64846 Rev. *I Getting Started with CapSense
®
Figure 3-17. IIR Filter Finger Touch
Unfiltered
Filtered
Unfiltered
Filtered
For more information about how to implement an IIR filter, see the code example CSA Software Filters with EzI2Cs
Slave on CY8C20xx6.
3.4.3 Median Filter
Median filters eliminate noise spikes most commonly associated with motors and switching power supplies. In a
median filter, a buffer of size N stores the N most recent samples of the input. The median is then computed using a
two-step process. First, the buffer values are sorted from smallest to largest; then, the middle value is selected from
the ordered list. The buffer is scanned for the median with each update of the buffer. This is a nonlinear filter. The
general equation for a median filter is:

 
   

Equation 20
Figure 3-18 and Figure 3-19 show the results of a median filter on real CapSense data using the general filter
equation with N = 16.

 


Equation 21
Figure 3-18. Median Filter Noise Spike
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