Creating First Order IIR Lag Filters in the PSoC® 3 and PSoC 5 Filter Component - KBA85693

Version: *A

 

Question:

How can I implement first order IIR lag filters in the PSoC® 3 and PSoC 5 Filter component?

 

Answer:

First order IIR filters may be implemented in the Filter component by using the biquad filters with some zeroed coefficients. The Filter 2.0 component in PSoC Creator enables the use of direct-form I biquad filters, which are second order IIRs. The following diagram shows the direct-form I biquad.

1.png

 

The following equation describes first order IIR lag filters:

 

2.png

 

Another way to write the difference equation for the IIR lag filter is:

 

3.png

 

To fit this lower order IIR into the biquad, we set the coefficients b1, b2, and a2 to zero, and then set coefficients b0 and a1 as described at the end of this section. Custom coefficients in the Filter component are in the order b0, b1, b2, a1, and a2. In the custom coefficients field, they must appear as:

 

k
0
0
-(1-k)
0

 

Just like the biquad sections in which they are implemented, these filters may be cascaded in the Filter component.