I want to determinate the cutoff frequency of a low-pass filter on CAN bus communication line. I found the following values on the Internet:
R = 100 ohm
C = 560 pF
When I calculate the cutoff frequency of this filter I find out that the frequency is 2.8 MHz.
In this example I have no idea of the bitrate on the bus. In my application I will use a bitrate of 500 kbits/s.
Now can anybody help me with determining the needed cutoff frequency on my application and explain me why I should take this frequency?
I have no experience at all with this.
SETUP:
As the CAN transceiver I use Microchips MCP2551.
Best Answer
I have no idea what you're trying to accomplish with a filter on the CAN lines, but that's not how CAN works. Don't do that.
What you should do is put 120 Ω between the two data lines at each end of the bus, and these data lines should be a twisted pair of about that impedance. That really should have been clear from even a cursory reading of the spec. The MCP2551 datasheet mentions 120 Ω terminating resistors, although in passing.
If you want to slow down the edges to reduce radiation, use the slope-limiting built into the MCP2551. Otherwise, don't mess up the bus by trying to "filter" things somehow.