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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.

Schematic of the filter

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.

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  • \$\begingroup\$ It's termination, not a filter. Filter depends on why do you need it. You can even work well without any filter. \$\endgroup\$
    – user76844
    Commented Dec 10, 2016 at 21:07
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    \$\begingroup\$ CAN does not need a filter, actually. You should place a 120R resistor across CAN_L and CAN_H if that resistor does not exist (that resistor is called Termination). \$\endgroup\$
    – Rohat Kılıç
    Commented Dec 10, 2016 at 21:10
  • \$\begingroup\$ Hi, Thanks for the answer. On the 2 outer end of the CANbus there I connected indeed a 120 ohm resistor. So there is no filter needed and probably because you read the difference between CAN-H and CAN-L. Is this also true in a very noise environment, for example an electric car? \$\endgroup\$
    – TMJ
    Commented Dec 10, 2016 at 21:35
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    \$\begingroup\$ I've looked through the paper you linked, and I don't see your circuit anywhere. So why did you think it would work? \$\endgroup\$
    – WhatRoughBeast
    Commented Dec 11, 2016 at 2:31

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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.

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  • \$\begingroup\$ Thanks for the reply. I allready implemented the terminating resistors. I was a bit confused by the following PDF where they explained to use RC low pass filters to protect for EMI and ESD. (onsemi.com/pub_link/Collateral/AND8169-D.PDF) \$\endgroup\$
    – TMJ
    Commented Dec 10, 2016 at 22:34
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    \$\begingroup\$ @TMJ That app note contains an example where the CAN lines are tied together with two resistors, with a cap to ground between. That's for bus termination, quite a difference from placing both resistors in series with caps to ground and not to terminate the CAN lines. If you want to filter EMI you should instead use their "common mode choke" solution - I know from experience that it works well. \$\endgroup\$
    – Lundin
    Commented Dec 13, 2016 at 8:41

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