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I'm trying to use the split termination on a CAN bus, but I'm a bit confused about the calculation for the split termination capacitor. It is described in Figure 10 in the Texas Instruments app note SLLA270 Controller Area Network Physical Requirements but when I use the values of R = 60 Ohms and C = 4.7nF, I geta cutoff frequency of 564.379 kbps and not 1.1 Mbps like they describe in the app note. enter image description here

I'm confused as to how they got 1.1 Mbps and why ? Isn't the RC low pass filter here created by the resistor and capacitor (60 ohms and 4.7 nF) ?

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2 Answers 2

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60 Ohm in parallel with 60 Ohm is 30 Ohm. RC-Filter with 30 Ohm and 4,7nF has 1.1MHz corner frequency.

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  • \$\begingroup\$ Somewhat more detail re the 60//60 effect would be helpful. It's obvious once you see it but can be unclear otherwise. \$\endgroup\$
    – Russell McMahon
    Jul 28, 2020 at 0:55
  • \$\begingroup\$ Could you explain a little more why they go in parallel ? And if the resistors go in parallel why don't the capacitors ? \$\endgroup\$
    – VanGo
    Jul 28, 2020 at 5:21
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In the worst case scenario, a CAN message will contain alternating bits (...01010101...), which will produce a square wave with a fundamental frequency of HALF the baud rate:

baud rate vs fundamental frequency

This ignores the higher harmonics to produce a clean square wave, however.

See this post about CAN circuitry: https://resources.altium.com/p/can-bus-designing-can-bus-circuitry

In my case, the network should have a 1mbit/s baud rate. Assuming the worst-case scenario, when the network is transmitting a sequence of alternating bits (01010101) the signal will be a square wave of frequency 500kHz, or equal to half the baud rate.
We know the resistance at 60 ohms, we can thus calculate the capacitor. enter image description here

This post seems to suggest that it's not actually important to place the cut-off frequency above the baud rate anyway: https://e2e.ti.com/blogs_/b/industrial_strength/posts/the-importance-of-termination-networks-in-can-transceivers

Typical concerns I have heard with this termination technique include, “Will this filter my CAN bus signals?” and “Do I need to place the corner frequency above my data rate?” The simple answer to both questions is no. Since the capacitor does not place a direct current (DC) load on the differential bus signal – it only filters the alternating current (AC) signal and the common-mode signal – and the differential signal is what determines the bus state, you do not need to set the corner frequency of the filter above the data rate.

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