# How does the data rate of a CAN bus affect the value of the capacitor used in a split termination scheme?

I'm helping to redesign the CAN bus of an electric car, and we're interested in achieving higher data rates of up to 8 Mbit/s. We have a transceiver selected, a CMC to reduce inductively coupled noise, and we are now looking at termination schemes as the next step.

We came across the split termination scheme which, according to section 4.5.2 of this TI document and various other sources, can help stabilize the common-mode bus voltage and filter out high frequency noise. A typical value for the center tap capacitor is listed as 4.7 nF for a data rate of 1 Mbit/s if I'm understanding correctly. Is the relationship as simple as solving for the capacitance with the given equation? If so a value of 620 pF would be suitable for 8 Mbit/s, but the common-mode voltage itself shouldn't be changing at such a high frequency should it? Would this value work?

• "the common-mode voltage itself shouldn't be changing at such a high frequency should it?" There is no way to know without measuring your environment. It depends entirely on what equipment is operating nearby. That's why Figure 10 of your linked document set f as a variable. It's up to you to decide what's necessary and appropriate. – WhatRoughBeast Jul 24 '18 at 19:45
• 8 Mbit/s? How long are those cables in the electric car? Extrapolating, the maximum for the total length (sum of all segments) is 7.8 meter. – Peter Mortensen Jul 24 '18 at 21:00
• It's a small race car, so the cable length shouldn't be an issue – Michael Jul 24 '18 at 22:43
• Tell me the car model so I could steer clear of it. Even with high tolerance of CAN bus you'll be looking at more error frames than actual data, IMHO. – Maple Jul 24 '18 at 22:53
• @Maple Good point. We still have to build with certain standards and on a timeline however; a lot of the groundwork has already been laid in terms of system architecture and changing communication protocols/hardware very drastically runs the risk of the delaying the project. Additionally, the car not only has to perform on the track but is be judged for its design, and fiber optics may be overkill. That said, 8 Mbit/s might be overkill too. – Michael Jul 24 '18 at 23:50