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CAN Bus: EMI capacitor values
PJ Bain:
Hi all,
Using 3 of the TJA1042 CAN transceivers for a new design in conjunction with a i.MX8. The project managers have requested that the 3 channels be capable of 1Mbps each, with a default of 250kbps. I have all the circuits designed except for the EMI capacitor values. These are 2 matched capacitors in the pF range between CANH and GND, and CANL and GND.
From what I can find, the value of these will change depending on the bps. Given they want the potential full 1Mbps available for use, I assume I use the value suitable for this (as it's the smallest). I am struggling to find documentation on how to calculate the values though. I think it is around 35pF from a couple examples I have found. The TVS I am using has a Cd of 25-30pF so if this is the case, I may not need the capacitors at all.
Any links to a resource which shows how to calculate these values based on the kbps?
Thanks
Peter
free_electron:
Much will depend on your cabling.
The cables form a capacitor as well. length dependent , node tap dependent ...
Are you using common mode chokes ?
PJ Bain:
The cabling s something I have no control over unfortunately :( I don't even know if they have designed that yet. I do know they place the 120R termination in the cabling where required. I would have preferred a split termination on the PCB, but this device may not always be at the end of the bus.
No I am not using any common mode chokes.
Rerouter:
A good starting reference, pretty much the "Maximum" is 35pF for 1Mbit, so you need to make sure your values are less than that. again its only a maximum, you can be much lower than this without issue, as the termination resistors in general will chew up most of it as its a differential bus.
Just If you can, make sure both Can High and Can Lows protection devices use a common ground point, as that capacitive shunting is generally going to end up being between the 2 data lines via the center ground point.
To make clear, you never had to add physical capacitors, just control how much capacitance there is in your protection devices/
https://www.onsemi.com/pub/Collateral/AND8169-D.PDF
SparkyFX:
Twisted pair cabling would help... but as you got no control over this, line length drastically limits baud rate on CAN.
The other thing is: as your bit timing registers are software, you usually define when to sample for all nodes on the bus, given you know the line length and physical properties of the whole net. Ringing should not be that much of a problem ... i mean afterall it would need to ring differentially and across the terminators.
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