Designing with the SN65HVD234D, what are recommended values for serial resistance as well as bus capacitance? The layout in the datasheet (Figure 41) shows them but does not recommend values.
Thanks
Angela
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Designing with the SN65HVD234D, what are recommended values for serial resistance as well as bus capacitance? The layout in the datasheet (Figure 41) shows them but does not recommend values.
Thanks
Angela
Hi Angela,
Figure 41 shows some series resistors on the RXD and TXD lines, although in most applications these are not needed. They can be useful for impedance matching if long PCB traces are used (in which case they are typically small values, e.g., ~20-30 Ohms) or for improving EMI by reducing the edge rates (in which case they may be up to ~1 kOhm in value). R5 and R6 are CAN termination resistors, and these should be 60 Ohms each. This gives 120 Ohms in total between CANH and CANL at the terminating node. (There should be exactly two terminating nodes on a CAN bus in order to provide proper loading as well as impedance matching, so nodes that do not implement termination would not include these resistors at all.)
The bus capacitances (C8 and C9) provide high-frequency noise filtering but also slow down the CAN signaling, so there is a trade-off to evaluate when picking these values. A typical target would be around 100 pF on each line. This could increase if more filtering is needed or would decrease if the application requires very high data rates or large numbers of nodes on the bus. The capacitor that is used as part of the termination network (C7) provides filtering for the common mode component of the CAN signal without degrading the differential component, and so it can be a relatively large value. A typical value would be 4.7 nF.
I hope this helps - please let me know if you have any other questions.
Max
Hi Max,
Thanks for your answer, it is very helpful. Just a couple other questions: For the split termination resistors R5, R6 is it critical they are 60 ohm? Will 60.4 ohm work? Are R3, R4 typically the same value as R1, R2? Are C1, C4 typically 100pF as well or another value?
Thanks,
Angela