CAN Transiver Termibation Resistor Calculation Reference

Hello Dinesh,

The termination resistor in a CAN network servers a couple of different purposes. One is to pull the CANH and CANL signals together when the bus needs to transition between the dominant state (which is driven by the transceiver IC) and the the recessive state (which is high impedance). Another is to provide a resistance at the end of the line that is matched to the characteristic impedance of the transmission line used. This helps to prevent reflections that would typically result as a signal is incident upon the end of a line (which would be high impedance if left unterminated). Since the cabling often used for CAN is a twisted pair with differential characteristic impedance of 120 Ohms, this value is common for termination.

Note that most CAN transceivers specify their driver output voltages and timing characteristics based on a load of 60 Ohms. This is because the bus is typically terminated via a 120 Ohm resistor at each end, meaning the effective bus resistance is halved.

There are some cases where the termination resistance may vary slightly. For example, with bus topologies that are non-linear where there isn't a clear pair of ends to terminate, some people may choose to use more than two resistances but to increase their values. In this case is it important to make sure that the total effective resistance is still close to 60 Ohms in order to match the conditions over which the transceiver is specified to perform within the CAN standard's guidelines.

I hope this helps, and please just let me know if you have any further questions.

Max

Dinesh,

When it comes to matching impedances, the trace impedance should be considered but also the cable impedance. (For most applications, the signals propagate relatively short distances over these PCB traces but longer distances over cables.) This is a value that is generally provided by the cable vendor.

Besides impedance matching, the other concern is the loading effect of the resistance. As the bus resistance becomes small, the CAN driver may become overloaded and its output voltage amplitude will reduce. As the bus resistance becomes larger, the transitions between dominant and recessive states slow down and the communications rate is limited.

Since the resistance affects several different things, it is usually best to target staying close to the "standard" operating guidelines of using 120-Ohm transmission lines along with dual 120-Ohm termination. This way the impedance is well-matched and the loading conditions correspond to the application conditions over which a CAN transceiver is characterized. Then, if some slight deviations are needed they can be evaluated individually. For example, if PCB constraints dictate that your trace impedance must be 137 Ohms then you can decide if it makes more sense to trade-off on transition times to match this higher impedance (in which case you should makes sure the cable impedance is similar, otherwise there is not much benefit to just matching the PCB) or if it makes sense to trade-off on impedance matching in order to achieve faster rise/fall times that better align with a transceiver's specifications.

These sorts of in-depth trade-off analyses are generally more appropriate for applications operating closer to the boundaries of normal operation - e.g., applications at very high rates, with very long cable runs, with high node counts, etc. Most "typical" applications have enough margin that some variation in impedances (such as 137 Ohms versus 120 Ohms) or other non-ideal conditions will not cause communications issues.

Max