Termination impedance of CAN bus?

[zenerbjt]

Dear Engineers,

We have a micro on a board which is receiving data from a different board. The micro has a CAN transceiver next to it, through which it is fed the incoming data. What would you say is the optimal termination components to connect to the CANH and CANL pins on the CAN transceiver.

The CAN transceiver is TCAN332 by ti.com

https://www.ti.com/lit/ds/symlink/t...tCh0s2QkEEAAYASAAEgLhZPD_BwE%26gclsrc%3Daw.ds

Would you say that a termination impedance of a 120R resistor in series with a 220n capacitor is too capacitive? (ie, this across CANH & CANL)

 

[FvM]

Doesn't work, CAN requires resistive termination.

 

[zenerbjt]

Thanks.....so 50R in series with CANH or CANL is ok?

 

[stenzer]

Hi,

have a look to the linked datasheet in your initial post, it also covers the termination topic. See section 11.2.1. The Datasheet states a termination resistance of 120 \[\Omega\]. See figure 36, which shows a standart termination as well as a split termination.

BR

 

[zenerbjt]

Hi,

Yes the datasheet suggests a 120R resistor between CANH and CANL. However, this would mean a power dissipation of 1.2W in that resistor.

Power = v^2/R = 12^2/120 = 1.2W

 

[KlausST]

Hi,

isn´t the differential CAN bus voltage just a few (3?) volts?

Klaus

 

[FvM]

No idea where you got 12 V, the dominant output voltage of the linked driver is 2.1V. There are two points about CAN termination:
The bus needs a resistive (parallel) termination to realize the rezessive voltage level, not necessarily 120 ohm.
For higher speed busses or longer cables, you want termination with cable impedance at both ends, usually 120 ohm.

 

[schmitt trigger]

Not only the differential voltage is only about 3 volts when in the dominant state, but in the recessive state, the dissipation is zero.

A Canbus spends the idle time in the recessive state, and during actual transmission, it would be safe to statistically assume a 50-50 dominant- recessive split.

 

[stenzer]

You should really have a closer look to the datasheet, the differential output voltage levels are listetd in section 8.5 on page 6.

BR

 

[zenerbjt]

Thankyou, so that's some 37mW in our 120 Ohm shunt terminators.....we are sending a PWM speed control signal to a BLDC via CAN Bus...it hardly seems like an RF transmission line situation, so i imagine that for us, the termination resistance is purely to achieve the recessive state? (ie by discharging the CAN bus voltage).

I appreciate that when the CAN bus is used to its fullest extent of bandwidth, then the termination impedance should vary with bus length, and vary with bit rate.

The bus needs a resistive (parallel) termination to realize the rezessive voltage level, not necessarily 120 ohm.

Thankyou...so our contractor, who has terminated it with a series RC circuit (120R-220nF) shunting the CAN bus, appears totally bad news.

 

[schmitt trigger]

Bad news indeed
The capacitor will probably prevent te recessive states to achieve valid voltage levels.

 

[zenerbjt]

..thanks, we do have a 120R at one end, and the RC at the other...again bad news?

 

[stenzer]

Hi,

I would say yes,, still bad news. The temination on both ends is used to avoid reflections. So if the bus is not properly terminated you can expect superimpositions of your intended CAN signal and the reflections. Have a look on standing waves in transmisson line theory.

Further, you would require a certain time to charge and discharge the 200 nF capacitor, which may not be possible if operating the bus at high frequencies (short switching times).

BR

 

[stenzer]

Hi,

Thankyou...so our contractor, who has terminated it with a series RC circuit (120R-220nF) shunting the CAN bus, appears totally bad news.

I assume by a series RC termination a Standard Termination as shown in the datasheet in Fig. 36 is meant, where an additional capacitor (220 nF) is placed in series with \[{R}_{ Term}\]. If so, the easiest solution would be the replacement of thr capacitor by a 0 \[\Omega\] resistor, or using two 60 \[\Omega\] resistors (\[{R}_{ Term}\] = 60 \[\Omega\] & \[{C}_{220 nF}\] = 60 \[\Omega\]).

BR