TCAN1044V-Q1: Is it ok to place capacitor at CANH/L line to mitigate emission?

Ito-san,

Generally it is OK to place smaller capacitances on CANH/CANL to serve as RF filters.  Capacitive loading does affect the timing of the CAN waveforms, though, and can introduce bit timing asymmetry (since the recessive-to-dominant transition is driven by the transceiver but the dominant-to-recessive transition is more dependent on the RC characteristics of the bus).  The maximum capacitance that could be supported per node would be a function of the data rate, number of nodes, capacitive loading of wiring harnesses, and timing margin supported by the controller device.  Common approaches are to use <100 pF per node to limit the impact on timing.  At very high speeds like 5 Mbps it may be better to operate without additional capacitance at all.

Regarding conformance requirements - these are very varied.  Many times the system-/ECU-level requirements would specify a maximum node capacitance (either single-ended or differential), though, and you should be careful not to exceed this value.  Let me know if this isn't clear or if I can help provide further details.

Max