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TCAN1042HGV: CSplit Value

ReedKacz
Expert 3657 points
Part Number: TCAN1042HGV


Hi Interface Experts,

My customer is designing a TCAN1042 with split termination using 60 ohms and a CSPLIT cap as shown in the datasheet figure 17.

After reading this post  https://e2e.ti.com/support/interface/f/138/t/290299 I have concluded the this capacitor is a low pass filter typically between 4.7nF and 100nF.

If the goal is just to low pass filter common mode noise, is there any downside to increasing the capacitor even larger than 100nF, say 10uF?

Lastly, a design equation would be very helpful. Please provide the best equation to use for CSPLIT calculation.

Thanks,

Reed

  • 0
    TI__Guru 62920 points

    Hi Reed,

    There's generally not much of a downside to higher values (besides size/cost).  But, there may not be much of a benefit either.  The capacitor creates a low-pass filter on the common-mode component of the differential bus which is useful for rejecting higher-frequency noise.  The effective corner frequency would reduce as the capacitance increases (meaning a wider range of noise is filtered), although the exact value would be difficult to quantify since it can be difficult to model the effective "source" impedance of coupled noise.  For cases where nodes operate at different ground potentials and therefore would have offsets between their CANH/CANL output voltages, higher capacitances may mean that it takes longer for a transmitting node to charge the bus up to a common-mode level that is optimal for it.  So, emissions reductions for nodes operating at close to a "nominal" ground would be traded off for potentially higher emissions on nodes operating with more extreme ground shifts.

    Max