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HVDA551-Q1: Output differential voltage is not stable when the cable is long

Wayne Wang20
Expert 4360 points
Part Number: HVDA551-Q1

Hi team,

My customer is using HVDA551QDRQ1. The other side of HVDA551 is a PLC and the terminal resistor is 120 ohm. HVDA551 VCC=5V and VIO=3.3V.

When HVDA551 is sending data out, we measured its differential output voltage between CANH and CANL in below 2 conditions.

Condition1: Cable length is 50cm. HVDA551 differential output voltage is 0V/2.2V

Condition 2: Cable length is 100cm. In most cases, HVDA551 differential output voltage is 0V/2.2V. But we can capture that sometimes it drops to 1.8V. And this 1.8V level won't show up when the cable length is 50cm. 

So could you please help check why the differential output voltage will sometimes drop when the cable is long? Thanks.

Best regards，

Wayne

  • 0
    TI__Guru 62920 points

    Hi Wayne,

    From the screenshots it looks like you must have at least a couple of different CAN transceivers communicating on the bus.  Is it possible that the high-amplitude portions are coming from the transceiver on the node where the oscilloscope is connected and the low-amplitude portions are coming from the transceiver on the other end of the cable?  (Cables will generally add some loss to the signal due to their series resistance as well as their parasitic capacitance/inductance, which can affect higher-frequency signaling.)

    Max