TCAN332: common voltage test, ACK is not normal

Hi Xiaobing,

I agree this looks a bit strange. Do you see the same "dip" in voltage every time this condition occurs (i.e., when the node with a lower ground reference sends an ACK at the end of a frame transmitted by the node with higher ground reference)? Or, does it only occur sometimes? And, does it only occur when a ground difference is applied? Besides the ACK bit, is the lower-voltage node able to transmit dominant and recessive levels correctly?

Are any other components present on the CAN bus in addition to the termination resistances you've shown (e.g., protection diodes, filter capacitors, etc.)?

Do you see similar results if you swap the transmitting and receiving nodes (or change the ground offset from +3 V to -3 V)?

Regards,
Max

Hi Robertson 

1、It always exists when the common mode voltage is more than 3V , the "dip" will disappear if the common mode voltage is very low(i.e., it is less than 1V)  ,I mean as the common mode voltage increases, the ACK drops  more severely.

2、The lower-voltage node can transmit dominant and recessive levels correctly  except ACK  .

3、If I change the ground offset from 3 V to -3 V ，  it still exists .

4、No other components present on the CAN bus in addition to the termination resistances and filter capacitors .

Xiaobing,

Thank you for the information. Can VCC be monitored while this happening? CANH is getting pulled to the CANL during the ACK, and I think it has something to do with the bus recessive level shifting suddenly (from the offset). Does this problem exist when the offset is 0.5V?

Regards,