Hi!
I use a LTC1685 transceiver pair in an application. To protect the input/output A and B pins of the LTC1685 I use a common mode chocke and TVS diode CDSOT23-SM712. However, the capacitance of the TVS diode is 75pF and the system fail to transmit/receive at 52Mbit/s if I use these TVS diodes. If I remove them the system works fine. Hence two questions are raised:
1. What is the ESD performance of the LTC1685? Can I use is "safe" without any extra TVS diodes?
2. The loading from the TVS diodes should be 75pF. Assume 50R cable the capacitance is about 100pF/m. The length of the cable that loads the LTC1685 should be approximately the length of a rising/falling-edge about 3.5ns, assume propagation speed about half light speed, 1.5E6 m/s, the corresponding length is 3.5ns*1.5E6 = 0.52m. So loading should be 100pF/m * 0.52m = 52pF. A total of 75pF + 50pF = 125pF. Maximum load capacitance is 500pF according to datasheet. According to this coarse estimation, it should not be any problem to use the TVS diodes. Maybe I miss something?
Best regards
Joakim
