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DP83620 - AC coupling and termination detail

Tom Domanski24
Expert 3345 points
Other Parts Discussed in Thread: DP83620

Hi,

Here is a customer question:

We are currently using the DP83620SQE and plan to reuse this in my new project that we spoke about.  In reviewing the data sheet I came across a question that I would like your help answering.  We use the DP83620SQE to interface to a fiber transceiver.  The data sheet on page 63 shows a recommended 100Mbs circuit.  I have two questions regarding this circuit. 

  1. First, is it necessary to AC couple the transmit lines to the transceiver? 
  2. My 2nd question is, given that the transmit lines are AC coupled, the typical termination voltage would be Vdd-1.3V = 2.0V.  What is recommended is Vdd-2V = 1.3V, which is typical for DC termination.  Can you tell me why it was done this way?  As another point of reference the TI DP83620 TP&FX Demo Board schematic uses basically the same circuit as is shown in the data sheet (refer to page 7).

Regards,

tom