KINTEX-7 SERIAL TRANSCEIVER FIELD DAMAGE

[jay.ikerman@adtran.com]

PROBLEM STATEMENT: In our product design portfolio we're using various variants of KINTEX-7 such as XC7K160T ; 325T ; 410T. In all cases we have concerns about field returns with QUAD115 & QUAD116 transceiver damage, isolated to the RX pairs. The symptom is an unresponsive channel with leaky or low impedance on die protection diodes. These ports have to transit to the "outside world" directly with no other transient protection or series impedance.  We understand that absolute maximums on these nodes are -0.5Vp to +1.26Vp.

QUESTION: Most of our applications are short cable runs (< = 3 meters) to adjacent equipment that's hardwired (no SFP). (1) Is any other user experiencing low level but recurring field failures to the RX pair of serial transceiver?; (2) Are you also connecting directly to the outside world with no series capacitance inline? (3) Your comments of revelation please. :o)

[karnanl]

Hello jay.ikerman@adtran.com 

Using Transceivers without AC coupling could be very risky.
Is there any particular reason to use DC coupling on your system ?

1. Perhaps your protocol do not use 8B/10B or 64B66B , so your serial data could have very long run-length ?
2. Or perhaps your board is too small, hence you cannot put additional AC capacitors on your board ?
   
   ( Other that those two reasons (a, b), I cannot thinking of any real advantage using DC coupling )

Without AC coupling, There would be no protection against common mode currents flowing to RX pins, any ESD from “outside world” will hit RX pins directly, which may exceeding absolute maximum value stated in datasheet.
You also need to ensure that common-mode voltage on TX side is the same with RX, or else common-mode current will constantly flows during normal operation.

Thanks & regards
Leo