AC/DC coupled LVDS on REFCLK

Hello,

I haven't implemented any Cyclone IV on a board (Only CI, CII, StxIII and StxIV up to now, using their LVDS and/or XCVR feature) but I would say:

1- To work properly, the inputs of a differential line receiver (clock or data) basically need to be properly biased and to see sufficient differential swing. In other words, the input differential signal Vcm and Vd must within the specified range for Vicm and Vid => So if your TX Vcm is within the RX Vcm range, the answer is YES you can DC-couple them.

2- I don't know whether the VCCINT value of Cyclone IV is compatible within the Vicm range nevertheless I would NOT recommend you to apply exactly the same biasing voltage to both inputs because this may lead the input buffer to oscillate when no signal is applied.

A pair of 2-resistor divider (one for the positive input, the other for the negative input) should be preferred and calculated in order to have a small DC-offset between the two polarities (some tens of mV is enough). More over, you can add a decoupling capacitor located as close as possible to the upper resistors feed point to filter ripple from the biasing voltage. Don't worry about "noise injection": when properly designed, power planes offer very low impedance above DC which prevent other "clients" from being contaminated.

Generally, when your signal is a clock, I would rather recommend AC-coupling because the coupling capacitor acts as a DC-block but also as a high-pass filter that can attenuate any clock disturbator below the clock frequency.

When your signal is a data, I would recommend DC-coupling as far as possible. This scheme allows for the straightest trace on the PCB layout which in turn is better especially for high data rates. Unfortunatly, the TX Vcm and the RX Vcm don't match most of the time, preventing DC-coupling from being implemented.

Oliver

PS: you are mentionning the "REFCLK" inputs which are part of the GXB blocks whereas LVDS TX and RX (True or Emulated LVDS) pins are located in different I/O banks. Though GXB and LVDS inputs are both differential, biasing and termination scheme do have some slight differences. Beware not to mix them.