A series-connected DC-blocking capacitor(a.k.a ac-coupling) is a commonly used method to change the DC bias level when interconnecting different devices and boards.
AC coupling is a simple high-pass filter that has the disadvantage of degrading the low frequency performance. The DC- blocking capacitor obviously removes the DC component of the signal as well as strips away all low-frequency contents in the signal. Whenever you pass a signal through any AC-coupled circuit there is always a small part of the signal that will be filtered. This filtering effect creates difference between the signal before and after filtering. In some cases this difference manifests itself as signal drifting also known as "DC-wander".
AC coupling works well only with signals that have equal numbers of ones and zeroes also referred to as DC balance. DC balance is needed in cases where ac-coupling is used to prevent DC-wander. DC-balanced datastreams are typical in most communication standards. However in the case of some specific or customized uses, this must be carefully understood before making a decison to use ac-couple.
Lattice offers many IO standards in our FPGA products. The LatticeSC/M , LatticeECP2M, and LatticeECP3 products with high speed SERDES I/O permits ac-coupling to be used with these CML(current-mode logic) input. This is possible due to on-die circuitry that re-biases the incoming data signal after the ac-coupling capacitor has removed the dc component of the incoming data. Without this rebias circuitry the ac-coupled signal would be swinging above and below ground which is beyond the limitations of the device technology.
The use of ac-coupling with FPGA only I/O is not as straight-forward as the CML IO mentioned previously. The Lattice FPGA input DOES NOT include any on-die bias circuitry. Therefore any signal that is ac-coupled on-board will also need to have on-board rebias circuitry to meet the input requirements of the input buffer. Without the on-board rebias circuirty the ac-coupled signal will be violating the device specifications as the input will be swinging above and below ground.