To buffer a LVTTL signal to FPGA I am using this TI buffer SN74ALVCH16827DGGR.The buffer IC is placed on a FMC based IO card mated to a ZYNQ mother card. The problem I am facing here is when an input signal is applied to the buffer IC, there is an output signal even when there is no power supply to this IC. The output enable signals are connected to PCB GND. I want to avoid this as it may damage the FPGA. Can someone suggest me with a solution?
IC behaviour is usually unspecified when no power is supplied. As commented, you will try to power the whole board with your input signals in that case, due to the ESD protection diode.
There are, however special IC families that are are rated for this (for texas device look for the "Ioff partial power down" feature). Anyway I'd recommend at least a protection resistor on the inputs. Difficult to say more without the schematic.