Use internal XADC reference
Current external XADC reference can be very noisy or create saw-shaped
ripple with 150 mV amplitude.
Xilinx datasheet says that noise amplitude should be below 250 µV, and
provides very specific PCB requirements.
This is important in case of e.g. DDR memory. Xilinx MIG uses temperature values provided by XADC to control IO phase delays to DDR SDRAM.