15a9cde274
golden_reference.py: update comment from 'Simplified' to 'Exact' to
match shaun0927's corrected formula.
fpga_model.py: fix adc_to_signed docstring that incorrectly derived
0x7F80 instead of 0xFF00. Verilog '/' binds tighter than '-', so
{1'b0,8'hFF,9'b0}/2 = 0x1FE00/2 = 0xFF00, not 0xFF<<8 = 0x7F80.