I recall at one point we had one of these NF1811 with a broken power suply pin. It was from a limited production run with the smaller micro 3-pin power connectors. Maybe check yours is not that one.
Long story short it still responded with only the positive rail but DC will gave a bad photovoltaic mode response and the AC had a large unstable oscillation that was only viewable on a high speed scope (if I recall right higher than the 125 MHz nominal bandwidth). I would check the power-in pins aren't bent/broken and also check the AC out on a higher speed scope (i.e. >=500 MHz).