Oh I see - Most of the Op Amps can only produce up to (VCC - 1.5V).
To get the full 5V - I need a supply of 6.5+ Vcc.
What are the downsides of using a DC-DC to get 6.5 from 5Vdc ?
5Vdc from the existing controller (mains powered), to the arduino 5Vdc, and then take from the 5Vdc to a DC-DC converter to get 7V with minimal power (OpAmp = low power).