I have an analog voltage input of 0-5V DC, and need a corresponding output of (IN + 5V) 5-10V.
My first thought is to use the +5V regulator to "step-up" the voltage directly, connecting the input to the ground of the regulator and using the regulator's output as my final output.
I have a +15V power rail that I'm using to power the 7805, but when I try to hook it up, the 7805 pulls its ground (the input) up to almost 10V. Is it bad form to use voltage regulators in series? I'm modeling my input using a variable PSU for now, so could it be a problem with current not allowed into the PSU?
UPDATE: The problem is definitely that the PSU will not accept back-current, as suggested in the answer by Oli Glaser below. I would love to use a voltage follower op-amp to buffer this current, but I don't know if I have/can obtain any op-amps that can survive that much current (I can't remember exactly how much, but I know I measured sink current in excess of 100mA when the regulator was connected directly to ground). Would a voltage follower transistor design be more robust in that way?