> 5V x (R2/(R2+R0)).  For some test values I've calculated, this
> leaves me with V2-V1 being around 1V-2V. Is there something
> other than a voltage divider I should use for this? I would prefer
> to keep the vrefs at +5V and ground since I will probably use
> other things on other A-D channels.

An A2D pin has a maximum recommended signal source impedance
of 10k. You don't mention which photocell you're using but if it is more
than 10k at any time then you'd need to buffer it

Perhaps this way, using a dual op-amp. One 1/2 to produce a DC
offset voltage and the other 1/2 as a summer amp to add and amplify
the DC offset and the photocell output. By making the DC offset and
gain adjustable you can make the photocell's output span between 0
and 5V at the A2D input

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