Could some electronics guru please clarify this a bit ? A bare-bones
> single slope ADC has an integrator, with its output being fed into a
> comparator. The comparator compares this input with a reference value,
> VREF. How is this reference value selected ? Any hints, suggestions
> would be of great help.
The reference value is chosen to optimize whatever circuit qualities are most important to you.
It needs to fit in the available supplies, be large in comparison to the comparator's offset and noise voltage, be small enough so that nothing is going nonlinear, be a convenient value for other parts of the circuit, etc., etc.
I would hazard a guess that if it's just a single-slope converter that the choice isn't terribly critical, because single-slope converters aren't that terribly good -- but I'm being cynical.