Ingvar
Thanks for the clever suggestion.Did you use it and with what results/observations?
I suppose that a 2,5V stable voltage ref is the obvious need BUT maybe a volage divider would be better because it will follow the VDD ripple.Is it so?
I think that having one AD channel with Vref 5V for measuring 0-15V and another AD channel configured with 2,5V Vref (following your trick with 2,5V bias in respect to ground) for measuring 15-30V is what I should try.

Pic_User
Thanks also.Maybe after giving up with the regular stuff and methods I might follow this route.
Bill