I am attempting to monitor DC voltages that can range from 0 to 250V with a micro controller. A simple voltage divider should be suitable for the broad range up to 250V, with somewhat coarse resolution. A second voltage divider for when the voltage is not more than 25V will give me improved resolution at the lower voltage levels. I have no problem using two distinct A/D ports on the micro for each of the dividers. Output of both dividers should be 0 - 5V. I've estimated the values of resistors for the two dividers.
My question is: How can I limit the voltage seen by the second divider so it never exceeds 25V even though it is monitoring the same potential source? I've attempted to utilize a zener in the circuit to clamp the voltage to not exceed 5V but it always seems to effect the linearity of the divider on both the very low end and high end of the ranges. I'm new to designing hardware circuits... as a software guy, I'm really starting to appreciate what's involved in developing analog hardware. Thanks for the help!