Use a resistor divider from the battery voltage as a reference for the A/D.
As the battery voltage changes the REF will change proportionally. Look for
an A/D with external reference capability.

Again use resistor dividers to scale down voltages and maybe a zener or two
for transient protection.

- -Mark

You typed to the world>>> ------------------------------

Date:    Wed, 2 Jul 1997 12:07:03 -0600
From:    Chris Atkins <tcatkins@AUDIOPHILE.COM>
Subject: Measuring fuel gauge level.

I'm trying to measure the voltage on my fuel gage to determine the fuel
level.

- -snip- -

I thought of using two A/D channels: One to monitor the
Battery and the other to measure the Fuel level. The
correct level could be found from the ratios. If anyone
has an idea of how to do this with one A/D channel, I'd
be interested.

Also, I'd like to hear anyone's ideas on a circuit to
interface this to a PIC to accurately scale down to safe
levels and protect from nasties like voltage spikes and
reverse voltages.

Thanks,