I am designing a small digital voltmeter and current meter to build into a
piece of equipment. I'm probably going to use a 16c73 or 16c74.
  I would like to be able to measure 0 to 40 Volts and 0 to 20 Amps. In the
lower range ( 0 to 25 Volts) I wanted .1 volt of resolution. If I feed the
input voltage through a 5.1:1 voltage divider network and  into an A/D pin
clamping the voltage at 5 volts I should get .1 volts per step ie. 255 =
25.5 volts, using Vdd = 5 volts as the reference.  My problem is what do I
do to read above  25.5 volts with lower resolution? I was thinking about
switching in a further 2:1 divider network with a FET when my A/D reads 255
and scaling the result in software to reflect the 2:1 division.
  For  measuring current  I am planning on using a .01 ohm shunt, ie. 20
Amps would yield .2 Volts. I would scale Vdd down to .2 volts (25:1) and use
that as Vref. For measuring voltage as described above  I would change the
A/D settings and use Vdd as the reference voltage.
  Can anyone suggest some way to simplify this project especially the
voltage measurement scaling. I would like to do away with the FET if
possible. Any comments or suggestions would be appreciated.
                                                       Jim