you should use a SAR 16 bit a-d and some sort of switched in divide by stage
then over sample and switch in the needed divisors using analogue switches
using the sample and resample function
as long as the resample frequency = 16 X 2 + ins depth and any needed lag
your quids in to get a 32 bit value as a decadanal resultant /32 bit value
youll get easily 100,000 volts or current or both to 1v or 1amppervolt
or you can apply divisors to V=/32/=I in a linear fashon as decadanol steppings
using 0- 10v as a sweep range = decade true results when digitising
adding switched in shunts and V &| I dividers is the way
a digital R2R and I2I ladders spring to mind using resistors
same way any really 32bit accurate lcd volt or current meter works
even 128bit values in v and i are possible but take 16 sar cycles X2
basicaly unless you want to wait 1 second
then an 8051 doest have the balls
better to use a dsp @ 400 mhz some fpga or something as the core
i remember 8051 is a synth core and runs at up to 1ghz on fpga??
im sure im right
this way you can have multiple r2r and i2i on say 4 or 8 or 16 a-d
and get a .0000 0000 0000 0001v or amp or 1000,0000,0000,0000 accruacy
here
http://mondo-technology.com/super.html
is a pic unit called a super probe
i built one and its hand for computer work and low voltate
but im sure you can extend the senario in your mind
and seek the libs and code you needs
together with adding some ideas from the above text
to conduce your project
if i can help let me know