>Thanks for this and I will certainly try this improved resolution. But with
>the 16C71/74 I understand that the range input is that of the supply (5
>volts) and so to get resolutions of 10 microvolts in 5 volts I need 20 bits.
>If I could scale the range to read 0 to 200 millivolts then 16 bits is
>enough. Alternatively I need 20 bits on an ADC.

USE THE Vref PIN TO BRING YOUR REFERENCE DOWN!!!  You can go to 2.5V
(Vcc/2, actually).  I've found best results with Vref < 3.0V.

>This ADC/PIC/etc needs to be able to fit in a tube(with a pH probe,
>conductivity probe and temperature sensor) which has external dimensions of
>12mm diameter and 300mm long. This is why I require something small.

Cool.  My brother was working as a waste treatment plant chemist for a
while, and I did a model of their treatment system.  We had pH,
conductivity, temp, and all kinds of other sensors in the system, and a PC
to control it all.  He ended up with a neat gadget for measuring certain
pollutants in a handheld size, but never marketed it.

Andy

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Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
          Hardware & Software for Industry & R/C Hobbies
        "Go fast, turn right, and keep the wet side down!"
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