Peter Grey wrote:
>
> I wish to use a pic to determine a signal which has a resolution of 10
> microvolts in 200 millivolts. It also has a requirement for any component to
> be in a very small package (ie surface mount and preferably in an 8-pin
> SOIC). I can get a 16 bit ADC but the range is usually that of the supply
> rail to ground. If I was to look at 10 microvolts in 5 volts then a 20 bit
> ADC would be needed. As you can see I would need serial I/O for the digital
> side. The ideal solution would be the LTC1298 but with a resolution of 20
 bits.
>
> Any ideas would be appreciated.
>
> TIA,
>
> Peter Grey
> Neosystems
> Australia

Try checking out Crystal Semiconductor's delta-sigma ADCs.
        http://www.crystal.com/
I have successfully interfaced them to PIC54s and 74s using modified I2C
routines.  They have 5V only devices with both 16 and 20 bit resolutions
and a variety of serial interfaces.  I'm having difficultly getting
delivery on military-grade parts but commercial/industrial parts should
not be a problem.

Matt