I'm currently working on a remote telemetry project.   One of the 
requirements is the ability to measure various DC (and possibly AC) 
voltages.   These may be above or below ground, ground referenced, or 
not.   I do have some flexibility with specifying what inputs are 
permitted (voltage), but don't have enough flexibility to say that "all 
signals must be ground referenced and must not be negative in reference 
to ground".  (Actually I might be able to do this, but I'm not going to 
be happy about it long term).

Oh yeah, I'm not thrilled about the prospect of needing a negative 
(below ground) power supply to do this since this is a cost sensitive 
application.  But if I must, I do realize that I could do this with some 
sort of level shifting opamp - but that isn't really the way I'd like to 
do this.

External adc's are not out of the question if needed...

What I'd love is a dirt cheap completely isolated adc, but I realize 
that this isn't going to happen.

Ideas?

-forrest
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