I asked a similar question a while back, and that gave me some great 
ideas, but I still haven't stumbled on quite the right solution.

Essentially I need to provide the functionality of a half-dozen or so 
"Voltmeters" on a telemetry project I am working on.   For each 
"voltmeter", I need to have a positive and negative input terminal - 
just like a regular meter, and be able to read the positive or negative 
voltage which appears across that pair of terminals.

Here's the problem:   The voltage sources may or may not be 
interconnected with each other and/or the voltage source for the 
telemetry instrument.  In addition, the voltage sources may be 
interconnected but at a higher reference voltage - I.E. a high side 
shunt or similar.   Think solar site where the instrument is powered 
from the same batteries that you are measuring voltage from - and that 
you're also measuring things like current shunts, and other devices at 
the site (voltage outputs from other sensors).

In short, I need the functional equivalent of an instrumentation 
amplifier which can be adjusted to measure input voltages over a wide 
range (+-60V) and like I mentioned above may or may not be 
interconnected electrically with the amplifier's power supply.   I'd 
also like to do my best to make it "idiot-destruction" proof.

Cost is an issue...  I probably have a couple dollar budget for the 
components for this section, although I could squeeze a bit more if I 
found the exact right solution (I.E. a multi-input SPI Instrumentation 
ADC which needs very few external components).

Accuracy isn't all that critical...  After all, I'm planning on feeding 
this into a 12 bit ADC.

Ideas?

-forrest

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