[Tfug] OT: PIC ADCs and differential inputs

[Choprboy]

OK... so I finally broke down and bought a Olimex PIC development board to 
start playing with. Far smaller in physical size than what I thought it would 
be and actually got more flash/RAM than I was expecting, as the PIC core had 
been upgraded in the latest rev. Suprising responsive on the network when 
free-running, though it is pretty easy to swap it with requests and stall it 
out.

So, I have run into my first problem. The PIC has multiple single-ended ADC 
inputs, but a common +Vref and ground on the devel board. For the hobby 
project I'm working on, I want to measure multiple voltage ranges, with 
different ground references. I.e. I want to measure 2+ 48V power sources (for 
instance, a battery stack and 1 or more supplies) which may be fully 
protected behind diode bridges, the 48V output to the load, and current 
shunts from/to the batteries, etc.

Circuit:
 +--*|==|*--+->|-------+
 |          |          | +48V bus
Bat       Charger      |
 |          |          +------------*---+48V
 +-------*--+-|<----+  |                Load
                    |  |                Connection
                    +--|---------+--*---Gnd
                    |  |         |
 +---*|==|*--->|----|--+         |
 |    current       |          -----
Pwr   shunt         |           ---  Chassis
 |                  | Gnd bus    -   Ground
 +--------*---|<----+
  * - measurement points

Measuring a single 48V power suppply is pretty straight forward, simple 
resistor divider network referenced to ground. But measuring multiple 48V 
where the ground reference of each is behind a diode creates a potential 
short circuit loop. Trying to then measure a 100mV current shunt with a 48V 
offset at the same time is pretty useless as there is no dynamic range in the 
ADC (works out to about 60mV per LSB at 10bit).

Does anybody have any handy-dandy circuits for this type of thing? What I 
really need is some differential ADC inputs, but as I said, the PIC only has 
single-ended. I can think of a couple ways to do it, some really ugly. It 
comes down to requiring no protection diodes on the measured source 
(everything on a common reference bus) and isolating the ground of the devel 
board from the chassis for a "simple" solution...


Adrian