I'm working on a data logger which is a little different than what I've
done before.

I have three independent circuits (Power, Control, and Monitoring).
Control and Monitoring _may_ share a ground, but this is neither necessary
nor preferred by the customer.

Power must be as isolated from Control or Monitoring as possible, as the
Power circuit gets a lot of noise on it and Control is susceptible to noise :-(

My project is to provide data to the Monitoring system (and ground and
power may both be shared).

Required data:

a) Average Voltage to Power circuit.  Power is a 6V-24V source to a 70A
load switched at 3KHz to control motor speed.  The motor has a nasty back
EMF spike.  We need to see the average voltage during the last 100mS.
b) Average Current in Power circuit.  Average current during the last 100mS.
c) Voltage of Control circuit.  Control is a 3-6V source for a 75mA-300mA
typical load, with occasional 1A loads.  The power supply is well able to
provide 1A with minimal drop for short periods.
d) Voltage of Monitor circuit.  Monitor is a 4.5-5.5V source (I'm
investigating whether they will use NiCd or Li technology).  It is a PIC
driving an Atmel serial DataFlash chip (which I'm not sure yet, probably
the 4M piece), so it will have bursts of "high" current.  Supply will have
no problem providing this.

All data points will be sampled at 10-30 Hz (selected by customer at
runtime, with no hardware changes permitted).  8-bit data is sufficient at
this stage, but they may want 10-12 bit in the future.

The software (PIC) is essentially done - I just borrowed another data
logger I did.  The hardware is a problem.

The Monitor circuit is supposed to be small (as small as possible) and
needs to run at least 1 hour on batteries (NiCd rechargeable or Li coin
cells).  Of course it's mobile, and will use an RS-232 interface to
transfer data to a laptop, either during operation or at the end of a
testing period.

Test points for the Control circuit is an existing jack into the system
(power and ground are on the connector, not a problem).

Test points for the Power circuit are another matter.  A connection to the
positive side of the supply is very easy, and to the negative side almost
as easy.  Connection for the ammeter function would be easily accomplished
on either the high side or the low side of the supply, but it needs to
introduce as little resistance as possible (they don't want to hurt
performance of the system).

Now my question:

How do I hook these separate analog signals up without sharing the ground
with the Power circuit?

Thanks.

Andy



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