piclist 1999\10\28\125706a >
Thread: Measuring Analog Voltages w/o Sharing Grounds
face BY : Maverick email (remove spam text)

>a) Average Voltage to Power circuit.  6V-24V
>b) Average Current in Power circuit.  Average current during the last 100mS.
>c) Voltage of Control circuit.  Control is a 3-6V source
>d) Voltage of Monitor circuit.  Monitor is a 4.5-5.5V source

>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 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.

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

Depends on what your design constraints are, obviously - how much additional
cost is acceptable, etc...  Do you intend to use the internal PIC A/D
converters or
an external part?  If you are using an external A/D, is that set in stone
or flexible?

This post intregued me because I've spent the last four years working
on EV telemetry and power systems, and we often face similar problems in
terms of noise,
coupling, floating voltages, and you name it...

Personally, I'd recommend you use true differential delta-sigma A/D
converters - and no,
before everyone jumps on me, these aren't necessarily out of the price
range on any reasonable
project - Analog's AD7705 is a relatively inexpensive (~$4 in small
quantity, I think) dual differential 16 bit D-S A/D that has an
SPI interface and works very nicely under extremely noisy conditions.
Because of the
integration-deintegration performed in the D-S process, noise and random
jitter in the data is reduced
significantly - to the point that one of these was used on the output of a
large (500W) switching power
supply to measure current through a shunt- and had consistent and accurate
average currents out to 10 to 12 bits
without a problem.  And with a main ground bus that can have ~2-3 volts of
ripple on it due to currents and inductive
kickback, this is no small feat!   Power consumption on these is very
reasonable for battery-based applications - I believe
they can be standby'd to 10uA or so...

If that isn't an option or you would prefer to use some other A/D
converter, just put an instrumentation amp on the
front end - these usually have two drawbacks:  they're expensive if you go
for really high-quality versions, and otherwise
their common mode rejection isn't the greatest at higher frequencies.
Since you're only looking for averages, though, you
could R-C integrate off all the high frequencies and leave the in-amp to
only convert the differential input signal you're
trying to monitor down to a "ground" referenced voltage for your circuit to
read.  I'm personally a fan of Burr Brown INA2128s, but
these are costly if you actually have to use them in a production circuit.
Analog also makes some very nice ones at lower cost, I
believe.  For your application, they also have one other drawback I just
thought of - they're usually current hogs.  On the order of ~20mA.

Just my two bits for the day...

ND Holmes

Nathan D. Holmes   spam_OUTmaverickspamdrgw.net, EraseMEndholmesEraseMEspamiastate.edu
  122 Shepard #3  Box 328  Gilbert, IA 50105  Iowa State University - EE
  http://www.drgw.net/~maverick   PH: 515-663-9368


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