12VDC isolated voltage measurement

[cdenk]

I would like to measure say 0-15 vdc from a marine battery with a PLC (TRI-PLC T00MD-888+) that has 0 - 5 VDC ADC input. Concerned with developing a ground loop since there is already a common ground that the PLC is using to control other items that might have chassis ground. Thinking there should be an off the shelf transformer or something to do the isolation. I can handle voltage dividing resitors or OP-AMP selection, just the isolation got me baffled.

[support]

You can use a multi-meter to verify if there is a fixed ground potential between the 0V of the marine battery and the PLC's 0V before the two are connected. If it is floating with respect to each other then the two power system are isolated from each other and may be brought to the same level by connecting the two 0V together. You can then use the standard resistor voltage divider to scale it down to 0-5V for the PLC.

[evanh]

Full isolation is indeed the most reliable way to go but is way more expensive and also has many more parts that can fail including another power supply.

Instead, you can use differential sensing and rely on full earthing to give a resonable common-mode reference.  With this method you still use twisted-pair for sense wires but need just one op-amp to convert to single-ended and it can do the scaling at the same time.  Best to use an "instrumentation" op-amp, it doesn't cost much more and combines three op-amps into a particular config for better common-mode rejection.  Also, sometimes these only require a single resistor for gain setting.  

The limitation of this method is the noise and particularly the signal levels must stay between the power rails.  If it floats away then you will get wrong readings.


Evan

[evanh]

To be clear, for non-isolated analogue measurements there needs to be three wires.  Two of them (The pair) are for meassuring the level while the third one (Usually connected between the zero volt power rails at both ends, this gives a good common-mode reference) is heavier and used to absorb any currents that might flow back and forth.

The key is to have no current flow on the sense lines.


Evan

[cdenk]

Thanks for the reply, do you have a part number for the instrumentation Op-AMP? I prefer the full isolated method since the PLC will be switched between the 120VAC supply and battery backup, though I will have to study the circuits sensors, and controlled devices to see whether everything is isolated.  The wires are in 30 feet of metallic conduit, there are 3 CAT-5's in one conduit, and 4 #12 stranded plus 10 #18 stranded (to controlled devices, automotive relays) in another. Planning to use the two of the #12's for indoor mounted battery charger, could use one of the other #12's for the ground, or maybe 1 or 2 of the CAT-5 twisted pairs.

[evanh]

Looks like you are in luck, I found that there is cheaply priced chips that perform isolation via capacitors instead of the more expensive transformer method and whats more they do it completely internal to the chip so you only need deal to power supply and scaling.  

http://focus.ti.com/docs/prod/folders/print/iso124.html

You should be able to wire it up using the battery sense wires as the input supply.  This will be quite accurate due to your circuit drawing a constant current and there not being any possible earth loop.  The only concern there is whether it's happy to operate from a single supply rail.


Cheers,
Evan