Measuring voltage with arduino ADC without connecting grounds

[Watcher]

How would you measure an external voltage with the arduino ADC   (eg ups battery, or another supervised  PSU) and at the same time maintaining ground isolation between the two systems?

I thought of using an external ADC and then optoisolate on the digital side, however the ADC then  needs to be powered from the system that is being measured which is not a good idea.

Any other ideas?

[wilykat]

It won't be easy.  Even a straight analog signal still needs common ground for Arduino ADC to work correctly. Otherwise the input would seem to float and your reading would be all over the board.

Can you use another power source? Power the ADC externally, read analog from whatever you need to read (and share ground), send signal digitally via optoisolator to Arduino.

[DVDdoug]

The ADC is referenced to the Arduino's ground.   (It has to be referenced to something.)   

The Arduino's ground can "float" relative to earth ground (and other grounds) and in fact if you are running it from a power supply or battery the Arduino's ground is probably isolated from everything else.   

For example, a battery-powered multimeter doesn't have a "ground" although the black lead is probably connected to an internal-isolated ground.

But if you connect the USB port to a computer, that may provide another ground path so don't expect it to be isolated if you're using USB.

[JohnLincoln]

Isolated ADCs are available, such as MAX14001.

They contain their own DC-DC converter, which solves your problem about powering.

[TomGeorge]

I thought of using an external ADC and then optoisolate on the digital side, however the ADC then  needs to be powered from the system that is being measured which is not a good idea.

Any other ideas?

You don't have to power the floating AtoD from the system being measured, you can get DCtoDC converters that have input and outputs isolated.
Just check what voltage you are measuring to make sure the isolation is high enough.
Tom...

[Watcher]

Isolated ADCs are available, such as MAX14001.

They contain their own DC-DC converter, which solves your problem about powering.

Thanks everyone for your responses.
An isolated ADC sounds like an attractive option to look into.

[Koepel]

What about putting the complete Arduino at the external voltage side ? and use a isolated USB ? 35 dollars is a lot of money, I hope there are cheaper modules.

[allanhurst]

If you need good isolation and accuracy , a V-F convertor, optocoupler , and F-V at the arduino end would be very robust. Or ( perhaps better) you could use the arduino to measure the frequency directly.  0.1% accuracy after calibration should be easily achievable.

Look at eg the LM331. They're not expensive.

Allan.

[Smajdalf]

Allan why is this better than ADC - optocoupler - Arduino?

[allanhurst]

An alternative. And probably  cheaper.

Allan

[TomGeorge]

Hi,
What is wrong with switched capacitor method?

Tom...

[groundFungus]

The IL300 is an analog optocoupler.  

[Watcher]

The IL300 is an analog optocoupler. 

That look like an attractive cheap option too!
Thanks!

[allanhurst]

Hi,
What is wrong with switched capacitor method?

Tom...

Nothing - there are a hundred ways to skin this cat

Allan

[allanhurst]

The IL300 is an analog optocoupler

A nifty device but very  limited dynamic range.

Vishay's recommended circuit takes  an input voltage of 4 - 6 volts  and maps that to a 2.4 - 3.6 volt output.

Other techniques are  better.


Allan