DAQ grounding

[JÞB]

Baran, consider using an opto-isolator! Many of these provide for a 600-1500 volt isolation between the input and output refeances (grounds)

Using differential inputs does not help you!  The 6009 has input protection circuitry to prevent damage to the device if the voltage applied to any input is greater than x volts from the device referance.  You will damage the device if any voltage greater than the device specified voltage is applied. If the device ia an human this may cause pain or (HENCE the term "Ole Sparky") death, in electronic devices it merely breaks them.

In Short- you need a circuit to provide signal isolation from the "high-side" to the "metering side" of your system.

Glad to help!

[Baran]

I was thinking of using opto-isolator. But if my low-voltage circuit is connected to computer chassis; which is connected to my high voltage ground; then my two grounds are connected anyway; unless there would be an opto isolator between DAQ and computer.

[JÞB]

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@Baran wrote:

I was thinking of using opto-isolator. But if my low-voltage circuit is connected to computer chassis; which is connected to my high voltage ground; then my two grounds are connected anyway; unless there would be an opto isolator between DAQ and computer.

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Well, That is the differance between "on paper" and "practical solution."

The engineers that know this, get paid well

[djs_at_eaton]

Excuse me, Even Deejay, but your comment is not only wrong, it's dangerously wrong.  There are several ways for electricity to damage humans.  Whether electricity kills depends on many factors.  It's not "just voltage," and not "just current."