Filtering out common mode signals with a transformer for a DAQ.

[BigC]

I have an NI 6008 USB DAQ that I want to use to measure a differential AC voltage in the millivolts range at 60 Hz. However, there is also a common mode voltage of around 120 V AC associated with it. To prevent the DAQ from being damaged I want to use a 1:1 transformer to filter out the common mode signal, however I am having trouble finding a suitable transformer for this purpose, since most of these transformers that are available for purchase are only meant to be used in the 115 V AC range or above and/or are not rated for a frequency as low as 60 Hz. I was wondering if there is a transformer that I could use for this and where might I be able to buy it. If that's not a possibilty, what other alternatives exist that can filter out the common mode voltage? I need the component that filters out the common mode to be passive, so active equipment that needs to be powered will not be suitable for my needs. Any help would be greatly appreciated.

[BigC]

Something just occured to me. If I connect the DAQ to a laptop running off of its battery, will common mode signals be a concern? I recall reading that battery powered handheld multimeters do not suffer this problem since the signal source ground is isolated from the meter's ground. How is the DAQ grounded, is it shared with the computer's ground? I couldn't find any information regarding this in the spec sheets.

[Jim_S]

Hello,

The 6008 is truly not designed to read signals of that high of voltage. The reason is that the ground in the USB-6008 is connected to the ground to the computer. Since this isolation is not there, it will fry your 6008 and probably your computer if this is connected to it. I would highly recommend using different hardware to acquire this signal. We have several different cDAQ cards that can provide the channel to earth ground isolation in order to protect your card as well as you system. The USB-9201 can provide the isolation you need and can read this signal. For more information on isolation, here is a good knowledgebase that explains this.

[Seth_B.]

To specifically address the floating laptop case, this may be possible for a short while but isn't safe.  Without a clear ground for the laptop, if you touch any metal on it while in contact with any other group, you just became the best path to ground.  This could definitly hurt you and I strongly advise against it.

It's much safer to find an isolated DAQ card that is rated for at least 120V Channel-to-Earth Ground isolation, such as the screw terminal variant of the USB-9201.

[xcuresme]

Instead of finding an isolated DAQ, how about isolating the voltage source of the signal?