earthed

[JasonXCX]

I am about to buy a net book to use with my USB 6211. I have notice that some of them have an ac adaptater with 3 wire plug ( live neutral and earth ) and some with only Live and neutral.

I wonder why ? .

is the usb plug earthed ??? Sorry for this silly question but .....

(For partical reason I would prefer the unearthed one .)

[JasonXCX]

QUOTE (JasonXCX @ Dec 4 2008, 10:21 AM)

I am about to buy a net book to use with my USB 6211. I have notice that some of them have an ac adaptater with 3 wire plug ( live neutral and earth ) and some with only Live and neutral.

I wonder why ? .

is the usb plug earthed ??? Sorry for this silly question but .....

(For partical reason I would prefer the unearthed one .)

so sorry to ask such a silly question

[Neville D]

If your asking if the USB DAQ measurements are with respect to the building ground (earth), I think not.

If I remember right, the DAQ channels are considered to be isolated from this ground. Which is as it should be, since if you run your laptop off the battery, then where would "ground" be?

N.

[JasonXCX]

QUOTE (JasonXCX @ Dec 5 2008, 07:51 PM)

so sorry to ask such a silly question

I dont understand some adaptater ( the alimentation 220 V to 15V ) are earthed . With a PC I had before some problem with a ground ( earth) loop between the pc the acquisition device and a machine tool . So I am careful.

but I believe you are right and that with a laptop and National device I will have no problems. Thanks

[Neville D]

QUOTE (JasonXCX @ Dec 5 2008, 11:44 AM)

With a PC I had before some problem with a ground ( earth) loop between the pc the acquisition device and a machine tool .

Yes, for PCI daq cards, the ground reference is the PC ground.

For ground loop problems, I would always recommend using a DAQ card with isolated inputs. They take away all the headaches of ground loops and high common mode voltages etc. and are well worth the extra cost.

Alternately, you can add isolation to PCI DAQ cards with additional NI hardware. Look at the 5B series of NI products.

N.

[JasonXCX]

QUOTE (Neville D @ Dec 5 2008, 11:29 PM)

Yes, for PCI daq cards, the ground reference is the PC ground.

For ground loop problems, I would always recommend using a DAQ card with isolated inputs. They take away all the headaches of ground loops and high common mode voltages etc. and are well worth the extra cost.

Alternately, you can add isolation to PCI DAQ cards with additional NI hardware. Look at the 5B series of NI products.

N.

Thank you I am looking at this products

[AnalogKid2DigitalMan]

Well, there's a quick way to check with an ohmmeter- one lead to USB or DAQ ground and the other lead to the AC socket ground. But I bet they are isolated from one another.

-AK2DM

[JasonXCX]

QUOTE (JasonXCX @ Dec 8 2008, 09:03 AM)

Thank you I am looking at this products

Thanks very simple good idea, I find that no links between earth and output. I believe that the earth is used to a make a screen against HF emission from the power supply ; So I will use this small netbook with my usb device , small and very easy to carry everywhere.

Jason

[Rolf Kalbermatter]

QUOTE (AnalogKid2DigitalMan @ Dec 8 2008, 09:59 AM)

Well, there's a quick way to check with an ohmmeter- one lead to USB or DAQ ground and the other lead to the AC socket ground. But I bet they are isolated from one another.

-AK2DM

They probably are but there will likely be some capacitive coupling. This can be sometimes problematic for higher frequencies.

Rolf Kalbermatter