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Ground problem

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Ground problem

Postby Aleksandar » Thu Jul 13, 2006 4:18 pm

Hi !

I purchased my new PicoScope 3204 about two weeks ago. I found on this site and also in user manual a comment, that with external power supply should be possible to get power without a ground connection of PC.

I tried to connect power supply first and then the USB but whatever I do, ground connection to PC is still there. Is there any possibility to get a "ground free" PicoScope? Maybe some kind of USB adapter, which would connect only the data connections USB+ and USB- and NOT Vground and Vcc(+) ?

With other things I am satisfied, I didn't have so much time to try all functions, but it seems to bee a good device. Sometimes, specially if I change the sampling rate offen, software freezes, I will try to upgrade the software first.

Thank You in advance
Aleksandar
Aleksandar
 

Postby Michael » Wed Jul 19, 2006 10:06 am

Hello,

Thank you for your post.

The PSU does not actually isolate the scope from the PC ground. It will help to reduce DC offsets and noise caused by ground loops.

If you use a Laptop running on batteries, your USB ground will be isolated. Whereas running on a desktop PC the eart connection will be permanently present.

YOu can also use an isolted USB hub. This will allow you isolate the USB ground from the PC ground and eliminate the problem. Try Lindy for this type of USB hub: www.lindy.com

Best regards,
Michael
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Which hub?

Postby Olly B. » Wed Jul 19, 2006 11:44 am

Hi I have a similar issue using an ADC11/12 USB. Cany ou direct me to a suitable hub? I checked the Lindy site but could not find anything saying which were issolated (I'm guessing opto issolated?). Would this then have to have a seperate power supply ie not be a bus powered hub?
Thanks,
Olly
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Postby Michael » Wed Jul 19, 2006 11:48 am

Hello,

My apologies, please visit this website:
http://bb-elec.com/product.asp?sku=UISOHUB4&dept_id=145

And this:
http://opticis.com/product_5.htm

Best regards,
Michael
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Postby cpinkle » Wed Jul 19, 2006 12:09 pm

Hi!

Here I found a hub, I wyill use it to isolate my pc from my usb devices:

http://meilhaus.de/pdf/e_usbgt.pdf

Greetz Carsten
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Thanks!

Postby Olly B » Wed Jul 19, 2006 1:03 pm

Thanks for the lightning fast help guys!
Olly
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another option

Postby Aleksandar » Thu Jul 27, 2006 2:40 pm

Hi!

Thanks for Your answers, but all the solutions seem to be almost so expensive as the PicoScope itself.

Is it possible to make a kind of adapter USB -> USB, so that only data pins (Data+ and Data-) are connected, and Power pins (USBground und USB5V+) are not? Will the USB connection work when PC ground is not connected to PicoScope ground?

This kind of adapter would be easy to make (2 connections) and cocts nearly nothing (1 female USB connector and one male USB connector and 2 short wires). The question is only: Does the device need USB ground to be connected or not?

Code: Select all
USB Female (from PC)          USB Male (to PicoScope)

USBground                     USBground
Data-        ---------------- Data-
Data+        ---------------- Data+
USB5V+                        USB5V+


Thanks in advance,
Aleksandar
Aleksandar
 

Postby Sarah » Wed Aug 02, 2006 2:39 pm

Hi

Thank you for your post.

I would not recommend disconnecting the ground as I cannot guarantee that this will not damage something. Instead I think the isolated hub is a much safer option. If you wish to isolate at the input you could try isolating probes.

Best Regards
Sarah
 

Postby QMSteve » Mon Feb 02, 2009 12:28 pm

I would agree with Sarah that disconnecting the ground and +5V from the USB cable would be a singularly bad idea. The scope may operate OK without bus power, but still needs a reference for the two data lines (think of it as similar to RS485).
It is also worth remembering that most if not all USB isolators require an external power supply. This must itself be isolated from ground and, in the case of a hub with multiple outlets, it is very likely that plugging a grounded peripheral into any one outlet will ground both the power supply and the remaining outlets, so defeating the object.
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