Isolating a USB scope? - Page 1
Forum section: electronics / testgear

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 18, 2014, 07:20:29 pm
content: I have a few ADUM4160, usb isolation chips,http://www.analog.com/en/interface-isolation/digital-isolators/adum4160/products/product.html, I ordered these guys because im always working on projects that are powered on via USB. Whether it be probing around them with a scope/meter or swapping out resistor values. I have ended up blowing quite a few passive USB hubs this way, or windows will gripe at me the USB controller has tried to draw to much current, and I will have to restart my computer to get everything working again. Of course this only happens if I accidently short something.I only own a USB scope, and it is a little noisy down in the mV range. If any of that noise is caused by the PC'd USB, an isolater should take care of it. But what happens if I isolate my scope and then I try to probe a board that is plugged in to the wall or a non isolated USB? I.E something that references earth ground? I would rather not blow my scope up, I remember there was a video about this a while back.

author: Stonent
author_rank: Super Contributor
timestamp: May 18, 2014, 08:28:36 pm
content: Generally if you are isolated, you have less risk of blowing it up because you're not risking a path to ground, however, it does increase your chances of being shocked.A few months back a piece of test equipment at work had burned up a few USB cables in a short bit of time, because there was no ground isolation between the device and the tester so when there was a short, it would take the ground pin on the USB cable, across the computer's motherboard to the chassis to the outlet.Turns out the isolation was supposed to be there, but someone who didn't know there was an isolation transformer in the power supply tied the GND pins on the two isolated parts together to the chassis of the cabinet.If you do isolate your USB scope, you still have to power it, and by isolating it, you've disconnected it from the PC as a power source. So you will need to look into some kind of 5V battery pack otherwise you'll have to use an external powersupply which could reintroduce noise and remote the isolation.But they do make isolators:http://www.bb-elec.com/Products/USB-Connectivity/USB-Isolators/USB-Isolators.aspxWe have one device at work that's outside the building but connected to a computer inside. It runs using a USB to fiberoptic cable converter. That at least isolates it from the PC in case of a lightning strike.And noise in general could be from anything, lights, nearby electronics, storm activity etc.

author: Mr Simpleton
author_rank: Unknown
timestamp: May 18, 2014, 08:41:35 pm
content: These chips are USB 1 only... USB2 goes up to 480 Mbit/s and I have yet to see something that isolates USB2I tend to use my laptop running on batteries instead, after toasting the HPIB interface in my spectrum analyzer.

author: Stonent
author_rank: Super Contributor
timestamp: May 18, 2014, 09:04:10 pm
content: Quote from: Mr Simpleton on May 18, 2014, 08:41:35 pmOverkill, but it will do it.http://www.blackbox.com/Store/Detail.aspx/USB-Ultimate-Extender-over-Multimode-Fiber-4-Port/IC404A

author: Stonent
author_rank: Super Contributor
timestamp: May 18, 2014, 09:08:50 pm
content: Found this, much cheaper:http://microcontrollershop.com/product_info.php?currency=USD&products_id=3769Oops, misread the specs. USB 2.0 but only 12MBps

author: wraper
author_rank: Unknown
timestamp: May 18, 2014, 10:39:41 pm
content: Quote from: ronaldlijs on May 18, 2014, 10:31:22 pmUSB2.0 Full speed 12mbps, no high speed.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 19, 2014, 03:34:57 am
content: Those boards use the same chip I plan to use, the difference is he is using switching regulators for power, I had planed to use linear regulators for a cleaner signal. My scope is only 25 MegaSamples so im pretty sure there shouldn't be any bandwidth limitations at 12mbits. Ive also noticed If I spit 1mv out of a DAC and measure it with a DMM, everything is fine. When I use my scope I get like -4mv's. Could this have something to do with earth reference to ground through the USB, vs a floating ground reference like a meter?

author: function
author_rank: Contributor
timestamp: May 19, 2014, 05:16:48 am
content: I also made a USB isolator using an adum4160 and a TI DCP020505 so that i could get 400mA (Adum5000 is limited to 80mA). Unfortunately for you it's a switching supply, the claimed ripple is 20mVpp

author: euzer
author_rank: Regular Contributor
timestamp: May 20, 2014, 03:59:43 pm
content: Olimex do onehttps://www.olimex.com/Products/USB-Modules/USB-ISO/