Opto-isolation of PC based instruments, groundplane, and current loops. / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Opto-isolation of PC based instruments, groundplane, and current loops.

1 2 next

Move Thread

LAN_403

Patrick

Tue Aug 24 2010, 06:23AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

a seemingly simple question....i have a USB based o-scope, i wish to measure isolated analog signals,
from my Capacitive Voltage Divider (CVD) prototype (10,000:1). is it possible to opto isolate the usb port or should i just use a laptop, with out the power tail, and firmly ground the laptop to my groundplane of interest.

so far the opto isolators commercially availlable are as expensive as a Asus EEEpc. the fastest optos i have are about 90ns on and another 90 ns off. i dont know a lot about USB digital signals so help would be appreciated.
480Mbps is way past my usuall playground, and one last thing, does USB have to be 480/12 Mbps or can it be throttled down?

my CVD thread->

Patrick

radiotech

Tue Aug 24 2010, 06:43PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

RS232 can use a fiber optic link. You can get a USB/RS232 dongle.

Patrick

Tue Aug 24 2010, 07:47PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

my usb oscope burst transmits at 480Mbps bulk, so the limiting factor is the high speed. the common optos dont seem to go beyond 60ns which is too slow.

maybe ill just use my laptop with a good star-ground and TVS diodes + MOVs

doctor electrons

Tue Aug 24 2010, 10:40PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Optical cables are standard equipment with fluke scopemeters when you opt to buy
the scopeview software with your scope. It can be done, check out flukes website. Look up scopemeters!
I actually have a cable for usb to optical on the scope end. If you can figure out the other end you can just buy the cable and
youre set!!

Patrick

Tue Aug 24 2010, 11:47PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Whoa! doctor electrons, that would be totally usefull, let me look that up!

it seems to be rs-232 to USB at a max of 57.2k baud.
]oc4usbiiiseng0100.pdf[/file]

I need a USB to USB isolator.
]uisohub4_1104ds.pdf[/file]
i guess i cant get away from the 280+$, becuase i cant figure out which hi speed optos there using. ill buy this one i guess,
dam it.

Sulaiman

Wed Aug 25 2010, 01:58AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Your 'scope SHOULD be earthed for safety
so there's no need to isolate the laptop from the 'scope.

If your 'scope is not earthed then see me after class !

The 'scope serial interface (RS232 or usb) is probably isolated from the 'scope internally
- check it's specifications or measure with a dmm.

Occasionally in an electrically noisy environment you will have to run the notebook on it's battery, but most power packs are galvanically isolated from the mains.

doctor electrons

Wed Aug 25 2010, 02:23AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Seems to me that two of those cables with the emitter and receiver flip flopped but directly across from each
other would be usb to usb. I use that exact cable attached to my fluke 199 and it will show signals up to 200 mhz
in real time on the pc. I can bet there is some kind of delay between actual sample time and when it is displayed on
the pcs monitor but isnt that how scopes work in the first place? The signal is sampled, properly triggered for correct display,
and finally displayed. I will borrow a second cable from work tomorrow, piggyback em and see what the result is.
On a side note, only ground the laptop if you dont mind frying it under a fault condition. A path to ground is a path to
ground, whether it be through you (electrocuting you) or through your laptop (electrocuting it). Maybe a more simple
solution would be an isolation transformer slipped into that usb to usb cable. ??

Bjørn

Wed Aug 25 2010, 04:22AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

USB signals do not tolerate any delays to speak of, the maximum cable length is 5 m because of that.

USB chipsets must support lower speed to be certified USB so you don't have to run it at 480 Mbit/s if that is the problem.

I have seen cheap capacitive isolators that might be fine with 12 Mbit/s.

Patrick

Wed Aug 25 2010, 04:50AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Sulaiman wrote ...

Your 'scope SHOULD be earthed for safety
so there's no need to isolate the laptop from the 'scope.

If your 'scope is not earthed then see me after class !

yes i know, without proper ground signal quality and human safety are questionable.
but the ground for the oscope is at different potential then the ground for the laptop, thus a opto-isolated usb cord is needed (480Mbps or maybe 12Mbps), i have no use for rs-232.

EDIT: i intend to star-ground everthing else but the laptop.

Wolfram

Wed Aug 25 2010, 08:45AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

The cutting edge in signal isolation now is magnetic isolators. They are quite cheap, and very much faster than most optocouplers. Here

is a two-dollar 150 megabit magnetic isolator. The fastest optocoupler I found in a quick search was ten times as slow as this. I don't know what options there are if you want to go higher than 150 megabit.

The USB data lines are bidirectional, so isolating them could be challenging, though not impossible.

Anders M.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob