USB Logic Analyzer isolator and voltage convertor - Page 1
Forum section: electronics / projects

author: badSCR
author_rank: Contributor
timestamp: January 14, 2010, 04:33:45 am
content: Just got a "USBee SX Logic Analyzer/generator" (24Mhz sample rate), So fair I like it.But It is not electrically isolated from computer and it can only handle 5volts.So to fix that this is what i am thinking.Input: "Circuit under test" feed into a 4volt Zener with a Resister, feed to a op-amp that would drive an Opto-Isolator, That would then feed into a USBee ch.Output: A USBee channel feed to a transistor that would drive a Opto-Isolator, that would control two transistors(npn/pnp) that would feed the "circuit under test".The result hope to get is to be able to sample/generate signals up to 25volts.

author: Unknown
author_rank: Unknown
timestamp: January 14, 2010, 02:26:42 pm
content: The zeners will influence your signal badly. You will pay an arm and a leg for 24 MHz-capable optocouplers. In the end you'll probably figure that it will cost you more to protect that thing than a second analyser would cost you.The only real problem I have with such toy USB stuff is not that they aren't very robust (you get what you pay for). But because they aren't isolated you can not only easily kill them but also your much more expensive PC.I know people who use such non-isolated USB instruments only with an additional dedicated USB hub between the instrument and the PC. They hope that if they mess up something the instrument and the USB hub will die fast enough so that the PC isn't affected.It is just a hope. And when it comes to your own safety letting a USB hub decide about your life or death isn't very clever.I'd say live with the analyser as is. Save your money for a spare analyser and never ever get the analyser out if there is anything more than 48 V on your work bench (not just the part of the circuit you work on, but your whole setup).

author: charliex
author_rank: Frequent Contributor
timestamp: January 14, 2010, 07:32:21 pm
content: slightly OT but perhaps somewhat helpfulhttp://www.circuitsathome.com/mcu/usb/usb-isolator

author: jahonen
author_rank: Super Contributor
timestamp: January 14, 2010, 07:49:18 pm
content: Like already said, reliably isolating 24 MHz digital signals with low skew and low pulse width distortion is difficult (I'd say that forget it). But based on the voltage levels you are after, you unlikely need the speed? If that can be derated significantly, then the isolation is much easier. How fast signals you are after?Regards,Janne

author: Unknown
author_rank: Unknown
timestamp: January 14, 2010, 08:53:15 pm
content: Quote from: charliex on January 14, 2010, 07:32:21 pm12 Mbps full speed USB. If we ignore the USB protocol overhead, and on the basis that the particular logic analyser has no own memory but generates 24 MHz * 8 bit = 196 Mbps, which then need to go immediately over the USB bus, that IC is 16 times to slow.

author: charliex
author_rank: Frequent Contributor
timestamp: January 14, 2010, 10:41:37 pm
content: yeah its a lot lower than needed, haven't seen a usb 2.0 hi speed opto isolator.

author: badSCR
author_rank: Contributor
timestamp: January 15, 2010, 03:42:19 am
content: Quote from: Andrew on January 14, 2010, 02:26:42 pmOK zeners are bad. I was wanting to take the lazy way.I could use a potentiometer and my Digital Oscope to set the voltage, then hook up the Logic Analyzer.Unless there is another cheap way of doing this.Can not see this thing seeing more than 24volts.

author: badSCR
author_rank: Contributor
timestamp: January 15, 2010, 03:56:28 am
content: Quote from: jahonen on January 14, 2010, 07:49:18 pmA max of 24volts.I would like to be able to use the full bandwidth of 24 million samples per second.and if that means losing the isolation, OK.