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optoisolated power supply

A friend has asked me to look into the possibility of constructing a one-off Arduino-compatable 8 channel muscle/EMG sensor for experimentation purposes, and gave the documentation of the commercial (single channel) product available here:

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as a spec for the type of device he would like. The product's design seems to be released under a fairly permissive license, so one could theoretically whip up an 8 channel version and distribute it as one wished with no problem.

However...

The design as it stands has some problems, the most obvious of which is that it does not make any attempt to meet any requirements for medical equipment, or provide any form of isolation between the body and the power/ground lines of the host computer's USB port.

A purpose-built piece of equipment would use an isolated supply for everything and run the USB lines to the host through an optoisolator, but because the device is supposed to be used with an Arduino, which uses the USB ground as reference ground for signals coming in to its A/D converters, this makes proper isolation of the body connections somewhat more difficult.

To provide isolation, my thought was to use a pair of 4 channel optocouplers to couple the EMG signal from the input instrumentation amplifiers into the rest of the circuitry. Since the current requirements of the input amps are pretty low, their supplies could be isolated by using an "optoisolator power supply," maybe as simple as some photodiodes charging up capacitors. The instrumentation amp IC used here can run with as low as +/- 1.35 volt rails. Does anyone make

4 channel transistor optocouplers with the base terminals brought out, so they could also be used as photodiodes?

Please bear in mind that I realize medical standards are stringent, and I probably do not have the means to test whether a device so constructed adheres to all such requirements. I simply wish to provided an added level of protection for someone who is aware of the risks involved with such experimentation, however small. I myself do not intend to mass-produce the device or make it available for sale.

Reply to
bitrex
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I could be wrong, but would have thought even one noncompliant device would leave you exposed to liability.

What sort of output & efficiency would you expect from an optical power bridge?

NT

Reply to
NT

you could just buy a USB isolator, simple box usb in usb out, data and supply isolated

Reply to
langwadt

Why run the USB lines to the host through an optoisolator? You merely have to isolate the body from whatever plugs into the wall socket. Isolate the sensors.

Photodiodes don't make very good solar cells. Why not just use a pair of button cells to power the patient-side equipment?

Use optocouplers designed for this purpose. Find some app notes for optocouplers in medical equipment. Don't use guesswork.

Reply to
spamtrap1888

The risk is nil. The PC is grounded, or a laptop is floating, and the USB port is grounded to the PC chassis. Nobody is going to get hurt by being grounded, or connected to 5 volts.

John

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Reply to
John Larkin

e

depends on where you are, around here almost nothing is grounded so if you touch the pc chassis and something grounded, you get quite a shock.

lots of soundcards etc. lost their lives when connecting a cable from a pc to a tv grounded with cabletv

-Lasse

Reply to
langwadt

"The risk is nil." Indeed! Only Bitrex need worry about being sued :-)

And, yes, I have designed and built multiple units meeting medical isolation standards. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

You must be terrified to touch a faucet, or open a dishwasher... both are grounded. But maybe you never do either.

We are always impressed by your ability to make claims.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Reply to
John Larkin

Those appliances aren't "medical devices".

I'm sure you are ;-)

I even once designed (and built, discrete parts) a 5V/400AMP switcher than was VDE and medical qualified (for ADR Ultrasound). (I can't remember the official name of the medical standard... the hand-drawn schematics are in off-site storage... paper, ~1975BC (BC=before CAD :-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Joerg would know the standard. I'm sure everything he designs is a lot safer than a faucet or a dishwasher :-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Thanks for your reply, you bring up some good points - I'm working through reading such an app note currently. I had wanted to avoid using battery power for the sensors, but given that the complexity of the alternative seems to be increasing it is probably the best solution. Given the low current draw of the input stages, the useful life of even button cells would probably be quite long.

An ill-conceived saftey system may provide a false sense of security, and in some sense be worse than no protection at all.

Reply to
bitrex

You're not going to beat this UH401

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, just power your EMG gizmo and Arduino from a medical grade isolated supply.

Reply to
bloggs.fredbloggs.fred

All my water and drain lines are PVC. What kind of ground is that?

Reply to
Michael A. Terrell

formatting link
, just power your EMG gizmo and Arduino from a medical grade isolated supply.

Wow, those are a great deal. Thanks for the heads-up!

Reply to
bitrex

You can get some nasty blasts from grounded equipment during a storm, it's just not a good idea to have anything connected to a living body at the time.

Reply to
bloggs.fredbloggs.fred

not a good idea to have anything connected to a living body at the time.

DANGER

ZERO VOLTS

Reply to
John Larkin

That's a good attitude. 'Design and Development of Medical Electronic Instrumentation' byt Prutchi and Norris has a nice chapter on desgining safe medical device prototypes. The component selections are a little bit outdated, but the general stuff is still ok.

When using emdical electrodes, the impedance can sometimes be surprisingly low. This means that even battery voltages can cause dangerous currents if not limited by device design.

--
Mikko
Reply to
mi

Then never touch anything that's grounded. Faucets. Power strips. Dishwashers. Pipes. Printers. Computers. Bathtubs. Play structures. Trees. Dirt in the back yard.

The objective is a home experiment to pick up EMG signals, not a certified, insured medical product to be used in an OR.

Reply to
John Larkin

Depending on the level of protection provided, anything that is even remotely 'patient applied' must limit leakage currents to safe levels, even if the patient's body or other attachments are somehow elevated to live line voltage, with respect to safety ground.

Grounded terminals are a non-starter.

Standard interfaces that specify a ground are non-starters, unless the connectors cannot also be accidentally misapplied by the end user. Approvable hardware might look like (and work) in the standard socket, but there will be no through-earth contact provided by either side.

RL

Reply to
legg

Perhaps I'm limited in some way, but I can't think of a single sensible use for the advertised product that the OP's buddy want's duplicated.

At the same time, it'd hard to see how it might actually kill somebody, when used as directed. It often is.

The imagination of of others, in doing odd things with themselves and others, at home, doesn't seem to have any limitation.

RL

Reply to
legg

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