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PICList Thread
'[EE]: USB isolation'
2000\08\28@094533 by Raymond Ranwez

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Dear forum,
I'd like to transmit physiological information from a human body (ECG) to a
PC.
USB is a possible solution for the connection to the PC, but I need a 4KV
isolation between the human body and the PC.
Is it stupid to think to isolate the USB bus?
Did someone implement an USB bus isolation?
Thanks for your help, and best regards,
Raymond Ranwez,
Belgium

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2000\08\28@114316 by Dan Michaels

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Raymond Ranwez wrote:
>Dear forum,
>I'd like to transmit physiological information from a human body (ECG) to a
>PC.
>USB is a possible solution for the connection to the PC, but I need a 4KV
>isolation between the human body and the PC.
>Is it stupid to think to isolate the USB bus?
>Did someone implement an USB bus isolation?


Hi Raymond,

Since the ECG has so much lower BW than USB, it might be a lot
easier to use isolation closer to the headstage amplifiers. Then you
also have greater latitude in selecting how to transmit the data
to the PC - USB, RS-232, etc.

best regards,
- Dan Michaels
Oricom Technologies
http://www.sni.net/~oricom
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2000\08\28@131543 by Olin Lathrop

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> I'd like to transmit physiological information from a human body (ECG) to
a
> PC.
> USB is a possible solution for the connection to the PC, but I need a 4KV
> isolation between the human body and the PC.
> Is it stupid to think to isolate the USB bus?
> Did someone implement an USB bus isolation?

You didn't say what the data rate or distances are, so I will assume "not
too much" and "not too far".  The USB spec includes electrical details, so
USB by itself implies a direct electrical connection and is therefore not
isolated.  However, at somepoint the signals on your board will be normal
logical levels, probably just inboard of the USB drivers.  That would be one
candidate for opto-isolation.

Another possibility is to connect the PIC to the USB without isolation, then
let that PIC talk to the rest of your system thru opto-isolators, perhaps
via IIC, SPI, or normal serial.

Keep in mind that 4KV isolation also implies a fairly large physical spacing
on the board.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olinspamKILLspamcognivis.com, http://www.cognivis.com

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2000\08\28@180820 by Andrew Warren

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Raymond Ranwez <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU> wrote:

> I need a 4KV isolation between the human body and the PC. Is it
> stupid to think to isolate the USB bus? Did someone implement an
> USB bus isolation?

Raymond:

B&B Electronics (http://www.bb-elec.com) has an opto-isolated 4-port USB
hub; more information is at:

   http://www.bb-elec.com/product.asp?sku=UISOHUB4&dept_id=145

Building your own opto-isolation is pretty difficult, mostly because
it's hard to know which direction the messages are traveling.

-Andy


=== Andrew Warren --- aiwspamspam_OUTcypress.com
=== Cypress Semiconductor Corporation
=== Interface Products Division, S.D.
===
=== The opinions expressed above do
=== not necessarily represent those of
=== Cypress Semiconductor Corporation.

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2000\08\28@221122 by Bob Ammerman

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You isolation should probably be as close to the human as possible,
preferably between your sensor and the USB interface.

ob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2000\08\29@035739 by Snail Instruments

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Hi Raymond,

you should be aware of another aspects of USB - there is maximum length of
the cable - only 5m. Perhaps you want to allow your experimental objects a
bit more freedom. Besides the USB protocol is quite complex, probably not
worth the hassle for a one of (or a few of) project.

If you still go for USB, perhaps something like body -> analog -> a/d ->
pic -> SPI isolation -> USBN9602(3) -> PC should work well. The advantage
of SPI interface is that its signals are all unidirectional. You could also
isolate the a/d from the PIC.

Josef

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2000\08\29@092128 by Raymond Ranwez

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Dear Forum,

Thank you to all for your answers (especially to Dan, Olin, Andrew, Bob and
Josef).
The goal of my e-mail was to be sure that I didn't miss the *magic*
solution.

To be more complete on my project, the PIC is on the body side, and it
acquires ECG signal through amplifiers. It also performs calculations like
the R-to-R time (R peak in the ECG signal).
That side is battery powered.

Finally, I think that RS232 is also good candidate for isolation (between
the PIC and the digital to RS232 converter), and has a direct connection to
the PC.

(I have another big project, with many channels and a DSP on the body side.
For this project, I will probably use RS422 or CAN, that are good candidates
for isolation, and a box that converts it into Ethernet. But it is another
story...).

Once again, thank you for your help, and best regards,
Raymond

{Original Message removed}

2000\08\29@111352 by Barry King

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Raymond,

Of course, the point of the isolation is to prevent a failure from
allowing fault currents to flow to the patient.  Whether its battery
powered or not is a side issue.  Battery powered equipment doesn't
have line voltage isolation to worry about, but a dangerous fault
current is still dangerous if a battery is powering it!  So the fact
that its battery powered makes the electronics simpler, but doesn't
automatically make it safe, right?

My understanding is that AC powered patient-connected equipment is
that the amplifiers are powered through isolation transformers, and
that the power available on the patient side is deliberately very
limited.

Then the signal is moved across the isolation barrier, (voltage to
frequency, then opto-couplers, maybe?) to the processing electronics,
whatever they may be.

My basic point is that it seems to me that you want to isolate as
little of the electronics as possible, and move the raw signals over
to the PIC and PC side of the barrier.  You may find that you can do
the patient-connected part once, and deal with all the fault
tolerance, etc.

Then you can freely build on increasingly complex processing, with
whatever interfaces are appropriate, without having to isolate any of
those interfaces.  In fact, I'd be surprised if there are no off-the-
shelf components to solve the entire patient connected side, and give
you isolated signals to build from.

Another resource that might help is that the US (FDA) and European
Union are working toward harmonizing the requirements for Medical
devices regulatory approvals.  One area of these regulations is
isolation and patient connected electronics.  You may find the
standards documents helpful.  Look at http://www.cemag.com Compliance
Engineering.  They are covering this Medical device standards
development, and what it takes to meet the specs, and may have the
standards available.  If it was me, I'd sure build to the standards,
even for special-purpose or one-off equipment.

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2000\08\29@123916 by Peter L. Peres

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>isolate USB

I don't know about medical grade optical USB isolation, but I know about
medical grade opto isolated RS232 and RS485 interfaces. I can't quote
supliers though.

Peter

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2000\08\29@131725 by Richard Crossley

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Just out of interest, from the notes I have at hand.

IEC 601-1 amendment 2 sub-clause 19.3 states that the maximum permissible
patient leakage current under a single fault condition (S.F.C.) must not
exceed 50µA DC where a S.F.C includes the failure of any single
semiconductor junction (sub-clause 17a). This leads to the requirement for
some form of current limiting circuitry to be inserted between the patient
connection and any circuitry which could conceivably supply more than the
allowed leakage current under a S.F.C.


Richard.



> {Original Message removed}

2000\08\29@154326 by Oliver Broad

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I don't think it's stupid but because of the details of USB I think it would
be too hard, if you use a USB adaptor chip with an SPI or I2C interface you
could isolate at the SPI link using probably only 3 or 4 fast optos which
could be a lot easier.

Oliver.

{Original Message removed}

2000\08\30@121108 by Raymond Ranwez

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Hi Barry,
Yes, I agree that a battery-powered system doesn't mean that we have a safe
system.
Anyway, with a battery-powered system or not, I have to deal with the
"Patient Auxiliary Current" test from the IEC601-1 (clause 19.1).

But we a battery-powered system, I have only one isolation barrier (for the
data) instead of two: one for the data, and one for the power. And the tests
to comply with IEC601-1 are much more simple.

With the USB solution, and a battery-powered patient side, all what I need
is a medical grade isolation between the PC and the PIC (patient side).
And with USB, it is easy to give the power supply to the PC-side isolation
electronics.
(I think it is also possible to do it with RS232, using DTR/RTS to power the
PC side isolation)

If I want to externally power the patient side, I will need a medical grade
power supply (wall-mounted), or at least a medical grade DC to DC converter
(primary power is taken from the USB).

BTW I didn't find a free and downloadable copy of the IEC601-1, EN60601-1 or
equivalent standard on the net.
Why do we have to pay for that?

Thank you for your mail, and best regards,
Raymond


{Original Message removed}

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