USB and optoisolator (6N137)

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rinandrew
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2006/02/20 07:58:25 (permalink)
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USB and optoisolator (6N137)

Hi !
I'm using a small EKG device connected to a 18F4550 microcontroller, to send EKG data to a PC with an USB interface. Due to security reasons the PIC and the PC have to be eletrically separated by an optoisolatior.
I have 2 6N137 optoisolators, but I cannot find a schematic to connect them to the USB bus. Can anyone help me ?
 
Andrew
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    kalpak
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    RE: USB and optoisolator (6N137) 2006/02/20 08:29:20 (permalink)
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    USB is bidirectional bus. You cannot just stick in a couple of optos and get isolation. You need bidirectional data transfering isolation.
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    rinandrew
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    RE: USB and optoisolator (6N137) 2006/02/20 08:48:15 (permalink)
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    Hi Kalpak,
    thank you for your information. Have you any suggestion on how isolate the USB bus ? I cannot find nothing about it.
     
    Andrew
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    nex
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    RE: USB and optoisolator (6N137) 2006/02/20 09:09:40 (permalink)
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      Hi,
     
     You should make the separation between EKG sensors and PIC. I'm curious, what kind of sensors do you use ?
     
     Cheers !
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    kalpak
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    RE: USB and optoisolator (6N137) 2006/02/20 09:35:46 (permalink)
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    B&B electronics has isolated USB hub:
    http://www.bb-europe.com/product.asp?SKU=UISOHUB4
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    rinandrew
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    RE: USB and optoisolator (6N137) 2006/02/20 13:05:32 (permalink)
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    Thank you for your help !
    The isolated USB hub is to expensive for my low budget.
    However the EKG sensor is connected to a dsPIC (30F3012), that communicates with the 18F4550 through a SPI bus. The SPI bus seems easy to isolate with optoisolators.
     
    Andrew
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    msc41
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    RE: USB and optoisolator (6N137) 2006/03/04 15:24:07 (permalink)
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    I designed a similar device, what I ended up doing was to use a IL717 spi optical isolator and isolate an external ADC away from the the USB powersupply. Hope this helps.
    Mat


    http://www.piccoder.co.uk
    where everyone can upload and share pic code/ideas.
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    Guest
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    RE: USB and optoisolator (6N137) 2006/03/04 15:49:32 (permalink)
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    What I do on my EMG systems is to isolate the analog signal that comes from the EMG preamplifiers. I use very low power preamps, and get a 3500VDC isolation. The problem with isolation is that you need to use a isolated power supply to feed the circuitry on the patient side, and it is really not trivial to get FDA specs isolated power supplies, so the amount of circuitry you can place at the isolated side is small.
     
    What power supply are you using for the isolated section?
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    msc41
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    RE: USB and optoisolator (6N137) 2006/03/05 05:10:36 (permalink)
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    I'm currently running off a standard 9V battery, the life is pretty good, not fantastic, but im not drawing much more 15mA, have to confess its still a work in progress so battery life still needs investigating fully, but at the moment it is adequate for my purposes. Mat

    http://www.piccoder.co.uk
    where everyone can upload and share pic code/ideas.
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    RE: USB and optoisolator (6N137) 2006/03/05 08:32:32 (permalink)
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    Optocouplers have limited bandwidth.
    The USB interface runs above 1MHz.
     
    I would not use optical coupling above 200-300KHz, escpecially if the human body was involved.
    Also they are prone to optical ageing, which alters specs.
     
    Try to design isolation circuitry around parts like TI ISO150, ISO721, Analog Devices ADUM1100. These have response times 2-3 orders of magnitude faster than opticals, and cost about the same as the best optos.
     
    I would not implement the isolation barrier in the analog domain. Do it in digital, it's more reliable. With the above parts, you have no choice, they are digital only.
     
    Andy
     
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    davidcary
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    RE: USB and optoisolator (6N137) 2006/03/21 16:29:27 (permalink)
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    I agree with funberry's recommendation to implement the isolation barrier in the digital domain.

    I don't understand what the point of saying "Optocouplers have limited bandwidth." Doesn't everything have limited bandwidth? Please tell me about *anything* that has unlimited bandwidth.
    • 6N137: 10Mbaud
    • HCPL-060L: 15 Mbaud
    There is such a thing as a bi-directional isolator .

    --
    David
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    lbodnar
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    RE: USB and optoisolator (6N137) 2006/03/22 04:39:54 (permalink)
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    Doesn't everything have limited bandwidth? Please tell me about *anything* that has unlimited bandwidth.

    Illusions and confusions.

    Leo
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    davidcary
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    RE: USB and optoisolator (6N137) 2006/03/23 19:38:50 (permalink)
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    rinandrew mentioned:
    However the EKG sensor is connected to a dsPIC (30F3012), that communicates with the 18F4550 through a SPI bus. The SPI bus seems easy to isolate with optoisolators.


    I am working on a similar project.
    At the moment my napkin-sketch of the signal chain looks something like this:

    Several copies of
    * standard EKG contact electrodes and contact gel
    * instrumentation amp on top of the electrodes -- INA2126 or INA163
    * MCP3208 (8 input, 12 bit, 100 kS/s ADC)(probably overkill)
    * "small" microcontroller, barely smart enough to forward the data from the ADC to the "big" microcontroller.

    One copy of:
    * "big" microcontroller
    * batteries
    * 4 Mbps IrDA transciever
    * 4 Mbps IrDA-to-USB dongle
    * desktop/laptop computer

    I see that the dsPIC30F3012 includes a ADC (8 input, 12 bit, 100 kS/s ADC), exactly equivalent to a MCP3208.
    So I could easily use it to replace the MCP3208 + "small" microcontroller.

    What advantages do you see to the dsPIC in this application?
    To me, the biggest disadvantage to the dsPIC is that its data sheet still has "Preliminary" stamped all over it.
    I'd be willing to overlook that if it gave me some other significant advantage.

    The batteries and the IrDA transcievers give me plenty of isolation between the desktop/laptop computer and the EKG electrodes.
    Is there any reason to add more isolation between each ADC and the "small" microcontroller, or between each "small" microcontroller and the power/data cable that connects it to the "big" microcontroller?

    --
    David

    http://openeeg.sourceforge.net/
    http://wiki.asiaquake.org/openeeg/published/

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    plugwash
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    RE: USB and optoisolator (6N137) 2006/04/07 14:08:13 (permalink)
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    if you look in the datasheets for the pic you will notice they support an external usb tranciever and the lines to that can all be opto-isolated afaict.
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    kalpak
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    RE: USB and optoisolator (6N137) 2006/04/07 18:42:46 (permalink)
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    Do you have a simple scheme for optoisolating the USB?
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    plugwash
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    RE: USB and optoisolator (6N137) 2006/04/08 09:28:42 (permalink)
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    there is a basic block diagram in the bottom right of page 165 of the datasheet. I haven't needed isolated usb myself so i dunno how hard it is to actually develop that into a working circuit.
    #16
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