Ground loop isolation

Discussion in 'General Electronics Chat' started by vortmax, Mar 2, 2013.

  1. vortmax

    Thread Starter Member

    Oct 10, 2012
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    18
    I'm trying to figure out the best way to obtain ground loop isolation for a USB data acquisition card.

    The card in question will have an MCU and be powered from a computer over USB. The MCU will be used for GPIO and to read several differential ADC's. The problem is that the ground level of the equipment can be 10's of volts higher than mains ground, which the computer will be held at.

    I want to avoid sourcing power from the equipment, so I need an on-board, isolated DC-DC converter. I have found one (TI DCR01) that appears to do what I want, but I cannot find any spec for the max ground difference. All I see is an isolation flash test rating of 1kV RMS. Anyone know more about this?

    If this is fine, I would then just power my ADC's on the equipment-side rails, and pass the communication bus through level translators.

    Does this all seem like a reasonable approach? Anything I'm overlooking?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    An isolation test of 1kV RMS would indicate that it can tolerate that voltage on a short-term basis. It should certainly have no problem with tens of volts continuous.

    What type of bus level translators where you thinking of?
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Just isolating the power alone will not solve your isolation issue as the USB bus itself is referenced to the same ground, and that is the hard part to isolate. There are USB isolators, both complete units and chips available. Any ole wall wart can deliver isolated power.
     
  4. vortmax

    Thread Starter Member

    Oct 10, 2012
    103
    18
    I was thinking about MAX14842 for bus translators. It's a 6 channel (2 in, 2 out, and 2 bi-directional open collector) translator that will tolerate a 72V difference in ground reference.

    The USB power and data bus should be completely isolated as is. The data bus will only supply a UART through an FTDI chip to the MCU. The only things crossing to the other side are digital IO lines from the MCU which will be run through bus translators.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    1,222
    I have done this for serial port using optocouples and a isolated DC to DC converter. To power the other side of the optocoupler.
     
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