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how to achieve 8 isolated analog channels.

Discussion in 'Electronic Design' started by [email protected], Dec 20, 2004.

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  1. Guest

    In my 8 channel analog input board, i would like to have
    channel to channel isolation for 8 channels. in my circuit i take
    analog signal (4-20ma or 0-10v) from field and convert it into digital
    form, i also would like to have isolated power supply for each channel.
    To use seperate power supply for each channel is not possible(space on
    board,cost would matter!). can you suggest some
    circuit(configuration,ICs ) which will meet my all
    requirements(isolated channels,isolated power supply)?
     
  2. CBarn24050

    CBarn24050 Guest

    Subject: how to achieve 8 isolated analog channels.
    High impedence differential amplifiers more or less give you what you need.
     
  3. Guest

    any other ways to do it....
     
  4. Fred Bloggs

    Fred Bloggs Guest

    ISO-AMPs - run search...
     
  5. John Fields

    John Fields Guest

    ---
    How far do you want the isolation to go?

    That is, if you have eight floating outputs from the field feeding
    eight isolated analog inputs do you also need the analog outputs from
    your circuit to be isolated from each other and from the supply(ies)?
    Will you be using one ADC and multiplexing the inputs to it or will
    you be using eight isolated ADC's?
     
  6. Guest

    thanks for replying!!!
    if i use iso-amp at input end then,
    preferably i'll use one ADC(multiplexing inputs) or 8 channel 16 bit
    ADC, as eight seperate ADC would be costly matter.


    __
    sanjay
     
  7. If you need 8 isolated input and 8 isolated (12~24V/~30mA)? supplies
    to meet the requirements, that's what you need.

    Isolated-output DC-DC converters and isolation amplifiers are in
    order, or maybe you can design them yourself. Yes, it will be costly
    per channel and it will take up some board area. The reliable solution
    sometimes is like that.


    Best regards,
    Spehro Pefhany
     
  8. The 4-20mA inputs don't need DC-DC converters, they
    could be self-powered off the 4-20mA. Something simple
    maybe, such as an opamp and an IL300 optoisolator.

    Perhaps the OP could consider an isolated 8-channel
    4-20mA design, and deal with 0-10V inputs separately
    (as an optional extra, and for more money).
    The Analog Devices and Burr-Brown prices for analogue
    voltage isolator modules do give some indication of the
    technical problems.
     
  9. Guest

    Thanks for suggesting IL300,
    In application notes of IL300, +15 V suppy is used, but you say they
    could be self powered. how is that ?
    can you give more details?
     
  10. My assumption is that he *needs* the power supplies to power
    individual loop-powered transmitters on each input. This is the most
    general (and negative, and costly) assumption but will be the most
    reliable solution. Often a single isolated supply is used for several
    channels, but that's not as good, especially if the option is there to
    mix them up with voltage inputs at times.
    Which, of course, is nothing compared to the cost of field problems in
    this kind of system, as reflected in the prices of typical modules for
    the process control market.


    Best regards,
    Spehro Pefhany
     
  11. I was thinking of something along these lines.

    ----------------------------------------------
    One transmitter channel.
    | ~~~~~~~~~~~~~~~~~~~~~~~~
    \|/4-20mA
    |
    +-------------------+-----------+
    | | _|_
    | | _\ /_ IL300 LED
    \_|_ | T (2-10mA)
    /_\ 5v | [R4]
    | _ | |
    | +----+---|- \| |--+
    | _|_ | |Op >-[R3]--||nmos
    | D1_\_/_ | +-|+ /| |--+
    | | | | | |
    +------+------+-----+-----------+
    | |
    [R1] [R2]
    |100 ohms |
    +-----------+
    |
    \|/4-20mA
    |
    ----------------------------------------------
    One receiver channel.
    ~~~~~~~~~~~~~~~~~~~~~
    -+-5v
    |
    +---[R5]---------+--Vout
    | _ | |
    +--------|- \| |
    D2_|_ |Op >---+
    / \ +-|+ /|
    T | |
    +------+-----+----0v


    D1 and D2 are the IL300's optos. D1 and the opamp
    servo the IL300's LED to keep the D1 current equal to
    the R2 current, so measuring the voltage across the
    current sense resistor, R1.

    Run the LED over the range of about 2-10mA, making sure
    that the 5v voltage reference and opamp take less than
    2mA.

    The isolated receiver simply converts the current from
    the second opto into a voltage.
     
  12. Yes that is a consideration. ITSTM that on the last
    4-20mA job I did, everyone else assumed that someone
    else would provide the 4-20mA power..... me.

    [High cost of analogue isolator modules]
    True.
     
  13. There is usually one bulk 24VDC power supply for each unit of
    4-20 process inputs. All the loops are powered by the same supply.
    The input card may or may not provide isolation. Isolation is
    normally only needed if the loop supply is remote to the AI card/
    unit.

    For digital contact inputs there is again one bulk supply, usually
    of 100VDC, the input card supplies the dropping resistor and optos
    are used to pass the signal. 100 VDC is used to blow the contacts
    clean. 5v @ 2ma will not keep relay contacts in working condition.

    If the application is digital->digital over some distance then just
    plain 'ole optos are the safe way to go. The output of the driving
    gate supplies the power.

    Another method involves pulse transformers. The transformer secondary
    is across the contact. Closure is detected by monitoring primary
    current.
     
  14. Problems arise in systems where there can be several
    independant 4-20mA receivers in series on the same loop.
    This could arise where there is a control receiver plus
    independant receivers for safety monitoring or display.

    One receiver (usually the PLC) will demand that it occupies
    the easy place, (down on 0v, little or no isolation req'd),
    so all other receivers in series on that loop have to have
    a common mode capability.
     
  15. Rolavine

    Rolavine Guest

    Subject: Re: how to achieve 8 isolated analog channels.
    You could use a mos analog switch controlled through opto couplers, and then
    use one isolation amp for all 8, if you don't need them that fast. You could
    use 8 indivdual iso amps, that is not so impractical if you choose the CP Clare
    ones cause they are pretty cheap.
    Sorry if this has been said before, I missed it.

    Rocky
     
  16. Guest

    Thanks for suggesting clare chips!
    I give analog signals to 8 input channels then i convert it to digital
    form using ADC then i give this count to microcontroller.

    i.e....|iso amp|--->|?mux?|-->|ADC|-->|opto isolation|-->|uController|

    can i get good accuracy?
    If i want channel to channel isolation then... upto what extent should
    i give isolation?
    What should i use for my scheme
    a)mux & 1 ADC or
    b)8 seperate ADC's to achieve channel-channel isolation???

    if i use a)mux & 1 ADC then... can i call it as channel channel
    isolation??
    if I use b)8 ADC (12/16 bit) then...which is cheap & good accuracy ADC

    how to get isolated power supply for each channel from single source(of
    5V)
     
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