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Connecting a grounded single ended source to a differential input amplifier via coaxial cable.

Discussion in 'Electronic Design' started by [email protected], May 16, 2007.

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  1. Guest


    I would like to know what would be the best way to connect a grounded
    single ended signal to a floating differential input amplifier?
    The diff. amp is powered from an isolated power source. The chassis of
    the box which contains the diff amp is grounded to the same earth as
    the sensor.
    The sensor output connects via coaxial cable to the diff amp. The
    shield is earthed at the sensor side and the other side connects to
    the diff amp negative input. The inner core of the coax connects to
    the positive of the diff amp. This is done to avoid ground loop
    currents in the shield.

    Is it ok to connect an unbalanced cable to a diff amp? I know I lose
    the advantage of common mode noise rejection.

    Is this better done with a single ended input amp? What about ground
    loop currents then?

    With the differential input, what kind of biasing resistors are
    necessary for diff amp input bias currents?

    Thank you for any help!

  2. John Larkin

    John Larkin Guest

    Don't see why.
    Ground loops? From where? The amp supply is isolated.
    If you do this, something has to establish the common-mode voltage!

    What's the signal bandwidth here?

  3. Joerg

    Joerg Guest

    Worst case there is always Mini-Circuits and their little transformers :)
    Since there ain't too much happening in politics right now you could
    have mentioned split grounds here, then we'd have another hot-blooded
    discussion ...
  4. PeteS

    PeteS Guest



  5. Guest

    Hi John, thanks for the reply.

    Signal bandwidth is up to 100kHz, but propably a lot less.

    I say I would lose the advantage with common mode noise rejection
    because I connect a coaxial cable, which shields the inner core, to
    the diff amp,
    and hence the noise on the 2(1 shield and 1 inner core) conductors are
    not the same. A twisted pair would have been better- but the customer
    already has 100s
    of meter of coax laid out. Or am I wrong here?

    The customer wants to connect the shield to the chassis of the
    measurement instrumentation, and that would create a ground loop, so
    he will short isolated ground
    to chassis ground. He claimes that gives better readings. He thinks
    the noise on the shield needs a good return path, and connecting to
    the instrument chassis on one end and earth, which is already a very
    good return path, on the other end is a good idea.

    I already have a diff amp, so that's why I am leaning towards it, I
    also thought it would be a beter solution. The biasing resistors would
    connect down to isolated ground.
    What do you mean something must establish the common-mode voltage?

    Why do you think the single ended amp would be better? Obviously, then
    the customer should not short the shield to the chassis.
    I can make my diff amp single ended by connecting the negative input
    to (isolated) ground- this is done with a mosfet switch with 1ohm Ron.

    Any other suggestions as to how to best measure grounded signal
    sources with isolated amps?

  6. Guest

    Try wrapping the coaxial cable around a ferrite toroid. This turns it
    into a balun, also known as a transmission line transformer. Googling
    for "balun 'transmission line transformer' " threw up a number of
    links. The first one

    actually describes more complicated variations on the standard 1:1
    balun as does

    I had to dig down quite a way to find a link that even talked about
    the standard 1:1 unbalanced-to-balanced application

    My favourite reference is to Matick R E "Transmission-line pulse
    transformers - theory and applications" from the Proceedings of the
    IEEE volume 56, pages 47-62 (1968) but you need a university library
    or a subscription to IEEE Explore to get hold of it.
  7. Fred Bloggs

    Fred Bloggs Guest

    It is not a "ground loop," it is a major line imbalance and uncontrolled
    reference return...

    That is probably right but not for the reason he thinks. The chassis to
    isolated ground connection eliminates noise current injection into the
    IA input due to potential differences between the two grounds. This will
    be going on internal to the chassis.

  8. Fred Bloggs

    Fred Bloggs Guest

    That used to be called a "coaxer" and it is a very high frequency common
    mode choke, not a balun...I have used this method and it works well.
    The largest interference will most likely arise from external current
    noise sources magnetically inducing a differential mode voltage into the
    coax. He has no option except to voltage-to-current convert the sensor
    output and terminate in a low impedance at the IA in that case, assuming
    he has to use coax and maintain a wide signal bandwidth.
  9. Paul Mathews

    Paul Mathews Guest

    It's easy to show that common mode rejection absolutely depends on
    having the impedances for both differential inputs exactly the same.
    Provided that both inputs are maintained within the common mode
    voltage range, the diff amp doesn't care, so to speak, whether the
    signal is single-ended or what some folks call 'true' differential.
    There is much practical information on this subject available at these

    For audio signals, the best results can be obtained using

    Paul Mathews
  10. Guest

    Thank you all for the replies- the whole problem is still unclear to
    me though.

    Also, this is not an RF application- it is an accelorometer sensor app
    with a single ended voltage output.
    The fact that the diff amp runs from an isolated supply make matters
    worse, sine the customer wants to connect his
    signal gnd to the diff amp chassis- which is at the same ground as his
    signal gnd- but the diff amp runs off its own isolated supply.

    Would it be wrong to just connect the shield to the one input of the
    diff amp and the inner core to the + input of the diff amp? And not
    connecting anything to the chassis.
    John says a single ended amp would be better. But the single ended amp
    would also be isolated (all the input channles run from an isolated
    supply in this measurement system).
    Most solutions show a 2 wire + shield cable- I have only 2 conductors,
    the shield and the shielded inner core.

    Is it so difficult to measure a ground single ended signal with an
    isolated diff or single ended input amp?

  11. whit3rd

    whit3rd Guest

    Just put a resistor from the 'ground' wire of your signal cable to
    the local ground. That will keep the common-mode in range if the
    distant end's 'ground' connection is ever allowed to float. Any
    other signal-conditioning (filters, clamps, etc.) will be the
    same regardless of the wiring scheme.

    Do be careful, though, that the 'ground' wire of the signal cable
    doesn't connect IN MULTIPLE PLACES to any kind of building
    frame or other grounded metal; minor ground-fault currents
    can be guided through your cable and the resulting I*R voltage drop
    can swamp a small signal. The resistor-to-ground at the amplifier
    end should be chosen to be higher value than the wiring resistance
    so it won't mess measurements up. Much.
  12. I think that if your signal comes in over a cable longer than
    a meter or two, the few uH of common mode inductance added by
    this kind of balun is not going to make much of a difference.

    Low frequency current flowing on cable shields (common mode
    current) ends up leaking in because of the cable shield
    resistance, so shield currents are to be avoided. Baluns don't
    help against low-frequency common mode current. Transformers
    and differential amplifiers do. The choice of which is better
    depends on the signal bandwidth.

    Would you hire Bill or me, John? ;-)

    Jeroen Belleman
  13. Jamie

    Jamie Guest

    in cases like that, you are losing the balanced use of the input.
    In my opinion, you should be tying the shield side to common.
    Just use either input of the amp if polarity isn't a problem,
    and tie off the other input so that it won't detect any noise.
    I do think if you tie the unused input to common also, you'll
    be all set.
  14. Guest

    My 'common' is isolated and floating with respect to the sensor's
    Tying the shield to common, will short my isolated ground and break
    the isolation barrier.
    And a current might flow in the shield, inducing noise in the inner
    core, because of potential differences in
    the 2 'common' points.
    Remember, my input stage is floating.

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