Can I measure a floating voltage with an instrumentation amp?

Discussion in 'General Electronics Chat' started by JustWood, Jun 23, 2010.

  1. JustWood

    Thread Starter New Member

    Jun 16, 2010
    3
    0
    I am trying to measure the current through a precision 1 ohm resistor.
    To do this, I have an INA126's (TI instrumentation amp) inputs connected across the resistor.
    I am feeding the output of the amp into the analog input pin of a microcontroller.
    I know nothing about the circuit I'm measuring, and I don't know if I can connect the "negative side" of the 1ohm resistor to the same ground as the in amp or the microcontroller.
    I am pretty sure the in amp won't be able to measure a floating voltage like that, right?
    If not, how can I do this?

    Thank you!
     
  2. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    You have to confirm that either the sense resistor is electrically connected to the measurement circuit, or it is isolated. Then you have to characterise the type of connection - if it is electrically connected then you need to understand whether it is for instance within a bounded range of voltage from your 0V point - if it is isolated then you need to understand if any parasitic current could flow in any way via your measurement circuit.

    If you have a good floating circuit connected to the sense resistor then it is fine to connect to the IA - imagine a battery connected to the sense resistor.

    Ciao, Tim
     
  3. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Oops, that first sentance was a bit clumsy. The reference to 'isolated' is for a floating circuit connected to the sense resistor. The reference to 'electrically connected' is for a circuit that has the sense resistor in it, but also has other electrical connections to the measurement circuit (ie. IA and uP) - possibly through devious paths.

    Ciao, Tim
     
  4. JustWood

    Thread Starter New Member

    Jun 16, 2010
    3
    0
    Well the "circuit" I'm trying to measure is just a DC power supply of some sort (It's a metal box you plug into the wall, and out the other side comes a red and black wire carrying a DC voltage). It also generates a frequency, etc (on a separate pair of wires). I do not know if the black DC wire is connected to earth ground.

    The only two wires from my circuit touching the other circuit at all should be the + and - terminals of the IA connected to the two sides of the resistor.

    My circuit is connected to the wall through a transformer though. So one side of the transformer goes into the wall, and the other side is center tapped providing a +, gnd, and - (which are then rectified, regulated, etc.). As long as my power supply transformer isolates me from the wall, I should be fine, right?

    As in I can safely connect the IA "-" terminal to ground (at least through a big resistor?)
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You are probably correct in that the DC output is not referenced to earth ground. And that the transformer gives safety isolation from the line.

    Why not use a meter to check the DC output, though? That has to be simpler than an instrumentation amp. If you want to get experience with an IA, mterering the DC level is good to do so you know what to expect.

    The convention should have the black lead out of the supply as the negative, so the red lead should be positive with respect to the black. This is another useful function for a meter - to eliminate surprises. Anyway, once polarity is established, attaching the black lead to your circuit ground gives a reference, so the red lead's voltage may be applied to any old amp.
     
  6. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    It seems to me the answer to your question depends on whether the input to the instrumentation amplifier is a differential input -- and, if so, how much the inputs can be floated (either positive or negative) with respect to circuit ground. Then, as beenthere said, use a voltmeter to determine the voltages with respect to ground on either side of the precision shunt resistor over the range of currents that will be supplied and you'll know whether things will work or not. Thus, your key tool is the datasheet for the amplifier.
     
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