4 wire measurement vs wheatstone bridge

Discussion in 'General Electronics Chat' started by Ravi Kanth, Aug 10, 2015.

  1. Ravi Kanth

    Thread Starter New Member

    Aug 10, 2015
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    I have to measure changes in resistance to the 3rd or 4th decimal, for ex 3.0145.
    I am using a 4 wire measurement with a very good constant current source. I have a 16bit DAQ device to measure the voltage.

    I keep getting questions on why one would be used over the other. I have trying to get this answer from various forums but I still keep getting doubts.
    Would love if somebody explained this and which to use as plainly as possible. A discussion on the advantages and disadvantages would be great as well.
     
  2. #12

    Expert

    Nov 30, 2010
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    A Wheatstone bridge is used to form a proportionality which is somewhat isolated from variations in the power supply voltage. A constant current source is used to create a current which is not dependent on the power supply voltage. Both circuits do the same thing in different ways. The only thing left is what parts are available and how good you need it to be. Then you bring in the math.

    I am not all that good with counting bits that I think this is a fun thing to do for breakfast, but other people here are. Want more answer? Respond here.
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In the overall measurement system, the current source and A/D reference voltage both are sources of error that are independent of the A/D converter's quantizing error, non-linearities, etc. If your current source and A/D voltage reference both have greater than 16 bits precision, then 4-wire will give you the most repeatable results. Watch out for thermocouple potentials (forget the real term) caused by junctions of dissimilar metals when two wires are connected together.

    ak
     
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  4. Ravi Kanth

    Thread Starter New Member

    Aug 10, 2015
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    So combining what I understand from both these answers is -
    1. If I have a good current source and a 16 bit precision acquisition device, I could get comparable accuracy/precision as a wheatstone bridge.
    2. A 4-wire measurement will give me the most repeatable results( Though I do not yet understand why?)


    3. This is more of another extension of my question -
    Considering I have the DAQ device and the current source and am free to build a wheatstone bridge as well, what would one choose.

    I apologise if the question seems repetitive and trivial but I am really under pressure to understand this and just reading up the basic theory doesnt seem to help me. Is there any known web resource which compares these two systems of measurement?


    Thank you so much for explaining this so far. Much appreciated.
     
  5. snav

    Active Member

    Aug 1, 2011
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    AFAIK 4-wire measurement from DVM already has the precision current source and A/D conversion plus elimination of connection resistance. very useful for low resistances. Wheatstone does not eliminate connection resistances.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    This might convince you why the 4 point method is better. At least you don't have to deal with an inverse lookup table.

    261.gif
     
  7. Ravi Kanth

    Thread Starter New Member

    Aug 10, 2015
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    I must admit I would need a little more explanation on what I am looking at?
    I understand its a comparison of the voltage drop measurements between wheatstone bridges with voltage and current excitation sources and the 4-wire system but what does it exactly tell me?
    I apologise again for sounding thick skulled.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Aside from the device under test (DUT), a Wheatstone Bridge has four variables, while a direct 4-wire measurement has only one. That alone is reason enough to rule out the bridge.

    Also, remember that a bridge measurement system is based on the idea that 0 (zero) can be detected with great precision when absolute parameters (voltage, resistance, etc.) cannot. The original Wheatstone bridge had a large analog meter with the null position in the center so that the needle could swing either way, and a big fat 0 marking the center position. By itself the bridge does not measure things, it measures ratios of thngs. To make your measurement to 16-bits, you need three greater-than-16-bit-precision resistors. That's less than .00076% tolerance.

    ak
     
    Last edited: Aug 11, 2015
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  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Look at the linearity of the three outputs. Both Bridges have a non-linear voltage vs resistance plot, while in the 4point circuit, the voltage is Proportional to resistance.

    Most folks forget that the proper method of using a four-arm bridge is to vary a standard resistance arm of the bridge until the V(a1)-V(b1) output goes to zero; not use it as an Ohmmeter.
     
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  10. Ravi Kanth

    Thread Starter New Member

    Aug 10, 2015
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    I must admit I did not think about the non-linearity, thanks for the graph. :) That makes a very convincing argument next time someone asks me that.
    However in my problem, I am dealing with very small changes in the resistance, mostly in the 3rd/4th decimal. What would be the minimum range to observe the non-linearity in your experience?

    I should add to this question sequence - Why would one still use the bridge configuration with strain gauges? Especially when the changes I see in the strain gauge resistance are similar to my sample's variations as well?
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You do the math.

    With a four arm stain gauge, all four of the arms are changing resistance simultaneously, two +Δ, two -Δ. The resistors are well matched for Tempco, and all four are subject to the same temp. variation.
     
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