Differential Op Amp Question

Discussion in 'General Electronics Chat' started by rainyday101, Feb 14, 2012.

  1. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    I have a hall effect sensor that is going to read 0 to 25 amps. The hall effect sensor has an output of Vs/2. I plan on using 5 Vdc for VS. This gives a null offset output of 2.5 Vdc at 0 amps of current. The sensor sensitivity is 37mA/1A. At 25 amps this would give a sensor output of 3.425 Vdc. Per my attached schematic, I am trying to offset the 2.5 Vdc with U1 to get a 0 Vdc output, and then adjust gain to get a 0 to 5 Vdc output. U2 is being used as a buffer and the 0 to 5 Vdc wil then be used as an analog input on my Arduino. I have learned/refreshed myself alot on op amps in the last 5 hours and cannot get this to work/an not totally sure what I am doing. I can never get the output at 0 or even close when I have a 2.5 Vdc input on U1 (0 amps from the sensor). I keep ending up with 2.5 Vdc output at U2 and U1 in this case. Any help would be appreciated.
     
  2. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    Will get back to you tonite unless someone beats me to it--the best circuit is simpler and easier to understand--you are not far off, but the way you are dealing with the offset will not work. It appears that you are using something like the Allegro ACS710KLATR-25CB-T--is that correct?
     
    Last edited: Feb 14, 2012
  3. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    The amp was a MCP660 by Micro Chip. Low power CMOS, single supply.
     
  4. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    Do you have -5V available?
     
  5. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    This is the simplest way to do it. By making the offset pot 1K, its Zint ≈ less than 250Ω so that its adjustment will not affect gain appreciably. On the other hand, a small resistance change to the 440K resistor will affect offset, so it must be dialed in carefully. Interaction can be avoided by changing the 440K to 100K and adding a 2nd non-inverting amp with Av of 2. 7.

    Tricky task to do without a negative power supply.
     
  6. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Which op amp should I use for this? LM324? 741?
     
  7. rainyday101

    Thread Starter Member

    Sep 24, 2009
    50
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    I can have -5Vdc available if needed. The circuit is being made from scratch.
     
  8. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    Early on, I thought that it would need a -5V supply, but it worked out OK without it.

    The 741 or 324 op amps will not do because their output does not go lower than 50mV, nor higher than about Vcc-1.5V.
    It needs a rail to rail output op amp--the MCP660 should do OK, but 32V/us is overkill.
    If selecting a new rail to rail op amp, go for about 0.5V/us or so.
     
  9. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    I just figured out the 741 or 324 would not work in Tina TI. I will look through their spice models and see what they have that is rail to rail and try your circuit again. Another thing that I tried was just a diff. amp with 2.5Vdc going in on V1 and the hall effect sensor voltage going in on V2. The hall effect output is 2.5 to 3.425 Vdc @ 37mV per amp. This gave me the diff Vdc of 0 to .925 Vdc on the output (V2-V1), which would just need to be scaled by another amp to get 0-5 vdc.
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    2,374
    999
    No. If the output is 37mA/1A, then at 25 amps, the output is 925mA. It looks to me this is throwing the whole scheme off, thinking the output is a voltage that's proportional to current. But the spec you've quoted says it's actually a proportional current. This will have an effect on the architecture.
     
  11. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    0
    The output is .925 Vdc + the 2.5 VDC null offset of the sensor. With no current the output of the sensor is at 2.5 Vdc, with 25 amps the output is 3.425 Vdc. (2.5 + .925)
     
  12. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    I assumed that this is a misprint since a constant current transducer (such as a Bell transducer) does not have a V/2 quiescent output. It seems very close to the 38mV/A Allegro transducer that I referenced.
     
  13. rainyday101

    Thread Starter Member

    Sep 24, 2009
    50
    0
    JimKeith,
    Just ran the simulation of your circuit and it works great! Thanks for your input.
     
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