Puzzled by AD623

Discussion in 'General Electronics Chat' started by tommydyhr, Mar 7, 2014.

  1. tommydyhr

    Thread Starter Active Member

    Feb 3, 2009
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    4
    Today I built a circuit which consists of the following blocks:
    Voltage regulator, wheatstone bridge with strain gauge, and an instrumentation amplifier. A schematic of which can be seen below (The potmeter R2 is our "strain gauge", which has an R_0=120\Omega.

    [​IMG]

    I've never worked with neither a strain gauge, nor an instrumentation amplifier, but I can't for the life of me figure out what's wrong with the circuit. My issue is that, no matter what I do, I can't zero the output of the op-amp. Not even when V_{in-}>V_{in+}, it never went below ~10 mV.

    Can anyone tell me what could potentially be wrong in the circuit? Thanks.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    pin 6 is your output pin.

    pin 5 is an output reference and should be tied to your ground. It can also host an offset voltage to move your output voltage to ground potential if your output load does not share a common ground with your input circuit.

    Edit: previous pin number refs. were wrong.
     
    Last edited: Mar 7, 2014
  3. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    I would start by lowering the value of R7 to 100 ohms and increasing your Zeroing pot, R1, to 30 to 40 ohms, or whatever you can find in a standard value around that figure. You may have just run out of adjustment range. My guess is that you have R1 set to put as much resistance as possible, but just can't quite reach zero. Lowering of R1 resistance will cause a greater degree of unbalance. Am I right?
     
  4. tommydyhr

    Thread Starter Active Member

    Feb 3, 2009
    39
    4
    That's a bit confusing. In the datasheet (and in the above schematic), pin 8 is for the feedback resistor, and pin 5 is the reference.
     
  5. tommydyhr

    Thread Starter Active Member

    Feb 3, 2009
    39
    4
    Unfortunately I already tried replacing R7, just to make sure. At one point, the inverting terminal was at 3 V whilst non-inverting input had 2.9V. At that time, the output should be at 0.000 V, right?
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I was wrong, the post has been changed to show the correct pins. My point was to tell you about the use of the ref pin to correct output voltage.
     
    Last edited: Mar 7, 2014
  7. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    Nothing is wrong. If you look at the OUTPUT section of the AD623 data sheet SPECIFICATIONS table you will see that the minimum output voltage with a single supply is 0.01V (10mV) as you measured.

    If you want the output to be at some other voltage at null and allow positive or negative excursions from there, then you can connect a voltage to pin 5. For example if you use two equal value resistive dividers to generate 1/2 the supply voltage and apply that to pin 5, then the output null point will also be at 1/2 the supply voltage and the bridge going off null will cause the output to go either plus or minus from that voltage.
     
    tommydyhr likes this.
  8. tommydyhr

    Thread Starter Active Member

    Feb 3, 2009
    39
    4
    Thank you, I had totally missed that line. I guess I took the "rail-to-rail"-term too literally. Back to the drawing board I go!

    Thanks everyone :)
     
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