Very basic instrumentation amp help

Discussion in 'The Projects Forum' started by cps13, May 7, 2014.

  1. cps13

    Thread Starter New Member

    Feb 25, 2013
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    Hi,

    I am after some help designing an instrumentation amp - this is not a college project etc so I am not cheating!

    I am trying to simulate the working of a TI INA333 instrumentation amp IC using multisim 13.0 with little success. Please can someone point out what I am sure is my obvious mistake?

    My understanding is that the IC should output 0-5V with a varying input.

    I tried to eliminate possible problems using a multimeter on a simple 12V battery but get no reading when simulation either.

    Many thanks
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    for one thing, the meter and 12 volt battery arent connected to the circuit.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,026
    3,236
    The battery (V1) can't be floating, it needs to have one end grounded.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
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    Same for V2.
     
  5. cps13

    Thread Starter New Member

    Feb 25, 2013
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    OK - so I have now grounded the battery and get an output of -599 micro volts when the dc input is set to 1V and R1 is 500 ohm.

    Any ideas?

    Thanks
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need to post your updated schematic, or we can't be certain of the modifications you've made. This will save time and give you better answers.
     
  7. cps13

    Thread Starter New Member

    Feb 25, 2013
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    I have attached a new screenshot. I have made a change in that it now has a wheatstone bridge attached to provide the input.

    thanks
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    All of the answers are in the datasheet.
    What is pin 4?
    What is the purpose of R5?
    What are the two input voltages?
    What is the input voltage range of the device?

    ak
     
  9. cps13

    Thread Starter New Member

    Feb 25, 2013
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    0
    Pin for is V- - mine is connected to ground
    R5 is your gain resistor - I don't understand why you have highlighted this
    input voltages are +1.8V to +5.5V - my input is 5V
    input range is are -100mV to +100mV - I have powered this as a normal wheatstone bridge is powered to provide a mV output.

    I realised I hadn't changed the value of R3 to 350 ohm. It was set to 1k. I have corrected this.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    You must have a different datasheet from the one on TI's website. Here is what I see.

    Pin 4 is the V- supply, but you have it tied to a 1K resistor to ground.

    R5 is not the gain resistor; the pot marked GAIN is the gain resistor.
    I highlighted R5 because it is in the negative power leg of the amp, where it should not be.

    If your intent was to adjust the differential gain of the amp by adding a resistance (R5?) to the reference input (pin 5), note that that will seriously compromise the diff in diffamp. If you are trying to add an offset to the output, note that the impedance at the reference input must be very low to maintain common mode rejection.

    Changing R3 to 350 ohms balances the bridge, but it better be *very* well balanced. With the 500 ohm gain pot at 50% rotation (250 ohms), the circuit gain is 401, so your max differential input voltage before positive clipping is 12.3 mV.

    ak
     
  11. cps13

    Thread Starter New Member

    Feb 25, 2013
    9
    0
    Does this circuit look anymore like it should do?

    Thanks
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Pin 4 must be tied to system ground so the diffamp input stage is referenced to something.

    ak
     
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