Instrument Amp Issue

Discussion in 'Homework Help' started by bnfdvn, Apr 11, 2014.

  1. bnfdvn

    Thread Starter Member

    Dec 8, 2008
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    I have a 4-20 mA current loop that I am running through two INA122 circuits in series. The first circuit in the loop has a 50 Ohm resistor across the input of the INA and the second circuit has a 40 Ohm resistor across the input of its INA. On the very bottom of the loop are a couple 1N4002 diodes in series to get the bottom of the loop above the common mode voltage.

    There are no gain resistors installed which sets the gain for the INA122s at 5. The bottom circuit (40 Ohm) works fine but the top circuit (50 Ohm) has a 0.1 Volt offset in the output. At 4 mA, the differential across the input is 0.2 V and at 20 mA, the differential across the input pins is 1.0 V. The outputs at these two points are 1.1 V and 5.1 V respectively. Vref (pin 5) is tied to pin 4 which is tied to 0 VDC.

    Does anyone have an idea of where the 0.1 V offset on the output could be coming from?? Thanks for looking!
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,673
    2,712
    At what?

    ...
     
  3. bnfdvn

    Thread Starter Member

    Dec 8, 2008
    21
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    At my post.
     
  4. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi,
    Looking at your description of the circuit and problem, its difficult see why you should have the offset.

    Can you try and short out the lower 40R and see if the 0.1V offset on the 50R IA changes.
     
  5. bnfdvn

    Thread Starter Member

    Dec 8, 2008
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    I shorted out the 40 ohm resistor leaving the diodes in the circuit and then just connected the bottom of the 50 ohm resistor circuit to 0 VDC. Neither change had an impact on the 0.1 V offset.
     
  6. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    Checking thru the d/s, nothing indicates why this should be happening.?

    I will continue looking for a possible cause, let you know.

    EDIT:
    I have run some LTS simulations of your circuit, no offset.

    Is it possible that your circuit layout is causing the problem,??
     
    Last edited: Apr 11, 2014
  7. bnfdvn

    Thread Starter Member

    Dec 8, 2008
    21
    0
    FYI - There are four identical circuits on my board. I put all four circuits in series (200 ohm total loop resistance) and they all behave the similarly. The offset voltage is not the exact same for all of them. On the input, the voltage difference between pins 3 and 2 is 0.198-0.199 V. The output of the IA in order from first in loop to last in loop is 1.084, 1.097, 1.080 and 1.098 V.
     
  8. bnfdvn

    Thread Starter Member

    Dec 8, 2008
    21
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    Layout could be an issue, I guess. I'm not sure what would cause the issue, though. 24 VDC power and 0 VDC traces are all 0.05" (1.27 mm) wide. Signal traces are 0.01" (0.254 mm)wide. The current input is stable, the 50 ohm resistors are 1% tolerance. The components are all close together. Anything else I should look at??
     
  9. bnfdvn

    Thread Starter Member

    Dec 8, 2008
    21
    0
    The output of the IA goes to an op amp LM358, input impedance of 10^10 ohms. I removed the op amp and installed a 1k8 ohm resistor to ground. Dropped the output from 1.09 to 1.07 v.
     
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