INA offset trimming problem

Discussion in 'General Electronics Chat' started by beedees, Jul 7, 2015.

  1. beedees

    Thread Starter Member

    Mar 29, 2015
    Hi everybody!
    I'm using an INA125 to amplify the output signal of a load cell. The INA125 is single supplied (5V), and I have an ADC that acquires voltage values between 0 and 3.3 V. The lower saturation voltage of the amplifier is 0.3 V so I decided to set an output offset which value is (0.3+3.3)/2=1.8 V in order to acquire both positive and negative values. So I have connected IAref pin of INA to LM317 output which voltage output is 1.8V.

    As you can see in the datasheet, INA's output is given by the expression:
    V_out = V_offset + G[(v_in+)-(v_in-)] = 1.8 + ....

    The problem is that I can't read 1.8V at INA's output when no load is applied (i.e. there is no strain)
    First I balance the bridge using a pot (I use an Agilent 34401a to read bridge's output) and then I should expect an output of 1.8V.
    When I don't connect gain resistor the output is exactly 1.8V, but when I connect gain resistor the output isn't 1.8 V and it is very unstable. I can assure that wheatstone bridge is balanced (2-3 microvolts) and it drifts 10 microvolts maximum.

    I can't explain this behavior. Any help is well-accepted! Thanks a lot!
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Care to show the circuit?
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    hi bd,
    Please post your circuit diagram, showing how the bridge is connected to the INA125.