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INA129 Offset Voltage.

paul miller44746
Intellectual 540 points

Other Parts Discussed in Thread: INA129

I am using an INA129 (http://focus.ti.com/lit/ds/symlink/ina129.pdf) in the ECG set up shown on the bottom of page 11, with a 47 pF capacitor added in parrell to the 390k Resistor in feedback on the last stage of the right leg drive.  I have also added an additonal gain stage with around 50 v/v as well as a DC restorator.  However, at the output of the IA I am seeing a DC offsetof -1.07V (consistantly).  This persists regardless of the gain, and regardless of the input signals.  This is resulting in heavy saturation of the rest of the circuit.  Can anyone help?

 

  • 0
    Intellectual 540 points

    I should also mention that the DC offset is completely insensitive to any change in the refernce voltage (shown in the Data sheet as the offset tuning pin), meaning, whether I hook it up to ground or to -5V there is no change.

     

     

  • 0 in reply to paul miller44746
    TI__Expert 7185 points

    Paul,

    Before hooking up the RL drive were you able to first confirm that the INA129 and the feedback integrator work as advertised?  If not, disconnect the RL drive and make sure that you get the correct output signal with a small sinusoidal input first.  You will need to provide a common mode input that is mid supply (Vs/2) and pulling the inputs up through a ~1Meg resistor to provide an input bias return path.  If you are using dual supplies you simply need to connect both inputs to a 1Meg resistor to ground to provide the common input bias and then use your test sine wave to provide your differential input signal.  After you confirm that the INA129 is amplifying signals correctly (without a DC offset), add the integrator and a small DC offset along with your test sine wave and confirm that the integrator removes it at the output.  Once you get this working, then remove the pull up resistors and add the RL drive back to the circuit.  If the circuit still does not work with the RL drive, you at least know where to focus your debug efforts.

     

     

  • 0 in reply to Matthew Hann
    Intellectual 540 points

    Matt,

     

    I have used the pull down 1Meg resistors on the input signal and this had no effect on the signal.  It still had a -1.07V offset.  Is there something else I can do?

     

    Paul

  • 0 in reply to paul miller44746
    TI__Expert 7185 points

    What are your input sine levels and are you using the integrator?  What are your supply voltages?

  • 0 in reply to Matthew Hann
    Intellectual 540 points

    Matt,

    I am using a 100 mV input, I have set the gain of the IA to 10. I am using +/- 12.  And what where do you want the integrator? I have one being supplied by the output of the IA and being fed back into the "Ref" pin of the IA.  

    Paul

     

  • 0 in reply to paul miller44746
    TI__Expert 7185 points

    Paul,

    Is this input a 100mV pp Sine wave?  What is the frequency you are using? 

    FYI--I want you to remove the integrator first and ground the reference pin to make sure we are getting the correct output at the INA129.  If you are using a +/-100mV sine wave input the output should swing +/-1V.  If you still have a DC offset, measure the voltage across the inputs to see if you are accidentally generating a DC offset elsewhere in your input path.

  • 0 in reply to Matthew Hann
    Intellectual 540 points

    Matt,

    Yes, the input is a 1kHz 100 mV sine wave.  The amplifier properly amplifies the signal but there is just a large offset at the output. I have grounded the refrence pin and it does not change the output.

    Paul

     

  • 0 in reply to paul miller44746
    TI__Expert 7185 points

    Paul,

    You need to determine if this is being generated at your inputs and amplified to the output.  A -1.07V offset at the output corresponds to a -100mV DC input voltage which should be easy to measure.  If you do not see this DC voltage at your inputs, you may need to check your grounding on the reference pin vs. your input common to make sure you do not have a ground loop.  Otherwise, it may be possible that you have a bad device as there are not very many things that can contribute to this type of a problem.

    Matt

  • 0 in reply to Matthew Hann
    Prodigy 55 points

    Hi Paul and Matt

    I have been facing the same problem with my device too. I am getting a constant negative offset on the output irrespective of any changes in the reference voltage. I am using the device to record EMG signals. Did you manage to figure oiut what was wrong the device. Please help.

    Cheers