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INA116 ultra-low input bias current opamp - huge offset voltage

Aaron Tan
Intellectual 270 points
Other Parts Discussed in Thread: INA116

Hello,

    I am using an INA116 on a breadboard and have connected the two outputs to each other and chosen a gain of 250. For some reason, I am reading an output voltage of -300mV, even though the inputs are connected to each other (i.e. input difference = 0V). Any ideas as to why this may be??

 

Thanks,
Aaron

  • 0
    TI__Expert 7185 points

    Aaron,

    With respect to the huge offset voltage you are seeing at the output of the INA116--this is a very common mistake.  The INA116 high grade device has an offset voltage of +/-2mV.  Offset voltage is defined as the voltage that would be seen at the output with the inputs shorted to mid supply.  If you put your INA116 in a gain of 250, you could potentially see as much as +/-500mV at the output due solely to offset voltage.  In a high precision application it is typical to calibrate the gain and offset of the INA by a 2 point measurement of the output with respect to Vin_min and Vin_max.  The real values of offset and gain are then stored in memory and used in the calculations to obtain the real readings of your sensor inputs.

    I hope this helps.

    Matt

  • 0 in reply to Matthew Hann
    Intellectual 270 points

    Ah, yes, Matt! That makes lots of sense - thank you!  Could you just please clarify what you mean by the offset voltage being defined as the "voltage seen at the output with the inputs shorted to mid supply"? What would the gain for this measurement be? And what is "mid supply"? Mine is powered by +12V and -12V pins.

    And also, what do the 2 values (  Vin_min&Vout   &   Vin_max&Vout  ) you get from your 2 point measurement give you?

     

    Thanks again,

    Aaron

  • 0 in reply to Aaron Tan
    TI__Expert 7185 points

    Aaron,

    The RTI offset condition is defined with the inputs short to a potential that is the center of the supplies, i.e. (V(+) - V(-))/2, which in your case is 0V or ground under a very specific supply condition (an extension of the orginal definition that I gave to you).  For instance, if you look at the top specification table for hte INA116, you will see that the nominal supply voltage for test is +/-15V.  If you change you supply voltage and or the common input levels relative to the center of the supplies, you can incur either a CMR or PSR induced offset change (for better or worse) the VOS condition.

    The 2 values that you measure, Voutmin and Voutmax give you the real gain accuracy (it is not going to be a perfect 250 due to the gain error of the device and the gain set resistor accuracy), from which you can extract the real gain slope.

    Gain = (Vout_max - Vout_min)/(Vin_max - Vin_min)

    With a calibrated gain and offset you can store these values in software to achieve accurate results that reflect your true sensor output.

    Matt

  • 0 in reply to Matthew Hann
    Intellectual 270 points

    Alright.. just to make sure I have this correct:

    Vout_max is the potential difference b/w the output and the non-inverting input (V+).

    Vout_min is the p.d. b/w the output and the inverting input (V-).

    To get Vin_max - Vin_min, you just measure the p.d. across the two inputs, i.e. (V+)  - (V-). What if this is too small to measure?

     

    Right?

     

  • 0 in reply to Aaron Tan
    TI__Expert 7185 points

    If your inputs are too small to measure, you need to use a known precision source (i.e. precision DAC or precision voltage source) to calibrate your INA116 gain.  As an example, on +/-12V supplies the device can swing linearly to about 1V from each rail.  If you give yourself some margin for VOS*gain (2mV*250) this puts your output swing at about +/-10.5V.  Therefore, the corresponding inputs that would produce these outputs would be 10.5/250 - 2mV = +/-40mV. 

    Keep in mind that gain calibration is necessary if you need super high % accuracy input measurements.  If you can live with a .1%-1% error to your output due to gain and gain resistor tolerance, this procedure is not necessary.  As you have discovered, however, it is very necessary to calibrate the offset which in high gain can account for a large percentage of your output error, especially at your lowest input levels.

    -Matt