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INA592: Offset voltage specification

Part Number: INA592

Hi,

I have a question about the offset voltage specification.

Vos is up +/-40uV when G=1/2  (or +/-80uV when G=2). From the datasheet, the upper part of the table is titled "OFFSET VOLTAGE (RTO)" but the specification in table says "Vos input offset voltage".

So, my doubt is: are these 40uV (or 80uV) present on output or on input?

Initially I thougth it was an output offset voltage, that should be equal to Vos_in*(1+6k/12k)=1.5*Vos_in  for G=1/2 (or to Vos_in*(1+12k/6k)=3*Vos_in for G=2).

It was ok for me since, in accordance to datasheet, the formula gives for G=2 an output offset voltage that is twice that for G=1/2.  If it is correct, negelcting bias currents, maximum Vos_in should be approximately 27uV (40uV/1.5 or 80uV/3).

But then I read "Vos input offset voltage" so I am a bit confused. 

Thanks,

Massimo

  • Hi Massimo,

    But then I read "Vos input offset voltage" so I am a bit confused.

    The verbiage is typically used in our datasheet. The input offset voltage figures are measured based on the input conditions stated above the table, where Vcm = -3V or Vcm = Vout = Vs/2, Rload = 10kΩ, Ta = 25 C, REF pin = GND etc.. Vos reported as referred-to-output (RTO) in this case, which has taken into account of G= 1/2 in the nominal configuration.  

    The same thing may be said about the Vos that reported with Gain = 2 V/V. These Vos figures are not correlated, even though Vos in G=2 V/V seems to be twice as large as Gain = 1/2 V/V. 

    If it is correct, neglecting bias currents, maximum Vos_in should be approximately 27uV (40uV/1.5 or 80uV/3).

    Regarding how to calculate the maximum Vos in an op amp, enclosed is an example in how it is calculated. 

    If you have additional questions, please let me know. 

    Best,

    Raymond

  • Hi Raymond,

    thank you for your answer. So, may I assume that the 40uV in the first table, despite labeled as "input offset voltage",  are in reality an overall value (that includes bias current as stated in note 1) measured on the output, since it is a "RTO" specification?

    In a certain sense, it is like to see the "effect" on the output of the input offset voltage and bias currents that are present on inputs (for a given gain G). Is this correct?

    Regards,

    Massimo

  • Hi Massimo,

    it is like to see the "effect" on the output of the input offset voltage and bias currents that are present on inputs (for a given gain G).

    This is correct. 

    I think that the reason specified in this way is because the Gain (1/2) is configured internally to the IC (The reversed INA592 configuration has a fixed gain of 2). In difference or instrumentation amplifiers, Vos figures are specified in either in RTO or RTI (referred-to-input) with a tested gain setting (more than one op amps at the input stages). This practice is different from how Vos figures are specified in our Precision Op Amp's datasheet. 

    If you have additional questions, please let us know. 

    Best,

    Raymond

      

  • Hi Raymond,

    thank you for your prompt reply. Now this topic is clearer to me.

    Regards,

    Massimo