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LOG114: Need to compensate for offset voltage

Richard Leath
Prodigy 240 points
Part Number: LOG114
Other Parts Discussed in Thread: LOG112,

I want to use the LOG114 to measure photo current over the range 1nA to 100uA (5 decades).  I plan to drive I2 with a 3.16uA reference current.   I know the LOG112 has better DC accuracy, but it is too slow for this application.

I would like to compensate (in firmware) for offset and perhaps gain error.  What is the best way to determine amplifier offset voltage?  At first I thought I could leave I1 open and read the output, but I don't know if the LOG114 will behave properly with I1 = 0.  I plan to use a precise external  2.5V Vref source to generate 3.16uA Iref.  Could I drive both inputs with 3.16uA and read the output?  Will that reading be a good number to subtract from actual readings to account for offset?  Is there a better way?

Thanks,

Richard

  • 0
    TI__Expert 6250 points
    Hi Richard,
    Our log amplifier expert is out of office until next week. We should be able to provide a more accurate answer at that time.
  • 0
    Guru 140200 points

    Hi Richard,

    there must always flow a current into both inputs of LOG114. You can see this from equation (1) of datasheet: Either i1/i2 is undefined, if i2=0, or the log(i1/i2) itself is undefined, if i1=0, because log(0) is undefined.

    The use of two identical input currents is a good idea, as log(i1/i2)=log(1)=0 then. Anything different from Vout=0 would be your output offset voltage.

    To calibrate the gain you would need to apply the two input currents set in an acurate ratio to each other, like I1/i2=10, or so. This would result in log(i1/i2)=log(10)=1. By this the factor 0.375 in equation (1) could be calibrated.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 240 points

    Hello Kai,

    Thanks for the advice.  

    Does the actual current value used for offset measurement make a difference?  I was thinking that the Log Conformity error difference between each of the 2 input amps could contribute to my offset reading.  

    But maybe that "error difference" is insignificant.

    For gain error measurement, should I try to use an input current range that is closer to my actual expected log measurement range (several decades) or just make the measurement over a single decade range?

    Thanks for the help.

    Richard

  • 0 in reply to Richard Leath
    Guru 140200 points
    Hi Richard,

    yes, it's a very good idea to use input currents that are closer to your actual expected log measurement range!

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 11135 points
    Richard

    We haven't heard back from you so we assume this resolved your issue. If you need more help just post another reply.

    Thanks
    Dennis