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Original question:

LOG114: Connecting a photodiode as the input current source.

LOG114: High Dynamic Range Photodiode Amplifier

Mihir Gupta65
Intellectual 2165 points

Part Number: LOG114

Hi Team,

My customer (in this thread  ) is trying to create a photodiode amp with the LOG114. In the SPICE model of the LOG114, the gain in the upper regions towards 10mA seems to become non linear. Do you have a suggestion about how this non linearity can be calibrated out? Can it be done digitally? If yes, how can one extrapolate the gain relationship in the upper regions?

Regards

Mihir

FAE - South Germany

  • 0
    Guru 140200 points
    Hi Mihir,

    a 10mA photodiode current is very unusual. Can you show a schematic of the circuit?

    Kai
  • 0
    TI__Guru* 93105 points
    Hi Mihir,

    Current above 1 mA cause substantial heating of the LOG114 logging transistors used in the A1 or A2 feedback loops and that leads to the less linear response in the high, 1 to mA input current range. The log conformity error in that current range is less predictable because of the device self heating which depends on the LOG114's thermal environment. Therefore, the datasheet refers the "Typical Characteristics" curves for the log conformity error for 1 to 10 mA. The curves are close for extrapolation purposes, but expect that each application environment may not result in exactly the same linearity response.

    The LOG114 EVM document discusses operation in this higher input current region starting in section 2.3 of the document. A simple analog solution is provided that results in improved linearity in the high current input operating region. You can find the LOG114 EVM document here:

    www.ti.com/.../sbou110.pdf

    Certainly, like most any analog signal chain involving analog to digital conversion digital calibration can be applied to correct for the nonlinearities. Since we do not know how a customer will apply the LOG114 such system designs we do not provide a ready made solution that involves digital calibration.

    Regards, Thomas

    Precision Amplifiers Applications Engineering