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LOG112: LOG112 Application Questions

Richard Elmquist
Mastermind 45250 points
Part Number: LOG112
Other Parts Discussed in Thread: LOG114

I have a customer asking if he can connect the output of a single photodiode to two different LOG112 amps. I told him that this would not be a good idea as the current would not divide evenly and the current from a photodiode is often so low that splitting it would make a reading impossible. Here is the response that he sent:

I perfectly realize that splitting the current will not be even but I am going to calibrate the log.amp output based on the known optical power applied to the photo-diode.

So the un-evenly splitting of the photo-current will be calibrated out for every of two log.amps.

I believe that the current can split differently depending on the level of the input current. Would this be correct?

I am concern about the stability of those log.amps with commonly connected input (about the stability of my calibration).

The TINA-based simulation shows no problems but I am not sure I take in account the important details / some parameters/. The TINA has the LOG114 – maybe you could send the model for the LOG112 or explain why the TINA simulator is not good for such task?

Would there be stability issues if he was to connect the photodiode in this way. I know that many of these amplifiers are very particular about capacitance on the input. Are there any issues that he should watch for if he connects the LOG112 in this manner?

Thanks for your help with this!

Richard Elmquist

  • 0
    TI__Guru* 93105 points

    Hi Richard,

    I have not seen a LOG amplifier applied in this manner before and haven't ever tried it. That said, when I think about it I am not necessarily finding a reason why it wouldn't work. Granted the two LOG112 device I1 (or I2) input will not be electrically identical, and because of that the current from the photodiode will not divide exactly 50-50 between the two amplifiers and their will be some other minor differences. However, the customer recognized that they will be a little different and is able to compensate for them.

    The LOG112 in normal photodiode applications does see the capacitance of the diode which can be tens to hundreds of picofarads. The LOG112 amplifiers do have their inherent input capacitance, and that of the additional LOG112 amplifier will be seen by the other. I suspect that will not affect the basic log amplifier function of the amplifiers, but will have some effect on the ac characteristics. Also, since each input now received about 50% of the current total log amplifier gain will be different from when it would receive 100 % of the input current.

    I examined the LOG112 Pspice model and do find it to be quite complex and that it models many of the LOG112 electrical characteristics including the input capacitances. Using the amplifier model I set up a TINA circuit with two LOG112 amplifiers driven by a single photodiode. The convergence time is lengthy and I had to add a high value resistor across the photodiode (R8) to get the circuit to converge, but once that was done the circuit appears to function as expected. You can can see the TINA schematic shown below. I don't think the real circuit would require R8.

    If the customer really thinks this circuit arrangement will satisfy their application requirements, I do think it is worth trying.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Mastermind 45250 points

    Thomas,

    Thanks for your reply!

    I will let you know if the customer has any further questions.

    Thanks for your help with this.

    Richard Elmquist