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LOG114: LOG114

vanu
Prodigy 40 points
Part Number: LOG114

I connected Lo114 in dual supply configuration with +5V and -5V as supply voltage.  The input of log114 is connected to a photodiode and the reference current is set to 1uA.  When i measure the voltage at 9th pin (Vlog out) instead of the output voltage swinging from -1.5V to +1.5V the negative voltage goes only upto -300mV.  I have no issues with positive swing.  What could be the error in the circuit.  I have followed the datasheet for designing the circuit.

Thanks

Vanitha

  • 0
    TI__Guru* 93105 points

    Hello Vanitha,

    A correctly functioning LOG114 should be capable of producing a +/-1.5 V output voltage when used with +/-5 V supplies, and having the necessary input currents applied that correspond to that range. The input current range for a -1.5 V to +1.5 V VLOG output when IREF is set to 1 uA, is 100 pA to 10 mA. The required input current can be found from the relationship:

    The 6.141 factor takes into account the LOG10 to Ln conversion and the 0.375 V/V voltage gain. Do keep in mind that currents above about 1 mA can heat the logging transistor and the log conformity suffers as the datasheet explains.

    If your input currents are correct, then there is likely something not quite right with the circuit design or layout. Did you make sure that the LOG114 underside thermal pad is connected to the V- supply? That is a common oversight that can lead to problems.

    If you can provide your circuit schematic we can look it over and see if there is any issue that might limit the VLOG negative swing.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Hi Thomas,

    Thanks for your reply.  I have tried to characterise my circuit to see how close my Vlog output is with the theortical value obtained from the formula. To my surprise there was lot of difference at the small current (in the range of 1u to 50uA) and after that the difference reduced. Find the excel sheet which has my test results.  Also another observation i made was that when i used a high  value input resistor of about 10M, the Vlog output was completly erroneous.  Does input resistor at I2 need to be of certain range??  I am using my I1 as reference terminal.

    I have tried to attach the excel document which has my test results.  Let me know if you are unable to access it, i can try to e-mail them to you.

    Regards,

    Vanithalog114 vlog output.xlsx

  • 0 in reply to vanu
    TI__Guru* 93105 points

    Hello Vanitha,

    It would be helpful if we could see your LOG114 circuit schematic so that we can make sure that everything is properly connected. Also, an image of your PC board assembly might be useful.

    Often, when there are issues obtaining the expected performance of the LOG114 in very, very low input current circuits the issue at hand is externally generated noise current making its way into the circuit and corrupting the results. In very, very input low current applications the transimpedance gain of the LOG114 can be extremely high and any induced noise current will be amplified along with the desired input current. These applications require careful shielding of the input current circuit, shielding of the LOG114 circuit, and elimination of any noise on the power supply lines. We install the LOG114 circuit in a shielded enclosure and power the circuit using batteries inside the enclosure. Otherwise, power brought in from the outside world can bring noise into the enclosed circuit.

    Take a look at the LOG114EVM User Guide. It provides some good, practical information about applying the LOG114.

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

    Regards, Thomas

    Precision Amplifiers Applications Engineering