Current Log Amplifier not working in software simulation

Hi. I am working on a low current measurement project using LTC6079. On the datasheet, there is a typical application circuit for "6 decade current log amplifier." I tried to simulate it in multisim, but i cannot make it work. The output voltage seems to be constant whatever input current I apply. I am not sure if I am tapping it to my test circuit right. How should i connect this to my circuit? Let us say the test circuit is just a 15V supply and a 15k-ohm resistor. Thank you!

The typlical application circuit is on this next image:

  • 0
    •  Analog Employees 
    on Nov 9, 2019 1:03 AM

    Hi engrfats:

    There is a typo in some instances of that schematic.  My apologies for not getting it cleaned up yet.

    Basically, the 100k resistor above opamp D should not be a feedback R, but rather an input R, making D an integrator.  So there should be a 100k resistor from the base of Q1 to the D -input, and no feedback R around D.

  • Did you ever get this figured out? I am attempting to simulate this circuit in LT Spice and am having the same issues you had. I moved the 100k per Glen's suggestion, but it is still not functioning. I noticed that there are two versions of this schematic. One in the datasheet that you have shown and captured, and one in App Note 105. That one has the inverting/non-inverting inputs reversed on B.

    ETA: Typo