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LOG114EVM: Output does not log scale

Hidekazu Machida
Prodigy 10 points
Part Number: LOG114EVM
Other Parts Discussed in Thread: LOG114

Hello,

I am trying this evaluation board.

It's rudimentary, but what's wrong?

  • +1
    TI__Guru 55466 points

    HI Hidekazu-san,

    If I attempt to perform the same setup, connecting directly a precision current source in between the I1 input of the log amplifier, an the VBIAS circuit, oscillations may occur at the low current settings, due to the parasitic capacitance of the setup (cables and instrument).  I can also reproduce intermittent results/erroneous measurements when attempting to connect a current source directly without an isolation resistor, where the log amplifier output is unstable, and the 2V VBIAS circuit may also be unstable.

    The recommended procedure is to use a precision differential voltage source (floating) with a series resistor, as recommended on the LOG114EVM.  Below is data using this setup. 

    Alternatively, you could also attempt to use the Keithley current source, but this time, place a series isolation resistor right at the I1 input.  The series resistor may help stabilize the VBIAS and LOG114 amplifier.  See bench data using a precision differential voltage source (floating) with a series resistor below.

    Thank you and Regards,

    Luis 

  • +1 in reply to Luis Chioye
    TI__Guru 55466 points

    HI Hidekazu-san,

    I went ahead and took data using the B2962A Precision Power Source with two series resistors.  I used the Agilent 3458 current meter to verify the input current.  At the lowest 10nA and 100nA current settings, I had to use two relatively large series resistors, or the LOG114 amplifier would become un-stable.  Below is the bench data.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 10 points

    Hi Luis,

    Thank you for the advice !

    It seems that it is necessary to change the series resistance value depending on the current range to be measured.

    Does it mean that a more voltage is required at the input to measure the weak current?
    Or is it just for current limitation?

    Thank you and Best Regards,

    Hidekazu.M

  • +1 in reply to Hidekazu Machida
    Guru 140200 points

    Hi Hidekazu,

    I think this has to do with the input offset voltage of LOG114. If you take a 1M resistor to apply 10nA you would need a voltage of

    U = 10nA x 1M = 10mV

    which is way closer to the input offset voltage of LOG114 than when using a 10M or even 100M resistor. With a 100M resistor you would need to apply 1V to generate a current of 10nA. And the closer you come with this voltage to input offset voltage of LOG114 the more the input offset voltage will degrade the precision of current generation and the more the above curve will deviate from a straight line.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 55466 points

    HI Hidekazu-san,

     The LOG114 is a high-speed logarithmic amplifier, and on the typical application is connected to a photodiode or sensor in close proximity to the device.   

    In this quick test, I did not spend time optimizing the bench set up, and provided a quick bench measurement connecting the precision source through relatively long cables.  The LOG114 will have the highest transimpedance gain at the lower current input signals, making it more sensitive to injected noise through the cables, and potentially there may be some sensitivity to stray parasitic capacitance of the long cables/test equipment .  In this case, Increasing the resistor may have remove some of the sensitivity to the source voltage/offset changes, as well as provided some isolation. 

    Because of the very high gain increase at low input current levels external noise coupled into the circuit becomes a challenge. For low input current level measurements, the LOG114 PCB board should be enclosed in a closed metal enclosure, and any wires in and out of the enclosure should be shielded. Attempt to reduce the cable length in the setup. 

    Another common issue at low currents is due to PCB board contamination on LOG114 Custom boards or LOG114EVMs than have been modified.  Leakage currents from solder flux contamination can disturb the log amplifier's operation at lower current levels. We recommend that the circuit be assembled using solder having water soluble flux. On our side, once assembled, we run the PCB boards containing the LOG114 circuit though an ultrasonic cleaning using fresh DI water each time, and dry the boards on a oven (or heat gun). 

    Thank you and Regards,

    Luis