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LOG114: Log114 behaving strangly

Christof
Prodigy 100 points
Part Number: LOG114
Other Parts Discussed in Thread: OPA376

Hello everybody, 

I'm using the LOG114  exactly as depicted in the Datasheet on page 11 in dual supply configuration, I1 derived from the internal 2,5V with 2,5MOhms. The design is powered by a single 3.3V voltage source. The +-5V are derived as described in Figure 12 on page 19 on the datasheet.

The current, that has to be measured, is created by a reference voltage of 1V followed by a variable resistor. If the resistance is low (~100k) i have no problem. However, the vlogout signal is very very noisy. This gets even worse if the current to be measured gets lower. It seems the chip starts to oscillate. 

Do you have any idea, what to do about it?

Best Regards 
Christof 

  • 0
    TI__Expert 6275 points
    Can you eliminate your excitation signal source and connect a fixed 100k resistor between pin 16 (VREF) and pin 3 (I2)? You can use other values as well. I want to eliminate your connections and source as the cause. By wiring direct and local to the PCB with LOG114 and using its own internal reference will help determine this.
  • 0
    Guru 140200 points

    Hi Christof,

    how do you generate the 1V reference voltage? Is it clean, stable and noise free?

    How far is the resistance away from the LOG114? Is there any cabling? Keep in mind that the LOG114 is a high speed logarithmic amplifier and doesn't like stray capacitance at the input pins 3 and 4.

    Another issue is the +/-5V supply voltge generation. For a test you could power the LOG114 by linear voltage regulators providing +5V and -5V. Do you see an improvement then?

    Scope plots would be helpful and a photo of your setup.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 100 points

    Hi Kai, Hi Tim, 

    thanks for your answers. It seems, that the mentioned stray capacity is indeed the problem, I was able to connect a 50Meg Ohm resistor directly between pin 3 and pin 16. Although the signal is still noisy, the ic itself doesn't start to oscillate and my ADC reads a voltage close enough to the resistors real value. However, without cables, the setup is of no use to me. Actually, not only cables are problematic in my design. The log114 is mounted on a separate board, which is connected to the actual sensing equipment through a pci-express edge mount connector, which introduces quite some capacitance! Is there any workaround to reduce the influence of the stray capacitance.

    Could a current mirror, closly attached to the input pin of the log114 mediate this issue, or would this also cause capacitance issues? Do you have any recommendations?

    Best regards 
    Christof

  • 0 in reply to Christof
    Guru 140200 points
    Hi Christof,

    in this thread I have proposed a remedy:

    e2e.ti.com/.../2858971

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 11135 points
    Christof

    We have not heard back from you so we will close this thread. Please reply if further discussion is needed.

    Thanks
    Dennis
  • 0 in reply to Dennis Goeke
    Prodigy 100 points

    Hi Dennis,

    sorry for the delayed response. I'm just now circling back to the project. The idea of using an external TIA for I->U to I conversion is quite interesting.
    I'm trying to simulate this in TINA, however, I can't find a suitable OP-AMP for the first TIA (that converts the current to the voltage). I'am expection currents
    as low as 1nA. Do you have a suggestion for me, which OP-AMP could be used to archive the proposed solution? I'm using the reference circuit from the 
    LOG114 page, removing the I2 source and replacing them with the TIA followed by a simple resistor.

    Thanks for the help!

    Christof

  • 0 in reply to Christof
    TI__Guru* 93105 points

    Hello Christof,

    For very low input current transimpedance applications CMOS and JFET input op amps are the best choices to minimize input current related errors. Since you are interested in converting a current level as low as 1 nA to a voltage output, an op amp having a few pA of input current should work well.

    The OPA376 is a high performance CMOS input op amp having a typical input bias current of 0.2 pA, and a maximum of 10 pA, at TA = 25°C. You can find the datasheet here:

    http://www.ti.com/lit/ds/symlink/opa376.pdf

    If you find there are other parameters that are important in your TIA application that the OPA376 doesn't quite meet, let us know and we can help recommend different amplifier.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 100 points

    Hi Thomas,

    thanks for the help. I followed your advice and implemented the OPA376 as TIA in my TINA simulation. The file i'am unsing is the SBOS301A from the website. When i do the dc transfer characteristics analysis, i vary the input resistor from 100Ohms to 1G, the result is however not satisfying. I'am trying to attach the file, what am i doing wrong?

    Best regards

    Christof 

    log114-tia.TSC

  • 0 in reply to Christof
    Guru 140200 points
    Hi Christof,

    I can't see a TIA in your scheme. Your OPA376 is wired as inverting amplifier with a gain of -1, instead.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 100 points
    Hi Kai,
    this is because the resistor R5 is my "current source" and changes during operation, acutally this is the sensor (it is a metal oxide gas sensor).
    Best regards
    Christof
  • 0 in reply to Christof
    TI__Guru* 93105 points

    Hello Christoff,

    I see in your TINA simulation that you have an input current to the LOG114 of 1 mA. Do note that the LOG114 log conformity begins to degrade with input current levels above 1 mA. This is evident in the graphs on Page 7 of the datasheet. If you can keep the maximum input current lower than 1 mA by increasing the resistance of the TIA series output resistor, better log conformity performance can be achieved.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Christof
    Guru 140200 points
    Hi Christof,

    what exactly is the sensor signal?

    Kai
  • 0 in reply to Thomas Kuehl
    Prodigy 100 points

    Hi Thomas, 

    thanks for the advice, actually i don't think my signal will exceed 1mA. 

    Best regards

    Christof

  • 0 in reply to kai klaas69
    Prodigy 100 points
    Hi Kai,
    the sensor acts a normal resistive resistor. However the values can be as small as 100 ohms and as bis as 1G ohms, so i'am looking for a way to compress the range. I thought a logarithmic amplifiert could be a good remedy. I'am also looking at the DDC 112 which integrates the current (but this is only a last restor, if the log114 doensn't work at all).
    Best regards
    Christof
  • 0 in reply to Christof
    Guru 140200 points
    Hi Christof,

    which sensor do you use? The MOX sensors I know, the TGS813 e.g., have only a variation of sensor resistance of about 100:1.

    Kai
  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Hi Christof,

    Do you have all the information that you need for your TIA/LOG114 application? If so, could you please close this e2e thread? If you have questions later, you can always start a new e2e post.

    Regards, Thomas

    Precision Amplifiers Applications Engineering