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LOG200: How to input 1nA current to I2 as reference current?

Xander
Prodigy 40 points
Part Number: LOG200

Tool/software:

Hello engineers, I would like to get a recommended design solution for LOG200 I2 reference current. My design range is from 1nA to 100uA as input range. Please give me a recommended I2 input solution.

  • 0
    TI__Genius 13520 points

    Hi Xander, 

    Our datasheet outlines typical application use case (Section 7.2) that can be set up for the LOG200 including the I2 input. The I2 input can be connected directly to the IREF pin which generates a 1uA current input reference. This allows for the device to have a low logarithmic conformity error across the input range of 1nA to 100uA for most temperature ranges, except when you start to get to high temperature (85C). 

    Section 6.3 of the datasheet also goes into more detail on the input pins. 

    Please let me know if you have further questions.
    Thank you!

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 40 points

    Let me repeat my question. TI's IREF does not meet my requirements. I need to input 1nA into I2, so I am asking how I2 can obtain 1nA. Is it through an amplifier or a resistor for VREF input? I hope you can provide an official recommendation for the input method.

  • 0 in reply to Xander
    TI__Genius 13520 points

    Hi Xander, 

    Thank you for the clarification. My understanding is that your I1 input range is 1nA to 100uA as you indicated in the design range above. In general, we recommend to set I2 current reference in the middle decades of the I1 current range. In this case, that would be about 1uA which can be set with IREF.

    Could you clarify why you are need an I2 = 1nA for your design? What voltage output range are you trying to achieve and what supply voltage are you using? 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 40 points

    The system I am using is a single power supply system with a current input range of 0-100uA. The minimum accuracy requirement within my system is 1nA. At the front end of my amplifier, there is a current mirror with a minimum output of 3nA. Therefore, I need the starting point to be 1nA, and it cannot have negative values. I hope you can directly address my question regarding the input of I2. What is the official recommendation from Texas Instruments for inputting 1nA? Please focus solely on the solution for my I2 input of 1nA and avoid discussing why I need to input 1nA.

  • 0 in reply to Ashley Kang
    Prodigy 40 points

    This is my schematic diagram. Can you help me find any problems?

  • 0 in reply to Xander
    TI__Genius 13520 points

    Hi Xander, 

    If you wish to input a I2=1nA, you can do so with the internal voltage reference and precision resistors to create the current you want. Please note that with this reference current, you may not see optimal performance of the LOG amplifier due to log conformity errors, as I have indicated with the typical plot in my first reply. This is why we recommend to use 1uA for I2 for a current range of 1nA-100uA for I1. 

    Adding a reference voltage will avoid negative values on your output. To level shift your output, you can connect the REF165 or REF25 to REFA depending on the voltage range you are trying to achieve. To calculate your output voltage, you will need to use the transfer function of the LOG200 and check the input range does not exceed the output swing range. I have calculated your use conditions in the two tables below. 

    Please let me know if you have further questions.
    Thank you!


    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 40 points

    Finally, could you please confirm if there are any issues in the schematic related to the LOG200, such as power supply configuration, resistor placement, etc.? Thank you.

  • +1 in reply to Xander
    TI__Genius 13520 points

    Hi Xander, 

    The schematic you have provided will need to be modified in order for the LOG200 to be in linear operation. In the current setup, both the input common-mode and output swing are out of specification and the part will not perform as expected. Additionally, you will see degraded performance in the higher end of your dynamic range due to the input current you are creating for I2.

    Please see the electrical characteristics specification table in the datasheet and listed below: 

    The recommended modifications are as follows: 

    • Necessary for function: Bias VCM to +2.5V and connect REFA to REF165 or REF25.
    • Strongly Recommended, Necessary for desired Log Conformity: Log conformity degrades significantly when I1 is >4 decades larger than I2. Connecting IREF directly to I2 will deliver the expected log conformity. Negative output voltage will be mitigated when I2 is greater than I1, since the output is biased up on REFA, and the amplifier is unable to swing less than (V-) + 0.3V.
    • Add bypass capacitors to VCM, REF25, REF165 per datasheet recommendations. 
    • Make sure bypass capacitors listed above and the capacitors connected to VS are as close as possible to the device in layout. 

    I have provided an example schematic to help illustrate the recommended modifications I have listed above. These changes are also described in more detail in both the datasheet and the application note I have linked below. 

    Application note reference: Perform Accurate Optical Current Sense Measurements Using the LOG200 Logarithmic Amplifier

    Please let me know if you have further questions. 

    Thank you!
    Regards,
    Ashley