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LMH6642: Question of LMH6642 Input Range

Wayne Chen
Guru 16055 points
Part Number: LMH6642
Other Parts Discussed in Thread: TL431, TLV3201

Tool/software:

Hi Sirs,

I am utilizing the LMH6642 as a comparator with single power rail 3.3V. The datasheet for the LMH6642 indicates that the maximum input voltage range is +/-2.5V. Does this imply that I can safely apply a common-mode input voltage ranging from +2.5V to -0.6V without any problems?

If this is not advisable, could you recommend a comparator that can handle an input voltage range from -0.6V to +2.5V with a single 3.3V power supply?



Thank you and Best regards,

Wayne Chen
07/10/2024

  • +1
    Guru 261500 points

    The absolute maximum ratings prevent damage, but do not guarantee that the device works correctly. (And the differential input voltage is the difference between the two inputs.)

    The CMRR specification shows that at 3.3 V, the input range is from −0.1 V to 1.9 V.

    I am not aware of any device that would allow 0.6 V below its negative power rail. Where does this negative voltage come from? Can't you shift it higher?

  • 0 in reply to Clemens Ladisch
    Guru 16055 points

    Thanks a lot. My plan was: +/-12V pulse signal -> TL431 -> +2.5V/-0.6V -> LMH6642 -> GPIO

  • 0 in reply to Wayne Chen
    Guru 261500 points

    What is the switching threshold? What propagation delay do you need? Do you have other supply voltages?

  • +1
    TI__Genius 13520 points

    Hello Wayne,

    How are you utilizing the TL431 in the system?  It appears you are setting the system voltage levels after the initial +/- 12V pulse; possibly using the TL431 as a comparator (open-loop).  Are you certain you need to drive the LMH6642 below 0V on a 3.3V single supply?  

    The LMH6642 is also a higher supply power device, handling a total supply voltage of 12V.  You could operate the LMH6642 on different split power rails if you would prefer, although I imagine 3.3V is what is present in your system.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Guru 16055 points

    Here is a section of my schematic, accompanied by the test results. The input signal is composed of +/-12V pulses with a 25us width (CH1). The TL431 is set to regulate the output voltage within a +2.0V to -0.6V range (CH2). I am in search of a comparator that can trim the rising edge of the TL431's output to make it compatible with the DAQ, which can handle up to +/-5V. The entire circuit operates on a single +3.3V power supply rail.


  • 0 in reply to Alec Saebeler
    Guru 16055 points

    I can resolve the issue by replacing the LMH6642 with a TLV3201 comparator and introducing a resistive voltage divider between the TL431 and the TLV3201.

  • 0 in reply to Wayne Chen
    TI__Genius 13520 points

    Hello Wayne,

    Using the TL431 you could shift the voltage up from -0.6V to -0.1V.  This would solve your input limitation issue on the low end.

    If you are using the LMH6642 as a comparator, could you not use it with this shifted input signal and take whatever the output is for +12V system input as your "HIGH" condition?

    What output signal do you need from the LMH6642 as a comparator?

    Could you not use the TL431, a resistor divider, and the LMH6642, as you mentioned with using the TLV3201?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Guru 16055 points

    Hi Alec, 

    The TLV3201 datasheet states that "Input pins are diode-clamped to the power-supply rails. Input signals that can swing more than 0.5V beyond the supply rails must be current limited to 10 mA or less". Does this mean that the device can tolerate an input signal of -0.6V, as is produced by my TL431 output?






    CH1: The input to the TL431 is a +/-12V square wave. CH2: The output from the TL431 ranges from -0.6V to +2.0V. I am researching a comparator to adjust the TL431's output back to a square wave form."

    Thank you and Best regards,

    Wayne Chen
    07/15/2024


  • 0 in reply to Wayne Chen
    TI__Genius 13520 points

    Hello Wayne,

    I have time to look into your circuit tomorrow & Friday this week.

    Thank you.

    Best,

    Alec