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LM4140 - Output resistance negative?

Rahul Chander
Prodigy 150 points
Other Parts Discussed in Thread: LM4140

Hello,

1) I just wanted to double check something with TI designers of LM4140.  Looking at the datasheet for the LM4140 reference IC, it seems that the output impedance vs frequency graph (figure 8) starts from a negative value.  If you zoom in, the thick line is clearly below the 0 (ohm) on the y-axis.  Is this a typo or does the reference behave like a negative resistance in the range 0-3kHz??

2) If another AC source is attempting to drive the output of LM4140CCM-1.2/NOPB with a Thevenin impedance of 982kohms and voltage 500VACrms (60Hz), what is the resultant output due to the superposition of AC source and LM4140?  I would expect that the LM4140 should just over-ride the AC source no problem, but I am seeing noise in the data which a 16-bit system easily picks up as 0.1mVrms.  I haven't been able to measure the "superposition result" due to challenges in dealing with high-voltage probing, but needed some ideas about LM4140's behaviour.  This should be explainable using the output resistance figure in the datasheet, which is where I am stuck at.

Thanks

Rahul

  • 0
    TI__Expert 3300 points

    Hi Rahul,

    We are currently looking into this and will get back to you as soon as possible. 

    Regards,

    Kartik

  • 0
    TI__Genius 13395 points

    Hi Rahul,

    Negative AC resistances can occur in these measurements.  It means that there is a negative voltage change for a positive current delta. 

    It will be very difficult to remove the signal you are talking about such that it will not be visible on a 16 bit converter.  By my calculation, even a 1 Ohm output impedance would still yield 500 uV of output noise.  You may need to add some additional filtering.

    I hope this helps.

    Mike

  • 0 in reply to Michael Hartshorne
    Prodigy 150 points

    Hi Mike,

    Thanks for explaining.  I understand that 1ohm can lead to significant portion of noise but I expect the impedance of the reference to be better than that.  Based on the graph, the impedance is of the order of 100mOhms.  

    Is it possible to know the exact number so that I can account for the observed noise, and also if the output impedance changes with the output capacitor?  I am using an output cap of 44uF.

    Thanks

    Rahul