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TL081: Loop stability

Zoey Wu
Expert 3570 points
Part Number: TL081

Hi  team,

Could u help to tell me that will the added capacitors(C2&C1) influence the loop stability of the system as attached?

  • 0
    TI__Guru*** 140126 points

    Zoey,

    It will influence loop stability in a undesirable way. Move C1 to pins 2 and 6 to get the desired filtering without hurting loop stability.

  • 0 in reply to Ron Michallick
    TI__Expert 3570 points

    Hi Ron,

    Thanks for your response!

    The C1 and C2 are parasitic capacitance of the clamp diodes and do u have any other advise that can counteract the influence of C1&C2?

  • 0 in reply to Zoey Wu
    Guru 140200 points

    Hi Zoey,

    there are several remedies:

    1. Mount the clamping diodes earlier, e.g. between R3 / R2 and R6 / R5.

    2. Take diodes with lower junction capacitances.

    3. Mount a phase lead capacitance between pin 2 and Pin 6.

    Can you show the complete schematic, including the protection diodes? And please tell against which scenario these diodes shall protect.

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 3570 points

    Hi  Kai,

    Thanks for your clear reply! The detailed schematic is as attached and one more question is that will the added capacitor between Pin2 and Pin6 influence the differential signal ? ps: 1k bandwidth is enough in this case. 

  • 0 in reply to Zoey Wu
    Guru 140200 points

    Hi Zoey,

    adding caps is no problem unless you introduce an imbalance. So I would recommend to add two low toleranced 220pF NP0 caps as shown below:

    zoey_tl081.TSC

    zoey_tl081_1.TSC

    zoey_tl081_2.TSC

    You may have noticed that I have changed V2 to allow a proper common mode input voltage for the TL081 of at least 4V above the negative supply rail.

    Another issue is that the common mode rejection is only high when all involved feedback resistors and capacitors are low toleranced and balanced. Because of that, +/-0.1% toleranced resistors and +/-1% toleranced 220pF caps are heavily recommended.

    Kai