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LM6084 opamp stability with very high source resistor

Bill Austhof
Prodigy 20 points

 Is this circuit going to be stable:

LM6084  configured as an integrator with 1 to 10pF  on the inverting pin and a 100pF  integrator cap.

A switch will be used to reset the intrigator.

  • 0
    TI__Mastermind 49966 points

    Hi Bill,

    It should be stable.

    Where it could go unstable (oscillate) is during reset when the switch is closed. With the switch closed, the op-amp is a pure follower (output connected to -IN) and it will see the 10pF as an additional output capacitive load (in addition to any existing output C load). Followers are the most unstable configuration, and the most sensitive to capacitive load. 10pF extra could push it over the edge.

    Place at least 100 ohms in series with the switch to keep the amplifier from driving the 10pF directly back through the switch. Simple integrator schematics tend to omit this important resistor.

    See Ron Mancini's article:

     http://www.edn.com/design/analog/4333099/The-nuances-of-op-amp-integrators

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Thank you for the information.

    I gested at the input capacitance and was way off. The PCB capacitance on input signal  is between 20nF and 90nF.  Also, the solid state switch has 2K ohms of resistance. 

    Still stable?

    Thank you in advance for your help! 

  • 0 in reply to Bill Austhof
    TI__Mastermind 49966 points

    Hi Bill,

    With capacitance that high, it will be unstable. You should put a few hundred ohms in series with the input to isolate the 90nF from the non-inverting node (the feedback cap goes to the input directly, tho). Start with about 100 ohms.

    Even with a 2K Ron - you should still use a separate series resistor because the mux/switch can have parasitic capacitances that are not "behind" the R_on resistance.

    Regards,