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ADA4941-1 opened input signal

Hello

I am considering Sallen-Key, Low-Pass Filter like figure 54 on the datasheet.

There is a situation that the input is opened.

Then the output would be unstable.

Should I put termination resistor?

If yes, which value can I use?

There is a little oscillation at output because of load capacitance.

How can I compensate it?

Regards,

on Mar 10, 2016 6:15 PM

power-man-san,

  What frequency?  What amplitude?

How much load capacitance?

I would suspect pc board layout first.

For a test, change the 1.69k resistor to 100 ohms.

Harry

  • I wanted to know the frequency and amplitude of the unwanted oscillation.

    WRT open input;, why not try a 10k pulldown on the input so there is something

    when the source is disconnected.

    Scope pictures?

    Picture of pc board?

    Harry

  • Hi Harry-san,

    When I removed the 2200pF, the oscillation is improved.

    Is it possible to inform me the load capacitance capability?

    Regards,

    Power-man

  • Power-man-san,

    There is no answer for "how much capacitance will this op amp drive".  Op amps have a finite

    output impedance.  So with a C, you have an R-C low pass which gives you phase shift and

    reduces the phase margin in your loop.  So it depends on what overshoot you can live with.

    10%??   30%??? 

    Unfortunately, the ADA4941 d.s. doesn't have overshoot vs. load capacitance graph.

    Fig 22 is not much help, but the fact that it has C>=0pF and 20pF tells me it can't drive much.

    Change the 33 ohms to 68-100 ohms and see if the frequency changes.

    Reduce the 1k/1k to 680/680 on the inverter stage.

    Drop the 7.5k feedback resistor to 100 ohms.  (don't worry about the offset)

    Harry

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