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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,

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  • FormerMember
    0 FormerMember

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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  • FormerMember
    0 FormerMember

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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