ADA4941-1 opened input signal

Question

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,

FormerMember · Accepted Answer

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

FormerMember · Answer

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

FormerMember · Answer

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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