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ADA4941-1 opened input signal



		

			

									
					    power-man
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Mar 9, 2016 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    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
					
      
		
      
			
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									FormerMember
							        
			
			
			
    
            
			                            
                        
                on Mar 15, 2016 7:40 PM
            
            
					
    
	
    

		
		
    
					
    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
							        
			
			
			
    
            
			                            
                        
                on Mar 15, 2016 7:40 PM
            
            
					
    
	
    

		
		
    
					
    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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