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ADA4637 OP Oscillate



		

			

									
						kent
					
							

			
				
									
											
							 		
		        	            							
			
			    	
			                    
                
				                on Oct 13, 2020 
				
			
			
		


				
		

		    		    
		    
		    	
			
Hi,


Have the below circuit with ADA4637, without or with input, have the below interesting issue.


       ----30M oscillation when C34 is 220pf, Oscillation disappear when C34 is 12pf. Why? Change R35 can't help.


      --I select other FET OP, there is similar oscillation as ADA4637 when C34 220pf. I change OP into no FET OP, there is no oscillation even C34 220pf.  Why?


  


Best Regards,


Kent

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	

		
	















	

	

					



			


				
	









	
    
	

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                                 Analog Employees 
                                
                            
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                on Oct 20, 2020 6:50 AM
            
            
					
    
	
    

		
		
    
					
    Hi Kent,
    

    According to the datasheet, the ADA4637 is a decompensated amplifier. This means that you must set the noise gain of the op amp equal to or greater than 5 to achieve stability. Upon checking your schematic, I can't verify the gain that you've set for the ADA4637.
    

    If you can't change the noise gain of your circuit to 5 or more, then you could also look into the ADA4627. The difference between the ADA4627 and ADA4637 is that the ADA4627 has lesser GBW and slew rate, but it is stable at a gain of 1.
    

    Thanks,
    

    Donnie
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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