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AD8676 Production
The AD8676 precision operational amplifier offers ultralow offset, drift, and voltage noise combined with very low input bias currents over the full operating...
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AD8676 on Analog.com

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AD8676 Unity Gain Stability



		

			

									
					    NORMGUO
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jun 7, 2021 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi team,
    

    I have a question about the unity gain stability of AD8676. I am not sure if the AD8676 can be used for unity gain configuration. I am currently using it in a configuration similar to the circuit below. 
    

     
    

    Simulation Result
    

     
    

    For AC Sweep settings I am using logarithmic from 1 to 5 Mhz with 1000 points per decade. It's a bit hard to see on this graph, but the curve starts to behave strangely before 1 Mhz. On the datasheet, it says the wide bandwidth is 10 Mhz typical. In addition, there is a graph of the closed-loop gain where the the curve starts to round off after 10 Mhz. 
    

    This leads me to guess that the AD8676 cannot be used for unity gain applications because it is not stable at unity.
    

    I apologize if my questions seems confusing. I am still a junior at university so my understanding is limited. If you could answer my question it would be greatly appreciated.
    

    Thank you,
    

    Norman Guo
    
									
			        
		
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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                on Jun 10, 2021 7:20 PM
            
            
												
						in reply to Hooman
					
									
        
	
        

		
		
        
					
        Hi Hooman,
        

        I ran the simulation using G =1V/V and I am still getting a sharp increase after 1.0 Mhz. According to the figure 
        

         
        

        I am using this exact circuit(unity gain follower)
        

         
        

        Shouldn't it start rolling off after 10 Mhz?.I performed the same simulations using an non-inverting amplifier for G= 10 and G=100 and they match the figure. 
        

        

        Thanks,
        

        Norman
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        •  Super User  
        
            
			                            
                        
                on Jun 10, 2021 8:35 PM
            
            
												
						in reply to NORMGUO
					
									
        
	
        

		
		
        
					
        Hi Norman,
        

        A macromodel is an "approximation" of the device behavior. It won't be exact.
        

        Your AD8676 LTspice simulation seems to have stopped at just above 10MHz. If you continue further in frequency, you'd see that after the ~1.8dB peaking predicted by the simulation, the output response does roll-off. 
        

         
        

        Many factors affect peaking including load resistance, layout, load capacitance, supply decoupling. The macromodel may or may not capture all these factors and again it's only a starting point approximation of the device behavior. Bench performance should be performed as the next step.
        

        Regards,
        

        Hooman
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
						
						
				
				
		
		
        
			
		
        
	
        

        

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