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LT6235 Production
The LT6233/LT6234/LT6235 are single/dual/quad low noise, rail-to-rail output unity-gain stable op amps that feature 1.9nV/√Hz noise voltage and draw...
Datasheet
LT6235 on Analog.com

AD8674 Production
The AD8671/AD8672/AD8674 are very high precision amplifiers featuring very low noise, very low offset voltage and drift, low input bias current, 10 MHz...
Datasheet
AD8674 on Analog.com

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LT6235 op amp hangs in LTSpice simulation

		
	
				

			

									
					    fossil
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 11, 2020 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    I'm trying to run an AC Analysis on a 9-pole 3 kHz low pass filter using the LT6235 model. The simulation does not seem to converge, even after a half hour. When I replace all of the LT6235's with LT6238s, it runs in a few seconds. When I replace the LT6235 with AD8674, it runs virtually immediately. The filter was originally designed in Analog Filter Wizard. At one point I saw an error message relating to "DC operating point not found". I put in ".options cshunt = 1E-15" just in case.
    

    The simulation seems to run fine with two other op amp models but not with LT6235. Is this a model issue or something a little more obscure?
    

    Thank you
    

    Bob
    

    2086.LT6235_3khz_SSB_filter.asc
    
			
    
			
									
		                             
                        

    
    
                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 amplifiers
                
                                                    
                 Precision Op Amps (Vos ≤1mV & TCVos ≤2uV/C)
                
                                                    
                 ad8674
                
                                                    
                 LT6235
                
                                        
                                            
                
                
                                
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      •  Analog Employees  
      •  Super User  
      
            
						
                            
            
            				
            
                on Feb 11, 2020 6:20 PM
            
            
						
			            		
      
		
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      Hi Bob,
      

      I was told that if you add the following spice directives to your schematic the simulation will run immediately:
      

      .options gminsteps = 0
      .options srcsteps = 0
      

      I tried it and it did work.
      

      There is also another technique of keeping the ESC key down while you invoke a simulation (running man) to do a transient analysis, and that also seems to work. I don't know what the effect of keeping / mashing the ESC key but it does seem to work.
      

      Regards,
      

      Hooman
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Feb 12, 2020 7:54 AM
            
            
												
						in reply to Hooman
					
										
			            		
        
		
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        Hi Hooman,
        

        I think this is very simply explained. I know that any type of analysis consists of successive stages. When running any type of analysis, the first step is always to search for a DC operating point, for example.
        

        These steps can be canceled by pressing the ESC key and this is written in the LTspice status bar in the lower-left corner of the screen. If the analysis is very slow, you can easily read it.
        

         
        

        I also experience difficulties when starting circuits with OP177, any analysis is slow and in the status bar I see that the process of finding solutions to equations and iterating through them is going on. I'm a little annoyed.
        

        Regards,
        

        Kirill
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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