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ADA4000 comparator



		

			

									
					    Jajajib
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 19, 2013 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi AD,
    
 
    is the ADA4000-1 can be used as a comparator instead of amplifier and is it a dedicated op-amp to work for this kind of use (e.g comparator)
    
 
    It seems that ADA4000 is highly sensitive working as a comparator and it doesn't like this non linear mode. 
    
 
    Moreover with a transistor on the output which could create high DV/DT for example.
    
 
    thanks for your help,
    
									
			        
		
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	
    
		
	
    















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
			                            
                        
                on Mar 14, 2013 3:46 PM
            
            
					
      
	
      

		
		
      
					
      Hi,
      
 
      Its it possible that there are a few sources of failure here. You did mention that the output actually rails to the maximum value it can on the low-side. This would tell me that it isn't a fault on the output, such as an ESD diode fusing to the low side supply due to an over-stress condition. 
      
 
      I have some questions:
      
 
      If you take your input and ground it, what does your output read? 
      
 
      What are the values of resistors you are using for your gain of -2?
      
 
      For a gain of -2, with 0-20V in, you would output 0 to -40V. As your supply on the negative side is limited to -15V, I would expect anything beyond +7V to rail the device. Once you go about 7V in, I would then expect the inverting terminal to follow (at an offset) the input: At 8V in, with Vout = -14, your inverting terminal would read about 0.66V. At 9V, the inverting node would go to about 1.333V, and continue to increase proportionately at 2/3V per V of Vin. When you continue to exceed the compliance of the device, you may be observing phase inversion of the part. 
      
 
      Really, without a schematic it is very difficult to debug the circuit. What I then would recommend is to look at our op-amp text book for more information: http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
      
 
      Cheers,
      
 
      -David
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
			                            
                        
                on Mar 14, 2013 3:46 PM
            
            
					
      
	
      

		
		
      
					
      Hi,
      
 
      Its it possible that there are a few sources of failure here. You did mention that the output actually rails to the maximum value it can on the low-side. This would tell me that it isn't a fault on the output, such as an ESD diode fusing to the low side supply due to an over-stress condition. 
      
 
      I have some questions:
      
 
      If you take your input and ground it, what does your output read? 
      
 
      What are the values of resistors you are using for your gain of -2?
      
 
      For a gain of -2, with 0-20V in, you would output 0 to -40V. As your supply on the negative side is limited to -15V, I would expect anything beyond +7V to rail the device. Once you go about 7V in, I would then expect the inverting terminal to follow (at an offset) the input: At 8V in, with Vout = -14, your inverting terminal would read about 0.66V. At 9V, the inverting node would go to about 1.333V, and continue to increase proportionately at 2/3V per V of Vin. When you continue to exceed the compliance of the device, you may be observing phase inversion of the part. 
      
 
      Really, without a schematic it is very difficult to debug the circuit. What I then would recommend is to look at our op-amp text book for more information: http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
      
 
      Cheers,
      
 
      -David
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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