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LT1818 Production
The LT1818/LT1819 are single/dual wide bandwidth, high slew rate, low noise and distortion operational amplifiers with excellent DC performance. The LT1818...
Datasheet
LT1818 on Analog.com

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LT1818 sustained differential input

		            
		

			

									
					    Sonny1983
					
							


			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 12, 2021 
				
			
			
		
    
		
     
				
		
    
		    		    
		    
		    	
			
    Hi,
    

    in a scenario where I want to use the LT1818 (output stage of scope differential probe) it can happen that the inputs stay at a differential voltage of about 4V for longer (when a wrong attenuation is selected or to high signal is measured). The inputs however always stay within V+ and V-. In front of the inputs are 100Ohm resistors. Amp runs in Gain 11 config (1k feedback). LTspice shows input currents in the range of under 10µA. The datasheet says to avoid this scenario because the amp may draw excessive current in the range of 50mA; however LTspice shows only 7.3mA and 13mA at +/-5V (output is 1Meg loaded). Is the simulation simplified there and doesn't show the excessive current draw problem, or is there no inherent danger of damage in my scenario? 
    

    Thanks,
    

    Simon
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 LT1818
                
                                                    
                 amplifiers
                
                                                    
                 High Speed Op Amps (Bandwidth = 50MHz)
                
                                
            
    
    
    

		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 12, 2021 10:58 PM
            
                                    
					
      
	
      

		
		
      
					
      Hi Simon:  You can believe the data sheet.  Apparently the fault condition wasn't modelled.  The high current is due to the fact that the opamp is trying to slew, when in fact it is clipped.  It may be possible to limit the supply current at the supplies, for example with 33 Ohm resistors, so that the supply voltages collapse somewhat during the fault condition.  Also, you can put protection diodes across the inputs to reduce the draw, but this will also reduce the maximum slew rate achievable if that's okay.   I believe the real threat to the part is thermal.  If you can use these methods to reduce the power dissipation, that should protect the part.  Don't rely on the opamp to limit to 50mA, as that is just a typical number.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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													0
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 12, 2021 10:58 PM
            
                                    
					
      
	
      

		
		
      
					
      Hi Simon:  You can believe the data sheet.  Apparently the fault condition wasn't modelled.  The high current is due to the fact that the opamp is trying to slew, when in fact it is clipped.  It may be possible to limit the supply current at the supplies, for example with 33 Ohm resistors, so that the supply voltages collapse somewhat during the fault condition.  Also, you can put protection diodes across the inputs to reduce the draw, but this will also reduce the maximum slew rate achievable if that's okay.   I believe the real threat to the part is thermal.  If you can use these methods to reduce the power dissipation, that should protect the part.  Don't rely on the opamp to limit to 50mA, as that is just a typical number.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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