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LMH6574: Input resistance of the LMH6574

Martin Gehrmann91
Prodigy 10 points

Part Number: LMH6574
Other Parts Discussed in Thread: OPA4872,

The input impedance of the buffered multiplexer IC LMH6574 is specified as 5 kilohms in the datasheet. 
That's very low for an input buffer. Is that really correct, or is there a printing error? 
Perhaps it should say 5 megaohms? That would be a plausible value for an input buffer. 
Question to datasheet LMH6574.pdf Which value is actually correct?

This is crucial for our design!



								


				
				

				
			

		

			
						
									
                			    			            
						
						
			
			
			
			                        
			

				

			


			

		


				    			

						

















	

	

					
    
	
    





		
	
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                          TI__Mastermind
                        22860 points
                      
    
                          
                   		
		
    
					
    Hi Martin,
    

    I was able to look at other devices in this family, and they also highlight values in the kohms. Unfortunately, this is a somewhat old device so we do not have its information in our latest database so I will need some time to ask about this spec. 
    

    Best Regards,
    

    Ignacio
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Ignacio Vazquez Lam1
					
									
    
	
    
	                                    
                      
    
                        
                          TI__Expert
                        5516 points
                      
    
                          
                   		
		
    
					
    Hello Ignacio , do speak with Rohit Bhat once , he has a way to pull the old data and check in design review files.
    

    Also OPA4872 is a similar device which has much higher input impedance. .
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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                          TI__Mastermind
                        22860 points
                      
    
                          
                   		
		
    
					
    Hi Martin,
    

    I went ahead and spoke with our designer, and we found some information on this device. He does not believe the input impedance looking into the buffers is 5kOhms as he believes it is likely in the >100kOhms. I would like to add that with these faster devices we would recommend having some kind of impedance matching (75ohms) at the inputs of the device, which means the impedance looking into this node should not affect/attenuate your incoming signal even if it is 5kohms. 
    

    Best Regards,
    

    Ignacio
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Ignacio Vazquez Lam1
					
									
    
	
    
	                                    
                      
    
                        
                          TI__Expert
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    Do you think the impedance matching would be required if one uses this devices for lower frequency say 10;s of MHz or so ?
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Shreenidhi Patil
					
									
    
	
    
	                                    
                      
    
                        
                          TI__Mastermind
                        22860 points
                      
    
                          
                   		
		
    
					
    Hi Shreenidhi,
    

    In most cases, 10MHz would typically not require any sort of impedance matching. However, this ultimately depends on the distance a signal will be traveling from the previous stage to the input of the LMH6574. If this is on a single PCB that has typical trace lengths in the centimeters range for example, then impedance matching is not required. Rohit and I had this conversation, and under the assumption this device would be used at speed, some kind of impedance matching would be recommended. Therefore, in theory the input impedance would not affect performance if impedance matching (ex. 75ohms) is required.
    

    Best Regards,
    

    Ignacio
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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