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The AD8274 is a difference amplifier that delivers excellent ac and dc performance. Built on Analog Devices, Inc., proprietary iPolar® process and laser...
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AD8274 - common-mode voltage limits

		
	
				

			

									
					    Lellozzo
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jul 29, 2020 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Hi, 
    

    I'm using the AD8274 to measure the voltage of a cell (2V to 5V), with a 0V +7V supply voltage, coming form a buck converter. I'm using the G=1/2 configuration. 
    

    It looks the maximum common-mode voltage I can apply is +16V, then the output voltage saturates to 7V. The minimum I can apply is 0.7V, but this is not a problem for me, since I will have only positive common-mode voltage. Is this normal for you ? I was expecting it to stand a higher common-mode voltage. 
    

    Can I connect the output to the adc directely or do you suggest to have a voltage follower before ?
    

    

    Thank you for your help. 
    

    Sincerely,
    

    Leandro
    
			
    
			
									
		                             
                        

    
    
                                            
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Jul 29, 2020 11:22 PM
            
            
						
			            		
      
		
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      Hello,
      

      You can have higher common mode voltage if you have wider supply.
      For your case which you uses 7V single ended supply and having the diff amp configured with gain of 1/2,
       the common mode voltage is limited to 16.5V.
      From the table, the max common mode voltage will be 3(Vs - 1.5).
      

      On the otherhand, your input is 2 to 5V.
      Considering the 2V input at the gain of 1/2, that will have an output of 1V.
      

      From the Spec table, for gain of 1/2, the output swing range is -Vs+1.5 to +Vs-1.5.
      For your supply condition, the minimum output would be 1.5V. Therefore you cannot achieve 1V out for input of 2V.
      

      You may want to take a look to LT1997-2.
      This is also a diff amp with attenuated gain.
      You could configure different gains as shown in page 15 of the data sheet.
      

      Your supply is within the supply range and the output could swing as low as 150mV.
      It has wide common mode voltage ad well.
      

      Thanks and regards.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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