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AD8479 Recommended for New Designs
The AD8479 is a difference amplifier with a very high input common-mode voltage range. The AD8479 is a precision device that allows the user to accurately...
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AD8479 on Analog.com

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AD8479 Ref pin unconnected



		

			

									
					    tae213
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 9, 2022 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi there, 
    

    I understand the relationship between common-mode voltage range and reference voltage when operating as a single power supply. But I came across that it also works fine when I leave the both ref pins unconnected, and provide input DC voltage 4V then I was able to observe output ~4V. Why dose this work when running as a single power supply(V+= 15V, V-=0V) even though ref pins are not connected to anything? I am not sure if Vcm formula provided in the datasheet is still valid in this case. 
    

    Thanks
    
										

    
				edited few words 
    
[edited by: tae213 at 7:14 PM (GMT -5) on 9 Dec 2022]
				
    
						
			        
		
    

				
						
        

    
    Tags: 
                                            
                 amplifiers
                
                                                    
                 ad8479
                
                                                    
                 common mode input voltage
                
                                                    
                 differential amplifier
                
                                                    
                 Specialty Amplifiers
                
                                        
                                            
                
                
                                
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Dec 14, 2022 1:52 AM
            
            
					
      
	
      

		
		
      
					
      HI,
      

      May I know your input voltages for -IN and +IN (Assumed it was 4V)? 
      

      Also, I would like to confirm on how you measured the output. Did you use the formula "OUTPUT = VOUT - VREF"? Did you measure it across Vout (pin 6) and Ref (pin 5) pins?
      

      When the REF pin is floating, it will have a very high impedance and the voltage reading on its pins is probably almost equal to the input voltage of IN+ (On this case is about 4V)
      

      With the said configuration, when the IN+ is 4V and the IN- is any voltage values, the output voltage reading on pin 6 is also within the 4V values and the OUTPUT value if based on the formula will be approximately 0V  (ranging from 0V to mV range).
      

      The theory of operation "VOUT = VIN+ - VIN-" will not be followed when the REF pin is floating. It is advisable to have your REF pin to be connected to a voltage reference source.
      

      You can also try to simulate it using the LTspice simulation software.
      

      Hope that helps.
      

      Regards,
      

      JE
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Dec 14, 2022 8:18 AM
            
            
												
						in reply to jeobico
					
									
        
	
        

		
		
        
					
        Input is a DC value of 4V and It's -IN pin is connected to the ground. I was actually wrong. I see that output offset is getting bigger when I increase input voltage from 1 to 4V. like when I provide input voltage of 4, I see the output 4.6V. I am measuring the output with respect to ground.  I could verify that ref pin has same voltage as input pins when ref pins are not connected to anything. But why is there huge inconsistent offset voltage at the output when increasing the input when using a single power supply and ref pins unconnected? 
        

        As datasheet suggested, I will definitely use reference voltage when running as a single power supply but I am still curious 
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Dec 16, 2022 3:09 AM
            
            
												
						in reply to tae213
					
									
        
	
        

		
		
        
					
        Hi,
        

        When the ref pins are unconnected, the device will act as a non-inverting amplifier. The use of single power supply has no effect on the voltage output as long as it is within the limits of supply.
        

        As for this device, we will apply the formula: Vout = Vin(1+Rf/Rin)
        

        Vout = 4V(1+(134387.6/1060310.61))
        

        Vout = ~4.506V
        

        Note: Resistor values are based on the data on the LTspice model
        

        Hope that helps.
        

        Regards,
        

        JE
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
						
						
				
				
		
		
        
			
		
        
	
        

        

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