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AD7690 Production
The AD7690 is an 18-bit, successive approximation, analog-to-digital converter (ADC) that operates from a single power supply, VDD. It contains a low power...
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AD7690 on Analog.com

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AD7690 Readings / Output code



		

			

									
					    Ant1882
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Sep 1, 2014 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hello,
    
 
    Hopefully someone may be able to help me out...
    
 
    I am testing out an AD7690 18-bit ADC for an application. Currently I can get a reading from the ADC, though it is not what I expect.
    
 
    Vref is set to 2.048 V and there is 1.45 V onto IN+, IN- is connected to ground.
    
 
    I assume that I should still get a code between 0x00000 and 0x1FFFFF for IN+, so I would expect to see roughly 0x16A80 (192800 decimal) for the 1.45 V, what I am getting however is around 0x1284B (75851 decimal). Working with the calculation of 15.625 uV per bit, (2.048*2 / 2^18).
    
 
    My question really is given that I have IN- connected to ground, can I still expect to be able to read a code as above? Or does it not work that way? If I am expecting the correct code then it may well be the communications that is the issue.
    
 
    Best Regards
    
 
    Anthony
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	
    
		
	
    















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
			                            
                        
                on Sep 2, 2014 11:01 PM
            
            
					
      
	
      

		
		
      
					
      Hi Anthony,
      
 
      For a differential converter like this it is best to use it in that fashion. We have plenty of pseudo-diff/SE parts like the AD7686 or AD7988-5 that can be used as you describe and are pin compatible. Generally the differential ADCs like the AD7690 require the common mode voltage to be maintained at Vref/2 or 1.024V in you case. Your common mode in the above described scenario is much lower (1.45V)/2 = 0.725V. I would try to keep the common mode within 100mV of Vref/2. Hope this helps.
      
 
      Best Rgds,
      
 
      Alan
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
			                            
                        
                on Sep 2, 2014 11:01 PM
            
            
					
      
	
      

		
		
      
					
      Hi Anthony,
      
 
      For a differential converter like this it is best to use it in that fashion. We have plenty of pseudo-diff/SE parts like the AD7686 or AD7988-5 that can be used as you describe and are pin compatible. Generally the differential ADCs like the AD7690 require the common mode voltage to be maintained at Vref/2 or 1.024V in you case. Your common mode in the above described scenario is much lower (1.45V)/2 = 0.725V. I would try to keep the common mode within 100mV of Vref/2. Hope this helps.
      
 
      Best Rgds,
      
 
      Alan
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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