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AD7747 Production
The AD7747 is a high-resolution, S-? capacitance-to-digital converter (CDC). The capacitance to be measured is connected directly to the device inputs...
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AD7747 on Analog.com

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AD7747 - Liquid Level Measurement



		

			

									
					    sureshdigant
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 30, 2014 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi Everyone,
    
 
                I am using AD7747 for liquid level measurement, I can able to read all the registers as well as capacitance properly. But the issue is the capacitance increasing 1fF(femto farad) for every few minutes. Presently i am using Cin = (((code-0X800000)/0X800000)*Cref) this equation for measuring capacitance. Could anyone tell me whether this equation is correct or not as well as how to measure the Cref.  End application is completed, once  liquid level sensor is done then we will move production level, if anyone give a appropriate answer that is very much help full for us.
    
 
    Thanks & Regards
    
 
    Suresh Kumar.S
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
			                            
                        
                on Dec 31, 2014 4:52 PM
            
            
					
      
	
      

		
		
      
					
      Hi Kumar,
      
 
      Your equation for Cin looks inline with that used in the evaluation software, it should be ok.  Cref is the internal reference capacitance. I have attached an application note that has some detail on this parameter (page 2) which should be true for the AD7147 except for instead for 4.096 you should use 8.192pF.
      
 
      Alternative, to my knowledge another way to calculate Cin is to convert the hex code to decimal, divide by 2^23 and then multiply by 8.192pF.
      
 
      Have you ruled out temperature and humidity to account for the 1fF change? It is a very small change and could be as a result of small environmental changes.
      
 
      Regards,
      
 
      Eric.
      
			
      

		
		
		
				CN0129.pdf
			
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
			                            
                        
                on Dec 31, 2014 4:52 PM
            
            
					
      
	
      

		
		
      
					
      Hi Kumar,
      
 
      Your equation for Cin looks inline with that used in the evaluation software, it should be ok.  Cref is the internal reference capacitance. I have attached an application note that has some detail on this parameter (page 2) which should be true for the AD7147 except for instead for 4.096 you should use 8.192pF.
      
 
      Alternative, to my knowledge another way to calculate Cin is to convert the hex code to decimal, divide by 2^23 and then multiply by 8.192pF.
      
 
      Have you ruled out temperature and humidity to account for the 1fF change? It is a very small change and could be as a result of small environmental changes.
      
 
      Regards,
      
 
      Eric.
      
			
      

		
		
		
				CN0129.pdf
			
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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