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AD7746 Production
The AD7745/AD7746 are a high resolution, S-? capacitance-to-digital converter (CDC). The capacitance to be measured is connected directly to the device...
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AD7746 on Analog.com

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AD7746 measurement problem



		

			

									
					    Sandman
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 15, 2015 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi , I'm trying to use the AD7746 to measure a capacitance about 30pF, I used the extension circuit, F is about  7.6
    
 
     
    
 
    but the AD7746 is feeding me with [00 00 00] (0x01, 0x02, 0x03) when the CAPDAC+ is enabled, and I tried different numbers, same issue. when I get the CAPDAC down to about 0, the CAP DATA REGISTER start providing numbers less than [80 00 00], which leads to negative capacitance reading.
    
 
     
    
 
    and I also tried to use the single-ended circuit in Figure 30 on AD7746 spec, still I got numbers less than [80 00 00].
    
 
    I'm wondering if it's possible my chip is damaged? Can anybody help me with this? Thanks.
    
 
    I got some pictures attached
    
 
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 Integrated/Special Purpose A/D Converters
                
                                                    
                 ad7746
                
                                                    
                 Capacitive-to-Digital and Touch Screen Controllers
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Dec 17, 2015 10:00 AM
            
            
					
      
	
      

		
		
      
					
      Hi Guan,
      
 
      Have you tried simplifying your setup.  Try removing the extension circuit and testing a known capacitance values.  Note, the capacitance range for this part is +/-4pF and you would need to set the zero scale value to calibrate for parasitic offsets in the application.  A zero scale offset within +/-1pF can be removed by using the capacitance offset calibration register, see page 23 in the datasheet.
      
 
      Regards,
      
 
      Eric.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Aug 2, 2018 11:18 AM
            
            
					
      
	
      

		
		
      
					
      This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

      Thank you,
      EZ Admin
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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