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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...
Datasheet
AD7746 on Analog.com

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AD7746 Calibration

		
	
				

			

									
					    supershiye
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Oct 27, 2015 
				
			
			
		
    
		
		                    	    
    
        	    
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                The user is experiencing issues with offset calibration on the AD7746, where the offset registers always show zeros after calibration. The support engineer confirmed that the offset calibration register is reloaded with the default value (0x8000) at power-on or reset, and suggested checking communication with the part and trying the CAPDAC for larger offset values. The user confirmed they could write to other registers and tried measuring parasitic capacitance without success, indicating a potential issue with the circuit inside the chip.
                        	    
    
                
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    I am working on a circuit design with AD7746. Now I am facing a problem is about the offset calibration. I write the register into calibration mode and when the calibration is done, the offset registers always shows zeros, like having never been calibrated. I also has an EVL-board, its calibration works well. Is that wrong to do calibration with connecting the capacitor? Or there is something special design in EVL-board?
    
			
    
			
									
		                             
                        

    
    
                                            
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Oct 28, 2015 1:29 PM
            
            
						
			            		
      
		
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      Hi Ye,
      
 
      Are you sure you are communicating with the part correctly on your board?  Can you write to other registers correctly?  The full range of the offset calibration register is +/-1pF, note the CAPDAC is used to offset larger values.  The offset calibration compensates for parasitic capacitance to "zero" the part, so connecting your capacitance is correct.  Do you know the size of your capacitor?  Have you tried to do the calibration without the cap? Note, the offset calibration register is reloaded by the default value at power-on or after reset, so you should check that this is not happening on your board.  The default value should be 0x8000 (register 13 and 14, see page 14 of datasheet). 
      
 
      Regards,
      
 
      Eric. 
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Oct 28, 2015 1:29 PM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      Hi Ye,
      
 
      Are you sure you are communicating with the part correctly on your board?  Can you write to other registers correctly?  The full range of the offset calibration register is +/-1pF, note the CAPDAC is used to offset larger values.  The offset calibration compensates for parasitic capacitance to "zero" the part, so connecting your capacitance is correct.  Do you know the size of your capacitor?  Have you tried to do the calibration without the cap? Note, the offset calibration register is reloaded by the default value at power-on or after reset, so you should check that this is not happening on your board.  The default value should be 0x8000 (register 13 and 14, see page 14 of datasheet). 
      
 
      Regards,
      
 
      Eric. 
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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