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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 
				
			
			
		
    
		
		                    	    
    
        	    
    Thread Summary
    
                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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                on Oct 28, 2015 6:15 PM
            
            
						
			            		
        
		
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        Hi, Eric
        
 
        Thank you for your replying. I am certainly sure about the offset calibration function works, and I could write the register correctly. But the problem is whatever I did with the circuit (open loop, short, with cap, etc), the calibration returns the same 0x8000 as you said, which means zeros. I have tried to use LCR measure the parasitic cap on the board, but it can't give out a value from CIN and EXC. It caused by circuit inside the chip. I am going to try CAPDAC by your advice.
        
 
        Thanks,
        
 
        Ye
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Oct 28, 2015 6:15 PM
            
            
						
			            		
          
		
										0
					
	
          

		
		
          
		
          
                                
                                                            
          
					
          Hi, Eric
          
 
          Thank you for your replying. I am certainly sure about the offset calibration function works, and I could write the register correctly. But the problem is whatever I did with the circuit (open loop, short, with cap, etc), the calibration returns the same 0x8000 as you said, which means zeros. I have tried to use LCR measure the parasitic cap on the board, but it can't give out a value from CIN and EXC. It caused by circuit inside the chip. I am going to try CAPDAC by your advice.
          
 
          Thanks,
          
 
          Ye
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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