Products Mentioned

AD7142 Production
The AD7142 and AD7142-1 are integrated capacitance-to-digital converters (CDCs) with on-chip environmental calibration for use in systems requiring a novel...
Datasheet
AD7142 on Analog.com

AD7147 Production
The AD7147 CapTouch™ controller is designed for use with capacitance sensors implementing functions such as buttons, scroll bars, and wheels. The sensors...
Datasheet
AD7147 on Analog.com

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Calculate the actual measured capacitance value

		
	
				

			

									
					    HuaWuque
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Apr 16, 2025 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
    
		      		            		        
    Category: Software
    
    		        		            		    
    Product Number:  AD7142 
    
    		        		    		    
    
		    		    
		    			
    
			    
    I would like to know whether it is possible to calculate the actual measured capacitance value (in pF) from the AD7142 or AD7147, for example by using the offset and the readings from the AD7142/AD7147. Is there a formula that can be used for this calculation?
    
			
    
			
									
		                             
                        

    
    
                                            
                 software
                
                                                    
                 ad7142
                
                                                    
                 Integrated/Special Purpose A/D Converters
                
                                                    
                 Capacitive-to-Digital and Touch Screen Controllers
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Apr 16, 2025 4:49 AM
            
            
						
			            		
      
		
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      Hi  ,
      

      While it is possible to calculate the absolute capacitance value, it will not be very accurate, a rough estimate could be gained by summing the Offset value and the CDC result value, the Offset resolution is 0.32pF, the CDC result resolution is 0.244fF.
      

      Multiplying the offset and CDC result codes by the resolution values and adding them together will give the rough estimate. Greater accuracy could be achieved by at least calibrating the zero capacitance value and also at a known capacitance value.
      

      For example if at the zero capacitance value, Offset = 17 and CDC Result = 2224, each code change in offset adds 0.32pF (The offset value must be incremented when the CDC Result is close to saturation) and each code change in CDC Result adds 0.244fF.
      

      So if a measurement gives the result, Offset = 25 and CDC Result = 40,000, the measured capacitance is (25-17)*0.32pF + (40,000 - 2224)*0.244fF = 11.78pF
      

      Regards,
      

      Maurice.
      

      

      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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