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Three electrode measurement for AD5933?

		
	
				

			

									
					    oscarcot
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 19, 2015 
				
			
			
		
    
		
		                    	    
    
        	    
    Thread Summary
    
                The user is using the AD5933 for bioelectrical electrode impedance measurements in a three-electrode configuration. The issue was discontinuities in the Nyquist plot and large errors at low frequencies. The solution involved recalculating the MCLK for each frequency, ensuring the excitation signal was within the ADC range, and adjusting the high-pass filter cutoff frequency to 0.14 Hz. This resulted in continuous and more accurate impedance measurements.
                        	    
    
                
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    Hi everybody,
    
 
     
    
 
    we're using the AD5933 for bioelectrical electrode impedance measurements. Usually, the electrodes have to be immersed in a saline solution for measuring, and a 2 point measurement (Electrode to Reference electrode) adds unwanted potential offsets to the measurement. For a more accurate measurement, a technique using 3 electrodes is employed:
    
 
     
    
 
    (Fig. 4 from this link)
    
 
    Ralph Group : projects : Fabrication and characterization of graphene electrodes for electrochemistry experiments
    
 
    Figure 4
    
 
    Is there any way to connect the AD5933 in such an arrangement to measure impedance? Do you have any additional suggestions for performing such a measurement?
    
 
     
    
 
    Thanks and Kindest Regards,
    
 
    Oscar Cota
    
			
    
			
									
		                             
                        

    
    
                                            
                 clock and timing
                
                                                    
                 Clocks & Timers
                
                                                    
                 Direct Digital Synthesis (DDS)
                
                                                    
                 ad5933
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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                on Mar 20, 2017 3:36 PM
            
            
						
			            		
        
		
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        The RFB is 20k ohms. I'm using the AFE below and in practice the Reference and Counter are not shorted together, but connected to electrodes in the biomarker solution like the examples Oscar gave above. 
        
 
        Edit:
        
 
        The red boxes which I tried to add in Paint are multiplexers and the red thing between them in the calibration resistor.
        
 
        
 
        For measuring the impedance, I start at 100 kHz and measure the calibration resistor and then the biomarker solution, then I manually step the frequency down to 63 kHz and measure the calibration resistor and then the biomarker solution, and I keep decreasing the frequency until 1 Hz. Then at the end of the sweep, the code calculates the gain factor and the actual real and imaginary values for the solution.
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Mar 20, 2017 3:36 PM
            
            
						
			            		
          
		
										0
					
	
          

		
		
          
		
          
                                
                                                            
          
					
          The RFB is 20k ohms. I'm using the AFE below and in practice the Reference and Counter are not shorted together, but connected to electrodes in the biomarker solution like the examples Oscar gave above. 
          
 
          Edit:
          
 
          The red boxes which I tried to add in Paint are multiplexers and the red thing between them in the calibration resistor.
          
 
          
 
          For measuring the impedance, I start at 100 kHz and measure the calibration resistor and then the biomarker solution, then I manually step the frequency down to 63 kHz and measure the calibration resistor and then the biomarker solution, and I keep decreasing the frequency until 1 Hz. Then at the end of the sweep, the code calculates the gain factor and the actual real and imaginary values for the solution.
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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