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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 2:42 PM
            
            
						
			            		
        
		
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        I'm currently using this AFE:
        
 
        The first stage takes the AD5933 200mV peak-peak signal to 0.18Vdc, 28mV p-p, the second stage is the buffer like you have above with the three electrodes (I'm not using a transistor for current), the third stage is for some gain, and the fourth stage moves the DC bias back to Vdd/2 for the AD5933 internal op amp. The gain is 1 for the internal op amp. 
        
 
        I've tested the circuit on the scope and verified the frequencies, DC and AC voltages, and ensured that the signal wasn't clipping. I'm sweeping from 100 kHz - 1 Hz, the PGA gain is 1, my calibration resistor is 747 ohms (interested range is roughly 200~2000 ohms), and I'm interested in the Nyquist plot (real and imaginary values). I've measured different values of "unknown" resistors just fine. I'm expecting graphs like the one below, where as more of the biomarker is added, the semi-circle should be getting larger. 
        
 
        Example from online:
        
 
        
 
        Here's an example of data I collected. The black lines are the attempt at circle curve fitting for the value of interest.
        
 
        
 
        Removed the last two data points (1.5 Hz and 1 Hz) so it's easier to see
        
 
        
 
        The circle shape starts out alright but then gets kinda wild. 
        
 
        This is the professional EIS device's results. They aren't the best as a whole (the circles should be getting larger as more biomarker is added), but the individual curves are nicely shaped.
        
 
        
 
        Do you have any further suggestions for making the AD5933 work like a potentiostat or work in a three electrode configuration? 
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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      • 
	
        



	
		
	
	



        
	
        
		
        
							
					
        
		
        
		    
		    		    
			
				
									
							        
			
			
			
          
            
						
                            
            
            				
            
                on Mar 20, 2017 2:42 PM
            
            
						
			            		
          
		
										0
					
	
          

		
		
          
		
          
                                
                                                            
          
					
          I'm currently using this AFE:
          
 
          The first stage takes the AD5933 200mV peak-peak signal to 0.18Vdc, 28mV p-p, the second stage is the buffer like you have above with the three electrodes (I'm not using a transistor for current), the third stage is for some gain, and the fourth stage moves the DC bias back to Vdd/2 for the AD5933 internal op amp. The gain is 1 for the internal op amp. 
          
 
          I've tested the circuit on the scope and verified the frequencies, DC and AC voltages, and ensured that the signal wasn't clipping. I'm sweeping from 100 kHz - 1 Hz, the PGA gain is 1, my calibration resistor is 747 ohms (interested range is roughly 200~2000 ohms), and I'm interested in the Nyquist plot (real and imaginary values). I've measured different values of "unknown" resistors just fine. I'm expecting graphs like the one below, where as more of the biomarker is added, the semi-circle should be getting larger. 
          
 
          Example from online:
          
 
          
 
          Here's an example of data I collected. The black lines are the attempt at circle curve fitting for the value of interest.
          
 
          
 
          Removed the last two data points (1.5 Hz and 1 Hz) so it's easier to see
          
 
          
 
          The circle shape starts out alright but then gets kinda wild. 
          
 
          This is the professional EIS device's results. They aren't the best as a whole (the circles should be getting larger as more biomarker is added), but the individual curves are nicely shaped.
          
 
          
 
          Do you have any further suggestions for making the AD5933 work like a potentiostat or work in a three electrode configuration? 
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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