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Right leg drive integrator



		

			

									
					    pauledd
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 24, 2019 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi
    

    

    I have a question regarding the driven right leg part circuit depicted in your ECG-circuit of your LT1168 or LT1920 datasheets.
    

    

     
    

    I understand this circuit so: C1 in the feedback path together with R3/R4 (15k)  forms an integrator that attenuates the input common mode signal
    

    with increasing frequency.  R2 in the feedback is for a stable biasing by restoring feedback at DC (where the circuit behaves like a high-gain inv. ampl.)
    

    What does "POLE AT 1kHz" mean? Is that the "cutoff" frequency of the integrator? How is it calculated? Is this correct?:
    

    1/(2π*((R3*R4)/(R3+R4))*C1) = 1061 Hz ?
    

    What does then R1 do next C1? What is its purpose?
    
										

    
				grammar
    
[edited by: pauledd at 6:42 AM (GMT 0) on 24 May 2019]
				
    
					
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			


				
	









	
      
	

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													+1
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      •  Super User  
      
            
			                            
                        
                on May 29, 2019 2:02 AM
            
            
					
      
	
      

		
		
      
					
      Hi pauledd,
      

      Sorry about the delay in getting a response back to you.
      

      

      Your questions and my responses below:
      

      Question 1 : What does "POLE AT 1kHz" mean? Is that the "cutoff" frequency of the integrator? How is it calculated?
      

      Response: "POLE at 1kHz" is due to the final stage R7, and C3. It's the overall circuit roll-off.
      

      LT1168 will have plenty of BW at a gain of 10V/V and the R7, C3 combination ensures that unused bandwidth beyond 1kHz is attenuated which also serves to reduce unwanted noise.
      

      Question 2: What does then R1 do next C1? What is its purpose?
      

      Response: R1 creates a zero in the transfer function at around 1.3kHz where the integration roll-off ends. I cannot immediately think why it was deemed necessary to stop the integration beyond this frequency!
      

      However, I've seen series R placed in series with capacitors in the op amp feedback loop used before. The reason I had been given was to isolate the capacitor beyond its usable frequency such as when the cap approaches its resonant frequency where it could misbehave / act inductive and cause instability. I cannot be 100% sure that's the reason for having R1 in series with C1 but it's the only thing that comes to my mind (unless other folks on this forum can think of other reasons?).
      

      

      Regards,
      

      Hooman
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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