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LTC6081 LTC6082 precision bidirectional current source



		

			

									
					    Besty
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jun 9, 2021 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    From the datasheet for LTC6082 (60812fd.pdf), there is an example of nanoamp current source.
    

     
    

    I couldn't find any detail explaining this circuit. How would I change it to make it operate between 1uA to few microamps (bidirectional)? Would change 10Mohms to 10kohms would greatly degrade the performance? I can see the bottom two OPs are part of an improved Howland current pump that is part of a the negative feedback loop. 
    

    https://www.analog.com/en/analog-dialogue/articles/a-large-current-source-with-high-accuracy-and-fast-settling.html
    

    What does the top OPAMP do with the FB? It'd be awesome if someone could write a follow-up article on this 3-OPAMP like Nick's article. Thanks.
    
										

    
				Clarifies it's still a bidirectional current.
    
[edited by: Besty at 6:09 AM (GMT -4) on 9 Jun 2021]
				
    
					
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
			                            
                        
                on Jun 9, 2021 8:42 PM
            
            
					
      
	
      

		
		
      
					
      Hi Besty:  It's a very simple circuit actually.  The bottom 2 opamps and their resistors form a difference amplifier, measuring the voltage across the 10Meg resistor and outing that voltage at the top opamp -input.  That is what closes the loop  So basically, the input voltage is divided by 1000 (100k:100 Ohms), and the circuit will put that divided down voltage across the 10Meg.  So the current in the 10Meg will be I=(Vin/(1000))/10Meg, = Vin/10Gig. 
      

      Yes the current can be bidirectional, because the opamp has a split supply.
      

      To scale for different currents, just scale the 10Meg.  Make it smaller for high currents.  The highest current possible will be 2.5V-Vload, divided by the resistance.  
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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