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LTC6081 Recommended for New Designs
The LTC6081/LTC6082 are dual/quad low offset, low drift, low noise CMOS operational amplifiers with rail-to-rail input/output swing. The 70μV maximum...
Datasheet
LTC6081 on Analog.com

LTC6082 Production
The LTC6081/LTC6082 are dual/quad low offset, low drift, low noise CMOS operational amplifiers with rail-to-rail input/output swing. The 70μV maximum...
Datasheet
LTC6082 on Analog.com

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

    
    
                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 amplifiers
                
                                                    
                 Low Power Op Amps (≤ 1mA/amp)
                
                                                    
                 LTC6081
                
                                                    
                 LTC6082
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Jun 9, 2021 4:42 PM
            
            
						
			            		
      
		
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      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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