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AD8642 Production
The AD8641/AD8642/AD8643 are low power, precision JFET input amplifiers featuring extremely low input bias current and rail-to-rail output. The ability...
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AD8642 on Analog.com

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Boosting AD8642 output current

		
	
				

			

									
					    Damaso
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jun 25, 2013 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    I'm testing a design in which I need to do a current to voltage conversion. The current ranges from picoamperes to 100mA (both positive and negative), so I use 4 different resistors and a switch to select each of them. I filter the corresponding signal (not shown in the image), use a variable gain (x1, x10, x100, x1000) amplifier and then adapt it and detect it.  
    
 
    Everything works perfectly fine till 20mA, the op amp output current limit. Can anyone please show me a simple power stage that can be used preferibly inside the feedback loop? The output voltage has to be between -5 and +5 volts and has to work for +-100mA. 
    
			
    
			
									
		                             
                        

    
    
                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 amplifiers
                
                                                    
                 AD8642
                
                                                    
                 High Voltage Op Amps =12V
                
                                
            
    
    
    
		
    

							
    
				
				
			
    
					
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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                on Jun 27, 2013 12:06 PM
            
            
						
			            		
        
		
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        The circuit works quite as expected for currents above 1uA. But when I connect the 500Kohms feedback resistor and try to meassure the current, power source shows a current consumption of at least 400mA in both rails. It seems like a start condition problem with that configuration of the feedback loop, because it doesn't happen if I select the 500ohms and change it to the 500Kohms during a meassure.
        
 
        I'm thinking that maybe what does work for high currents (mA) won't work or won't meet the expected behaviour for the uA, nA and pA currents. That's why i'll try and use two different op amps for above mA and below mA. I'll keep the AD8642 for the low current meassurement, thanks to its femtoampere bias current, but I'll need a new op amp capable of 100mA output current and +-12V sources. Would anyone please recommend a part for my project?
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Jun 27, 2013 12:06 PM
            
            
						
			            		
          
		
										0
					
	
          

		
		
          
		
          
                                
                                                            
          
					
          The circuit works quite as expected for currents above 1uA. But when I connect the 500Kohms feedback resistor and try to meassure the current, power source shows a current consumption of at least 400mA in both rails. It seems like a start condition problem with that configuration of the feedback loop, because it doesn't happen if I select the 500ohms and change it to the 500Kohms during a meassure.
          
 
          I'm thinking that maybe what does work for high currents (mA) won't work or won't meet the expected behaviour for the uA, nA and pA currents. That's why i'll try and use two different op amps for above mA and below mA. I'll keep the AD8642 for the low current meassurement, thanks to its femtoampere bias current, but I'll need a new op amp capable of 100mA output current and +-12V sources. Would anyone please recommend a part for my project?
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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