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AD8667 Bias Current Figures

		
		

		

			

									
						EHaag
					
							

			
				
									
											
			        
                                                			
				
			
		


				
		

		    
		    
		    		    
			
Good afternoon-


I'm planning to use the AD8667 in a high impedance measurement application, and I'm hoping to find some more data on input bias current. The datasheet gives maximum figures at 85 and 125 C, but only provides a typical figure at 25 C. While I can't see the ambient temperature inside the enclosure getting higher than 30 or 35 C, I hesitate to pull the trigger with the '8667 without a worst-case figure on bias current.


Cheers,


-Erik

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	

		
	











	

	

					



			


				
	









	
    
	

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    Hi Erik,
    

    You are right that we only specify typical values for bias current. 
    

    But the 45pA max bias current is specified across this temperature range −40°C < TA < +85°C. 
    

    This means that you should be able to use this value as worst case especially if you are sure that your temperature will vary across within this range. 
    


    Does 45pA too high for your application? What is your requirement and application so that we can give better recommendation for your circuit.
    

    

    Best regards,
    Emman
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    

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						in reply to Emman.A
					
									
    
	
    

		
		
    
					
    Hi, Emman-
    

    It looks like another member of the same DigiTrim family, the AD8662, DOES specify a maximum Ibias at 25 C. Why the discrepancy?
    

    The amplifiers will be used to measure leakage current from a capacitor under test. One will be configured as an inverting integrator with polystyrene feedback cap, and the other will be used as a (temperature compensated) inverting log amplifier that operates on the result of the integration. A range switch selects 10nA, 1nA, 100pA, or 10pA ranges. Absolute accuracy is less important than precision, as the readout is graphical rather than numerical. Up to +/- 15% error is tolerable so long as it is repeatable.
    

    The "gotcha" is that I require a bipolar supply of at least +/- 5V, otherwise I'd use something like the LTC6078, which has the added benefit of much lower current noise density.
    

    Thanks,
    

    -Erik
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    

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