AD8655 cannot simulate current noise

Question

Hi , 
 I could simulate the Input Voltage Noise Density but could not simulate the current Noise Density with AD8655 on Ni Simulator . 
 could not find information on Data sheet with regards current noise of this OPAMP 
 Thanking you 
 With best regards and wishes

FormerMember · Accepted Answer

Wallman16, 
 The current noise on most CMOS op amps is so low we cannot measure it. 
 You can use 0.5 fA/rt-Hz for cacluations. 
 Harry

FormerMember · Answer

Wallman16, 
 Current noise on bipolars is measured by putting a big resistor in series w/ the noninverting input 
 of a follower, measuring the output noise, and subtracting out the voltage noise and the resistor noise. 
 With a CMOS or JFET op amp, the resistor is 10-50 meg, so the resistor and input capacitance 
 form a pole at a few Hertz. So any 1kHz current noise number or graph is suspect. 
 Can you come up with a better way? The current noise will be a function of the transistor size. 
 What is your circuit that needs a number? A TIA?? 
 Harry

EZAdministrator · Answer

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