Current noise density from Ibias on op amp datasheet

AI Thread Summary
The discussion centers on the discrepancy between calculated input referred noise current density from bias current (Ibias) and the values provided in op-amp datasheets. The formula used, inoise=sqrt(2*q*Ibias), yields a lower noise current density than what is specified in the datasheet, raising questions about the underlying reasons for this difference. It is noted that while Ibias is consistent across op-amps, the input noise current density varies, suggesting other contributing factors. A user mentions receiving clarification from Texas Instruments regarding this issue and offers to share that information for others' benefit. The conversation highlights the complexity of noise specifications in op-amps beyond simple calculations.
jrive
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it is my understanding that the input referred noise current (density) can be obtained from Ibias via

inoise=sqrt(2*q*Ibias), where q=1.6e-19.

on op-amp datasheets, both Ibias and the input referred noise current are both specified. Yet, the value typically shown for the noise current density does not match the result from the equation above. What am I missing?

ie, Ibias=10pA.

inoise (calculated)=sqrt(2*q*Ibias)=1.7fA/sqrt(Hz).

the datasheet shows 4fA/sqrt(Hz). Where is the difference coming from?

From the equation for inoise above, I would expect all op-amps with the same Ibias specification to have the same input noise current density spec as well. This does not seem to be the case...why would that be?

Thanks!
 
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I posted the question on ti's website and got a response that explains where the discrepancy comes from. Perhaps i can post it here (paraphrasing it) for the benefit of others.
 
jrive said:
I posted the question on ti's website and got a response that explains where the discrepancy comes from. Perhaps i can post it here (paraphrasing it) for the benefit of others.

Good idea!
 

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