# Current noise density from Ibias on op amp datasheet

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!