Current noise density from Ibias on op amp datasheet

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Discussion Overview

The discussion revolves around the calculation of input referred noise current density from the bias current (Ibias) specified in op-amp datasheets. Participants explore the discrepancy between calculated values using the formula and the values provided in the datasheets.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents the equation for calculating input referred noise current density (inoise) as inoise=sqrt(2*q*Ibias), questioning why the calculated value (1.7fA/sqrt(Hz)) does not match the datasheet value (4fA/sqrt(Hz)) for an op-amp with Ibias=10pA.
  • The participant expresses confusion over the expectation that all op-amps with the same Ibias should have the same input noise current density, seeking clarification on the source of the discrepancy.
  • Another participant offers to share a response received from TI's website that purportedly explains the discrepancy, indicating that further information may clarify the issue.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there is an ongoing exploration of the reasons behind the differences in noise current density values. The discussion remains unresolved regarding the specific factors contributing to the discrepancy.

Contextual Notes

The discussion highlights potential limitations in understanding the relationship between Ibias and input referred noise current density, as well as the need for additional context from manufacturer specifications.

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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