Current noise density from Ibias on op amp datasheet

[jrive]

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!

 

[Greg Bernhardt]

I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?

 

[jrive]

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.

 

[Greg Bernhardt]

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!

 

[alphy]

Thank you for your question. I would like to provide a response to help clarify the discrepancy you have observed. Firstly, it is important to understand that the input referred noise current (inoise) and the input noise current density are two different parameters that are often used interchangeably but have different meanings.

The input referred noise current (inoise) is the equivalent noise current at the input of the op-amp, while the input noise current density is the noise current per square root of frequency. The equation you have mentioned, inoise=sqrt(2*q*Ibias), is used to calculate the input referred noise current, not the input noise current density.

The input noise current density is calculated using the following equation:

Idensity = inoise / sqrt(BW)

Where BW is the bandwidth in Hz.

Now, coming to the difference between the calculated and specified values for the input noise current density, there are a few factors that can contribute to this. Firstly, the equation mentioned above assumes ideal conditions and does not take into account any non-idealities in the op-amp. In real-world scenarios, there can be additional noise sources that contribute to the overall input noise current density.

Secondly, the specified value on the datasheet is an average value and can vary from device to device. The value for Ibias may be the same, but the actual noise current density can vary due to manufacturing variations.

Lastly, the specified value on the datasheet may also include the noise contribution from other components in the circuit, such as resistors or capacitors. This can also lead to a difference between the calculated and specified values.

In conclusion, the difference between the calculated and specified values for the input noise current density can be due to various factors, including non-idealities in the op-amp, manufacturing variations, and inclusion of noise from other components in the circuit. It is important to carefully analyze the datasheet and understand the context in which the values are specified. I hope this helps clarify your doubts.