frilance
Member level 1
Dear all
I've been playing around with simulated equivalent input current noise in LTSpice and I found out that, in the same way it works when you have two DC current sources in series, the maximum current noise you get is the one set by the lowest contributor.
For example:
In a simple circuit based on a DC current source attached to a 10K and a 10 Ohms resistor in series, the input current noise density shown by LTSpice is around 1.29 pA/Sqrt(Hz), even if the 10 Ohms alone should produce around 40 pA/sqrt(Hz). I can understand this if I think of that topology as its equivalent of a 10.010 KOhms resistor and this resistor should produce noise lower than 1.29 pA/Sqrt(Hz). But, my question is... does this mean, anything I put in series with the 10 KOhms resistor won't contribute to the input current noise density? For example, an OpAmp stage with a higher input current noise source.
I don't really see the underlying physical principle of this behaviour...
Any hints?
Thanks a lot in advance.
I've been playing around with simulated equivalent input current noise in LTSpice and I found out that, in the same way it works when you have two DC current sources in series, the maximum current noise you get is the one set by the lowest contributor.
For example:
In a simple circuit based on a DC current source attached to a 10K and a 10 Ohms resistor in series, the input current noise density shown by LTSpice is around 1.29 pA/Sqrt(Hz), even if the 10 Ohms alone should produce around 40 pA/sqrt(Hz). I can understand this if I think of that topology as its equivalent of a 10.010 KOhms resistor and this resistor should produce noise lower than 1.29 pA/Sqrt(Hz). But, my question is... does this mean, anything I put in series with the 10 KOhms resistor won't contribute to the input current noise density? For example, an OpAmp stage with a higher input current noise source.
I don't really see the underlying physical principle of this behaviour...
Any hints?
Thanks a lot in advance.