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  3. Unusual Input Referred Noise Characteristic

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Unusual Input Referred Noise Characteristic

Deepansh
Deepansh over 10 years ago

Hello,

I'm working on the design of a low power low noise amplifier. I'm using the switched op-amp technique,where instead of one input transistor,two input transistors are used, where each transistor is ON for only half the clock cycle (and is connected to the input) while the other is OFF. The other transistor is ON(and connected to input) for the other half of the clock cycle.This helps reduce flicker noise.

On doing a PSS + Pnoise Analysis,I'm running into an unexpected problem. The graph of the output noise is as expected, starting at 15 uV / (root Hz)on the Y-axis and then decreasing with frequency. However the input referred noise starts at a startling 2.3 V/(root Hz) and then decreases with frequency. This is a very high input noise and even more surprising, it is atleast 10^5 times greater than my output noise! 

Further, on doing: 

sqrt(integ(getData("out" ?result "pnoise")**0.1  20k))

to compute rms output noise, I get a  value of 2.98*10^(-5), which is indeed a very good value.This is the input noise graph:

As far as I know, this could only be possible if my op amp was attenuating signals, only then output noise would be lesser than input noise.However, looking at the Gain v/s frequency response of my op amp, I can't see any attenuation, i.e. gain is >0 dB everywhere. 

I am attaching the relevant graphs for reference. Can somebody help me find what the problem could be? Is there some problem with the PSS/Pnoise settings? The gain frequency plot is shown below:

P.S.:I intend to use this as a band pass amplifier, which is why you see a high pass characterisic, from low frequencies. I have used a feedback resistance and capacitance to implement the same.

Any help regarding this would be much appreciated.Thank You

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  • Deepansh
    Deepansh over 10 years ago

    This shows the input referred noise:

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  • Deepansh
    Deepansh over 10 years ago

    This shows the output noise:

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  • Deepansh
    Deepansh over 10 years ago

    Sorry, I'm having some problem uploading the graphs. The input noise graph is given below:

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