Hi everyone,
I was designing a filter and my team wanted to ensure the total noise to be as low as possible and hence searched for low noise op-amps. I'm actually having a problem with the term Input Voltage Noise Density, like for AD8605, the datasheet mentions Input Voltage Noise Density = 9 nV/sqrt Hz and the total noise in output is around a max of 580 nV/sqrt Hz. Another device, LT6203, where in the datasheet it's mentioned Input Voltage Noise Density = 2 nV/sqrt Hz but the output shows a total noise of around 26 uV/sqrt Hz max. I found this to be strange since I felt low Input Voltage Noise Density meant lower overall noise which isn't the case for my simulations as you can see.



LT6203 Noise Response



AD8605 Noise Response

I unfortunately do not have much command over the concepts of noise parameters and hence couldn't figure out the mistake from my side. Can anyone please help me out. The simulation files are attached to this.

Thank You,
With Regards,
Srinjay Bhattacharya

 

It might be helpful to understand what is going on, and although a bit simplistic I found the following to be helpful:

Ultimate Guide to Understanding Phase Noise (bliley.com)

 

What you are looking at is the combination of white noise which has a flat spectrum and 1/f noise which rises at 6dB per octave at low frequency. There is a parameter called "1/f noise corner", where white noise and 1/f noise are equal. Above that frequency the noise spectrum should be flat, and below that frequency the noise will rise at 6dB/octave.

I also note that you haven't mentioned current noise density and your source impedance.

 

What you are looking at is the combination of white noise which has a flat spectrum and 1/f noise which rises at 6dB per octave at low frequency. There is a parameter called "1/f noise corner", where white noise and 1/f noise are equal. Above that frequency the noise spectrum should be flat, and below that frequency the noise will rise at 6dB/octave.

I also note that you haven't mentioned current noise density and your source impedance.

Thanks for helping out.