Thanks for all the links & comments so far - I read them all and had a day of frustrating experimentation.
I tried:
- Different window types, taking in account their corresponding equivalent noise bandwidth factor in the density conversion.
- Bartlett vs. Welch methods.
- Using power spectrum and square root vs. amplitude spectrum only.
The resulting trend shapes all correlate, it's just the amplitude that varied. I would like to get my trends within the correct quarter-order of magnitude.
I decided to look at my LNA noise floor, estimate what the noise density at a certain frequency should be and see what method best matches it.
The 3 dominant sources of noise in the LNA is the input resistor, input voltage noise & +ve input current noise.
My LNA has a 1.5k resistor in the HPF before the opamp, corresponding to 4.9nVrms/rHz (290k). The metal film resistor wasn't selected for low noise, so this is likely to be higher.
As seen in my previous plots, my 10hz LPF filter starts a little too early, so I decided to look at the 6hz frequency point on the LT1007/LT1037 datasheet on the input voltage noise plot.
The input voltage noise at 6hz is approx. 3nVrms/rHz.
The voltage noise due to +ve input current noise (2pArms/rHz) at 6hz is approx. 3nVrms/rHz.
That results in a uncorrelated noise sources sum of 6.5nVrms/rHz. Likely to be a little higher due to input resistor not being wirewound or similar low noise type.
I looked at which method was near to 6.5nVrms/rHz, and found this:
where the noise density at 6hz is 8nVrms/rHz.
The method being: Bartlett, with no window into an amplitude spectrum, RMS averaged over 100x 10 second captures. The average metrics over 100 captures is 22.85nVrms and 116.3nVpp. The downside being the requirement for a high number of captures to get a smooth spectral density line (in comparison to the Welch method which gave very smooth lines but incorrect magnitude).
As a point of interest:
Shows the 10hz rolloff and a small amount of 50hz noise.
I'm aware the maths may not support this fully but I'm mainly after comparisons between devices, and a reasonable level of absolute accuracy (within quarter-order of magnitude of true value).