Noise density graph seems to be slightly offset

[NoiseFree]

Thanks for the fix! I can confirm that the measurement is not window dependent anymore.

But there is still an issue with the display. It seems that it's off by about 1 dB.

To profe that I measured the spectral density of an 1MOhm resistor at room temperature. The circuit is the same non-inverting amp as shown above with the 50 Ohms removed.

Originally posted by @NoiseFree in https://github.com/QuantAsylum/QA40x/issues/31#issuecomment-1544174770

[QuantAsylum]

Hi @NoiseFree, thanks for the data. On my end, the noise test circuit looks as follows:

The OPA377 is 7.5nV/rthz (typ @ 1k), which is -162.5 dBV. So, the circuit above should be dominated by the R noise, which in this case is 500K equivalent, which is should yield -140.92 dBV at 68F (my current temperature).

I measure the following, which looks pretty much right on to me (this is 900 to 1100 Hz). What do you think of the math above?

Is there a chance your gain setting resistors might have a contribution? If they are 1% or better, probably not. If you set the opamp gain to unity, does the error get better or worse?

PS. Here's a plot of QA403 noise density with input shorted, just to make sure the lack of amp wasn't clouding things:

[NoiseFree]

Hi,

thank you very much for the thourough investigation! It turned out that the gain wasn't exactly 60dB in my case due to a bad solder joint. Everything is fine.

Regards