Would you have Noise Spectral Density values (or plots) for the ADG451BR? I
don't see them on the datasheet.
If not would you have typical analog switch NSD values?
We don't have Noise Spectral Density values or plots for the ADG451.
Noise in our CMOS switches is not something we test or characterise for any of
our switches so we can't guarantee any numbers. The main consideration is
Johnson noise, therefore choose a switch with low Ron.
In most applications static switch noise is unlikely to be a consideration.
Noise from other sources in your circuit is likely to be far more significant.
The construction of the ADG451 (and most MOSFET switches) is a PMOS and NMOS
transistor connected in parallel.
For any MOS transistor there are essentially two voltage noise components.
Johnson is essentially thermal noise and is due to the channel resistance.
1/f noise is present in MOSFETs too and the corner frequency can be quite high
at around 10kHz. But the broadband noise is so low that the overall
contribution is likely to be small.
You can make a good approximation of high frequency noise (above the corner
frequency) from the Johnson noise equation:
En = sqrt(4kBRT)
Current noise in MOSFETs is extremely small and will be in the sub pA/sqrt(Hz)
for the ADG451.
Since noise sources add as route-sum-of-squares, any noise source which is 3 or
4 times larger than other noise sources will dominate and you can effectively
ignore the smaller noise sources.
This is not a quantitive answer to your question but it should convince you
that noise in the switch is unlikely to be a problem unless you are working at
low frequencies with other very low noise components.