Multiplexor and switch noise specifications

Q 

What noise performance can I expect from AD multiplexers/switches bearing in
mind that they will connect to
amplifiers having AC gains of around 1000. I'm looking for decent noise
performance but can't find any mention of this in your datasheet specs. If I
look at op-amp spec sheets I see nV and pA per root(Hz) but no obvious
equivalent for switches/muxes.

 

A 

Noise in our CMOS switches is not something we test or characterise for any of
our switches or multiplexers so we can't guarantee any numbers. When the switch
is on, it's noise is simply the noise associated with the Johnson noise of the
on-resistance of the switch, therefore choose a switch/ multiplexer 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 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 ADG413.

Since noise sources add as root-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.