MOSFET Analog Switch Question

The data sheet I'm looking at (Revision 01 Jan 2025) has this on the first page: "All devices feature true bi-directional performance in the on condition, and will block signals to the supply levels in the off condition." I've not so far read the data sheet in any level of detail. Keep your analog signals within v- to v+

Right. The switch will be made with parallel PMOS/NMOS.

It should be symmetric, happy conducting in either direction.

Don't let the signals go beyond the rails.

The data sheetx doesn't say that explicitly - they mostly just says that it is a CMOS part - but it is clearly implied.

Only in the sense of conducting both ways. N-MOSFets tend to be more conductive than P-MOSFets. If they make the P-MOSFets bigger than the N-MOSFets they can can balance the channel resistances, but not the channel capacitances.

That would reverse bias the isolation diodes and feed current into the rails, and elsewhere if you are unlucky. The effects can be unfortunate.

This might be useful:

Linearity is probably only a condition when looking at ~5V single supply analog switches.

RL

Yup. Otherwise, typically all the channels will get connected together by way of the substrate. It's a puzzle to debug, the first time it happens to you--it's always the channel you're not focusing on that's railed. ;)

If you're worried about linearity, do note the switch resistance vs. signal voltage curves, which are usually sort of moustache-shaped.

To avoid signal distortion, don't draw significant current through an analog switch.

Cheers

Phil Hobbs

Exactly. And the series fets will start to conduct before the ESD diodes clamp the swing.

Roger the first-time perplexity.

Don't load the switches if you want linearity.

I just said that!

Charge injection is fun too. Drives some opamps bonkers.