I managed to do a board with sufficiently few silly mistakes, to make it worthwhile to add components and do some tests,
Some waveform pics of precharge/AZ switching running in a state-machine -
first pic - uses a contrived sin-wave as signal input. And shows the digital pre-charge switch ctrl-line level shifted to the signal/boot voltage. It seems to work quite well over the +-10V input rage.
second pic - shows a test modulation of the precharge / AZ switching scheme. the precharge-switch selects the signal for the hi-sample, and then switches to boot to shield/protect the signal, when the AZ mux switches to take the lo sample.
The mux organization is changed a bit to handle all the inputs.
The two hi-mux's select the continuous signal of interest, then follows the pre-charge to select signal or boot, then the AZ muxes between the precharge-output and lo/zero signal.
I want to add a mode to test the pre-charge switching to reveal charge-injection bias in the same fashion as the previous tests done with the standalone '4053.
To do this a charge-capacitor (depicted on U402) can be switched onto the signal, so that AZ/PC switching can accumulate a charge, to be measured after a fixed duration/number of cycles.
This would form a useful self-diagnostic test, to run without any operator action.
The DC-source could also generate the signal inputs with a DC bias (eg. -10V,0V,+10V) to validate.
I chose to try surface copper-fills for soic-8 guarding rather than ring traces. At BOOT potential for hi muxes/precharge swtich, and gnd for az switch.
This is simpler due to how related signals get grouped by the 1ofN muxes, compared with discrete jfets placed about the board.
But it can be changed if needed.
I don't think the extra capactance due to the increased copper surface area is an issue. R405 can unload the BOOT driver output if needed.
The trick is to route signal placement carefully, to avoid any capacitive-coupling to the wrong copper fill.
Some schematic comments are more self-notes and are not reliable.