I added a second channel mux/pre-charge switch.
The idea here is to apply the same input switching strategy to alternative HI inputs, to improve 4-phase sample cycles.
This should work for ratio-metric mode (alternate dcv-hi, 4w-hi).
As well as for AG mode (alternate dcv-hi,dcv-source ) and should support an amplifier configuration option using jfets with high thermal effects/walk (if3602 ).
Another possibility would be alternate sampling of voltage and current inputs, although it's not clear how useful this is.
After adding the extra channel, it is interesting to see the extra symmetry created in the input section.
There is the possibility to use the second channel to also mux isolated LO inputs (eg. DCV-LO, 4W-LO) - even just for test purposes.
A further possibility would be to make the gnd-referenced node selectable.
A divider can create a CM node between the buffered boot hi/lo inputs.
And if this CM node is gnd-referenced via a selection relay, a differential voltage-range is avaliable with twice the input range.
If I understand correctly, Kleinstein has discussed this approach, and does something similar,
https://www.eevblog.com/forum/metrology/diy-high-resolution-multi-slope-converter/msg3827432/#msg3827432I think LF noise from the channel buffers, will be added when sampling in this differential mode.
But this noise ought to be mostly cancelled by the AZ zero subtraction.
In addition, the mode would only be used/useful when amplifier gain=1.
Does this approach makes sense?
The additional complexity once the second-channel precharge switching/ mux for RM, AG modes has been added, seems low.
It looks like it just needs an extra relay and divider.
And maybe some changes to the input protection.
The question there is more the U² part from the nonlinear switch resistance - unless the front end compensates for this by using positive and negative readings.
A single DPDT relay/or mux could invert the inputs, to support taking measurements in both polarities.
This would ease turn-over tests from external (isolated) input dc sources, and might be justified on the basis that it is a good check for measurement confidence.
But the switching action couldn't be performed per-cycle, without disturbing the inputs, unless they were tapped after the precharge switches.
And then it gets complicated with the gnd-reference.