AC analysis of a circuit including variable resistors
If you are building an anti alias filter get rid of the components called R - they are not helping and are also attenuating the signal across the whole of the baseband - you should consider several cascaded sections made like this: -







simulate this circuit &ndash; Schematic created using CircuitLab

Pick a value of R and C and do an AC analysis and, if the cut off frequencies are still too high make R bigger until they are not.

BTW it's not a great way of designing an anti alias filter and, it won't be as good as a couple of 2nd order sallen-key filters made with op-amps but nonetheless it should suffice.