Updated circuit diagram to Ver 1.2
The circuit has been revised to reflect the removal of the two series resistors. This is as a result of the observation by Hwansoo Suh questioning the function of these components. They are there to protect the input of the converter chip from excess common mode voltage. As the multiplexer will be destroyed by anything much over +3/-0.6 volts the resistor seems a bit pointless.
The PCB is yet to be updated. At this stage if you use the Ver 1.1 PCB any value between 0 Ohm to 1K will work. I will publish an updated version as time permits.