Board schematics
After many days, I finally made up my mind about the basic circuitry on the sensor board.
I tried to optimize power consumptions, but some sensors require a power supply of 5V and that's very difficult to accommodate in a low-power design.
Attached to this post is the first draft of the board. There are still some component values to define, but the main idea is there
The power is managed by the LTC3108. This circuit provides a 2.2V power supply, that powers up the MSP430. When the MCU senses (through the LTC3108's PGOOD output) that the 5V power supply is good, it switches on sensors (one at a time) by means of a ULN2003LV.
Because the MSP430 is powered with 2.2V, ADCs accept voltages from 0 to 1.5V, so I scaled all sensor output to that range by means of voltage dividers. I also added a 741 op amp connected as buffer to each ADC input in order not to load the ADC input.
I'm evaluating whether a resitor + capacitor is required at the output of the 741 op amp, but I don't think so mainly because of the very low sampling frequency