A deep dive into the pitfalls of using a P-channel MOSFET as an analog signal switch in a thermistor voltage divider circuit powered by a single AA cell. The key problem is MOSFET off-state leakage current (up to 1–10µA), which causes significant voltage errors in high-impedance circuits — up to 0.125V with a 249K thermistor. Two alternatives are explored: bipolar transistors (nanoamp leakage but non-zero saturation voltage) and dedicated analog switch ICs. Most analog switches share the same ~1µA leakage problem, but low-leakage options like the Maxim MAX4066 (1nA) and Intersil ISL43L series exist. Finding parts that also operate at 1.1–1.2V supply is an additional constraint, with the Intersil ISL43L110 and NXP NX3L series being among the few viable options. Practical component values for the thermistor divider, low-pass filter, and gain-switching resistors are also discussed.
