Work out adc sample time instantly with clear inputs, formula shown and shareable results.
The sampling capacitor must charge to within half an LSB, which needs (n+1) x ln2 time constants where tau is source impedance times sample capacitance. A 10 kilohm source into 8 pF at 12 bits needs about 0.7 microseconds — far longer than the minimum the datasheet allows.
ADC settling
tau = Rsource x Csample; sample time >= tau x (n + 1) x ln2
It sets the charging time constant. A high-impedance sensor read too quickly gives a reading biased toward the previous channel — the classic multiplexer crosstalk symptom.
Buffer it with an op-amp follower, or add a capacitor at the ADC pin much larger than the sampling capacitor to supply the charge locally.