The MAX309EPE+ from Analog Devices (formerly Maxim Integrated) is a dual 4:1 analog multiplexer — two independent SP4T switches in one 16-pin DIP. Each switch connects one of four inputs to a common output, with a 100 Ohm max on-resistance and 1.5 Ohm channel-to-channel matching. That matching figure matters when you're scanning multiple sensor channels into a single ADC: the gain error from Ron variation stays under 1.5 Ohm between channels, so you don't lose a bit of resolution to the mux itself. Turn-on and turn-off are both 150 ns. Crosstalk is -92 dB at 100 kHz.
Supply range and temperature — where it lives on the board
That dual-supply capability lets it handle bipolar analog signals without external level shifting — common in audio or industrial signal chains where the signal swings below ground.
Leakage and charge injection — the quiet details
Off-channel leakage is rated at 750 pA max, and charge injection is 2 pC. For a precision analog front end, these specs tell you how much the mux corrupts the signal when it switches. 2 pC of charge injection will cause a small glitch on the output — fine for most DC measurements, but worth noting if you're multiplexing a high-impedance sensor and the settling time matters. The off-capacitance (3 pF source, 26 pF drain) also contributes to settling, especially at higher scan rates.