EYE on NPI - Texas Instruments' TMUX821x Flat RON SPST 4-Channel Switches

Adafruit Industries

This week's EYE ON NPI is switching it up, with a look at some high-voltage-capable analog switches - Texas Instruments' TMUX821x Flat RON SPST 4-Channel Switches (https://www.digikey.com/en/product-highlight/t/texas-instruments/tmux821x-flat-ron-spst-4-channel-switches). A simple and effective way to switch and control up to 4 x 100V connections with up to 200mA each from any microcontroller or microcomputer. We carry dozens of different switches (https://www.adafruit.com/category/759), and DigiKey stocks easily 50K different switch configurations (https://www.digikey.com/en/products/result?s=N4IgTCBcDaIM4HcCWAXAxgCxAXQL5A0) from push-buttons (https://www.digikey.com/en/products/filter/pushbutton-switches/199) to toggles (https://www.digikey.com/en/products/filter/toggle-switches/201) but all of these require a person to push the button mechanically, whereas the TMUX series will connect and disconnect based on a separate digital signal, as low as 1.8V and as high as 48V. These are often used for switch audio or video, but are also often used in sensor measurement or test equipment. They're not good for power supply switching, and for digital signals there's often other ways of moving signals around, although they are sometimes used for the data lines of bidirectional digital signaling systems like USB. Like mechanical switches, the TI TMUX family of analog switches are 'bidirectional' (https://www.youtube.com/watch?v=z9GRiYPq7LM) - that means you can pass signal from A to B or from B to A. However, unlike mechanical switches there is no, well, mechanical connection. Instead, the switches are created by putting an N-FET and P-FET back-to-back. This means that there is no switch-bounce (https://en.wikipedia.org/wiki/Switch#Contact_bounce) or contact oxidation (https://en.wikipedia.org/wiki/Electrical_contact). Analog switches are pretty magical, and solve design problems that involve moving an analog signal around without having to hand-build a switch on board, but there are a few things to watch out for when 'modeling' how an analog switch varies from a mechanical one! First up, analog switches have fairly low 'Ron' resistance when the analog switch connection is made - on the order of a few ohms - and that is much higher than a mechanical switch. There is also 'very very high resistance, but not an open-circuit' when the analog switch connection is broken - on the order of a few hundred mega-ohms. (https://www.youtube.com/watch?v=n9EklrCrgH8) And don't forget you will need to have positive and negative power supplies beyond the voltages switched. So, if you are trying to switch 24Vpp signal, that ranges from -12V to +12V, you will need to supply them on the VDD and VSS lines. Also these are not electrically isolated, so add a separate opto/digital isolation (https://www.ti.com/isolation/digital-isolators/overview.html) circuitry if you want to keep your microcontroller fully disconnected from any high voltages. But compared to classic jellybean analog switches like the CD4066 ,(https://www.digikey.com/short/jw44n87t) the TI TMUX series of analog switches have improved circuit protections that will keep you from popping your chips (https://www.youtube.com/watch?v=RTPHjhjUl0E) by accident - especially since this series can be used with up to 100V. Check the datasheet for all details, but highlights include: built in pull-down resistors, protection against over/undervoltage as well as out-of-order voltage application, and latch-up immunity. If you're looking to switch up some analog signals on your next design, check out the Texas Instruments' TMUX821x 4-Channel Analog Switch (https://www.digikey.com/short/r3rhn270) family with 3 different configurations of NO/NC. And all are in stock for immediate shipment from DigiKey! Order tonight and by tomorrow morning you will be switchin' and swatchin' with these low Ron SPST analog switches.