2 Application

A common design of PLCs includes Analog to Digital Converters (ADC) where they sample analog readings from the monitored devices and output a digital response. They usually read current or voltage inputs from the sensors. A design implementation is shown where two ADCs are used, one for current readings and the other for voltage readings. The burden resistor is there to convert the current to voltage, which is then sampled by the ADC.

Having two ADCs is common for this application, but they do substantially increase the footprint of the PLC. A potential design is to remove one of them and reroute the signals in such a way where one ADC can sample both current and voltage readings. That is where the TMUX4821 can provide a design. It is a very low Ron (0.19Ω typical) beyond the supply multiplexer with powered off protection. Implementing this on the signal path of the burden resistor allows the ADC to switch between the current and voltage modes. When the switch is closed there is a path for the current to flow through the burden resistor allowing current mode readings by the ADC. The very low on resistance of the multiplexer results in a trivial change on the total resistance on the line preventing any additional error in measurements, while allowing for more functionality. With the switch open and the mux disconnecting the burden resistor path, the precision ADC can operate in voltage mode. With the switch closed there is a path to the burden resisting allowing to operate in current mode.

A single TMUX4821 can support two precision ADCs. Without the TMUX4821, four ADCs must be used, increasing the BOM greatly. Implementing the mux in such a way cuts in half the amount of PADCs in PLC systems.