1 MCU ADC Input Sharing

MCUs are often used to monitor analog inputs by measuring the voltage via the internal ADC of the MCU. Frequently, a buffer may need to be added to source current to the ADC in cases where the analog signal being monitored does not drive enough current for the ADC to register the voltage variations. The amount of these signals an MCU can register is limited by the amount of I/O pins incorporated in their architecture. In a scenario where an MCU has only a single ADC input available but 4 sensing elements that require monitoring, the input count would fall 3 short of the system requirements. This scenario can be resolved with a multiplexing stage between the MCU and the sensors, as Figure 1-1 shows. This eliminates the need for a second processor or investing in a larger, pricier MCU or extra buffers that may be needed saving time, money, and reducing system size and complexity.

Figure 1-1 Expansion of 1 MCU ADC Input to 4 Sensor Outputs for Monitoring Using a Multiplexer.

When selecting a proper multiplexer for your design it is important to first understand the equivalent circuit of the multiplexer, using passive components. Ideally, these switches will appear to be invisible to the circuit, as if they are not there. The reality is, as with any component, there is an accumulation of parasitic capacitances and resistances associated with the device in its use in real world applications. Figure 1-2 shows a simplified circuit model of a FET switch when the switch path is selected.

Figure 1-2 Simplified Circuit Model of FET Switch When Signal Switch is Selected.

The parasitic on-resistance (R_ON) and on-capacitance (C_SON/C_DON) create a low pass filter through the chip. Although the load impedance heavily impacts the bandwidth of the multiplexers, minimizing the R_ON and C_ON can help limit bandwidth limitations associated with the switch itself. The leakage current introduced can contribute to DC errors and should be limited as much as possible for high accuracy applications. Some of the important considerations when selecting a multiplexer for expanding the MCU are the voltage range of the input signals, on-resistance, on-capacitance, on-leakage, and charge injection. These are explained in further detail in the Multiplexer Specifications section.