On the Use of Decoupling Capacitors in Autonomous Sensors

Power consumption is a major design constraint in low power systems such as autonomous sensors. A strategy to reduce power consumption and extend autonomy is to use low power devices with shutdown control and enable them only at specific times. An alternative strategy is to turn off the supply voltage of those devices that are not being used at a given time. In this case, however, power supply decoupling capacitors might dispensed more energy than the devices they protect. In this work we analyze whether or not conducted interference justifies the use of decoupling capacitors in autonomous sensors. Based on a case study, we present simple models for conducted interference and power and energy consumption. The results show that interference in power supply lines because of microprocessor activity in autonomous sensors is often small enough as to make most decoupling capacitors dispensable for analogue and mixed-signal devices sharing the same power line. An energy consumption analysis leads to a criterion to select the best optimal power management strategy. In low latency systems, the best is to turn off the supply voltage when not in measuring mode. These results depend on the particular layout, and also on the PSRR of the particular devices used, but the models provided can help in deciding which devices need decoupling capacitors and selecting the best power management strategy.