Technology review
Digitisation to underpin growth in pressure-sensor demand
Power Management
Pressure Sensor (main)
MUX
Temperature Sensor (compensation)
Data Registers Sensor n All on one chip A block diagram of a digital pressure sensor construction.
In a situation where the requirements for a pressure sensor can change over time as new IoT-based applications become apparent, it makes sense to have front-end processing that can suit most use- cases – and to have that front-end integrated into the sensor package itself. Moving the signal to digital as close to the sensor as possible has other advantages.
There is often a complex relationship between the signal conditioning op-amps and the front-end amplifiers that feed into the analogue-to-digital converter (ADC). Sensor designers have the best understanding of anyone to determine which combination of front-end components provides the best conversion for the widest range of target applications. By bringing the digital conversion into the sensor module itself, they can provide this skill and understanding to the customer. Furthermore, the close integration of sensor and ADC reduces the risk of electrical noise coupling into the interface. This helps guarantee high-quality readings.
Power consumption is an additional benefi t of moving digital outputs as close to the sensor as possible. The progressive scaling of integrated-circuit feature sizes has brought with it tremendous improvements in energy effi ciency through the use of lower voltages. Power consumption has a quadratic relationship to supply voltage, so every fraction of a volt that is saved in a new circuit technology often pays off signifi cantly more in energy effi ciency.
Many IoT sensor nodes rely on the ability to shut power down to large sections of the circuit board in order to save precious battery energy between
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phases of activity. The periods of activity are largely when they take measurements and process the resulting data. Outside these periods, many sensors will be powered down. Some of them may stay active to allow them to trigger on asynchronous events unless they are designed to do so. Incorporating digital conversion and control logic into the sensor makes it possible to build more intelligence into that particular subsystem. It can decide to buffer recorded buffers until there is a major change in conditions that demands intervention from the node’s microprocessor.
Having the analogue sensors isolated into small subsystems connected by a digital bus or serial interconnect makes it easier to have system-wide, fi ne-grained control of power consumption. The ability to fi ne tune power control in this way greatly enhances the overall energy effi ciency of the IoT node’s design and opens the door to systems that make use of energy harvesting rather than relying purely on batteries.
There are further advantages of digital interfacing to sensors. The ability to move digital functionality into the sensor subsystems helps reduce overhead on the microprocessor even when it is running. If the processor cores needs to control its own ADC, internal fi rmware needs to be in place to set up conversions at precise intervals and move the resulting data into a memory buffer within a narrow time window to minimise the adverse effects of timing jitter.
A digital sensor can use its own control logic to organise and process samples. The host microprocessor can adjust parameters such
A/D Convert Low Power uC Data I/O
Power In
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