Sensors & transducers


capacity, impacting the sensor operating life. So, what can be done to optimise the operating life span of the single cell? When fully charged, at the beginning of its


life, a single cell is at 1.5V. This voltage drops progressively over time until it reaches its end of life at 0.9V. To maximise the lifespan of a single-cell battery, the application must operate between 0.9V to 1.5V to get the longest application operation time. Because other system components operate at 1.8V, it is important to select a DC-DC boost converter that maximises the active and standby current efficiency and operates within the 0.9V-to-1.5V operating range. Having high efficiency of 95 percent is not


the only consideration for efficient power conversion. Boost regulators must also be efficient across a wide current range. This enables a lower quiescent current (IQ) and reduces heat dissipation during operation. As the application spends most of its time in standby mode, it is critical that the boost converter is efficient during the light-load standby state to extend battery life. A shutdown feature can also greatly reduce power consumption by turning off portions of the circuit, which brings down the current consumption to the single nanoamp range.


Signal Chain Solution


Sensors typically produce a weak output signal in the order of microvolts whereas analogue- to-digital converters require a signal in the order of Volts. This makes the selection of a low-power, high-precision amplifier the next important consideration in the design. Two important aspects of low-power


amplifiers are current consumption and operating voltage since many sensors require bias current to maintain accuracy. This requires the sensor portion of the application to be on to maintain an accurate reading. Also, a low operating voltage from 0.9V to 1.5Venables single-cell battery operation, eliminating the need for a boost converter. Typically, there are tradeoffs when selecting


low-power amplifiers, which results in reduced accuracy. But there are low-power amplifiers that can maintain a high level of accuracy even at low operating currents and voltage. Some features of precision amplifiers include sub- microvolt (µV) input offset voltage, voltage drifts in the order of nV/°C, and input bias currents in the picoamp range. Combining a low-power microcontroller


with an integrated ADC creates a low-power sensor solution that maximises battery life while maintaining a small application footprint.


Ethanol SEnSor Solution MEaSurEMEntS


Beyond improvements at the device level, system architecture can also be optimised to achieve lower power consumption with the same level of precision measurement. To prove this, we will provide two lab measurements of an ethanol sensor solution using similar devices


34


Figure 3. A futuristic 1V sensor system solution is represented in the block diagram above.


and one theoretical measurement for a future sensor solution that shows the power savings. This experiment uses the devices listed


below which have identical duty cycles for ethanol electrochemical sensor measurements.


SPEC electrochemical ethanol sensor


MAX40108 1V precision operational amplifier/1.8V operational amplifier


MAX17220 0.4-5.5V nanoPower synchronous boost converter with True Shutdown


MAX6018A 1.8V supply precision, low- dropout voltage reference


MAX32660 1.8V ultra-low-power Arm Cortex-M4 processor


Single 1.5V AA battery lEgaCy 1.8V SyStEM


The 1.8V system solution shown in Figure 1 is powered using a single-cell battery, which uses an efficient boost converter to provide a 1.8V system supply to the ethanol sensor, op amp, and microprocessor with an ADC. The 0.1 per cent active-duty cycle is controlled by the microcontroller, which wakes up to take a measurement and then goes back into sleep mode. The sensor in standby mode utilises the


boost converter to maintain power to the sensor, op amp, and microcontroller in sleep mode. In the standby state, the system consumes 150.8µA of current. During the active state, the microcontroller wakes up and takes a sensor measurement. In the active state, the system consumes 14mA for a short duration. Because the active state only occurs 0.1 per cent of the time, the calculated average current of the combined active and standby modes is 164µA, which is typical of a real- world sensor application.


1V aMplifiEr SyStEM


In the 1V amplifier solution shown in Figure 2, both the SPEC ethanol sensor and the


MAX40108 1V operational amplifier are directly connected to the battery. This requires an amplifier that can operate down to 0.9V, maintain a high level of precision and maximise the battery life of the single-cell battery. The remaining circuit is similar with a boost


regulator that powers the microcontroller and supports circuitry at 1.8V. In this configuration, the current is substantially reduced to 81.9 µA, a reduction of 45 per cent, and an average current down to 95.7µA, which is a reduction of 41.79 per cent. As a result, the battery life of the system using the MAX40108 1V operational amplifier is almost double that of the legacy system.


futurE 1V Signal Chain SyStEM


In the futuristic 1V signal chain solution shown in Figure 3, the amplifier, ADC, and microcontroller all operate down to 0.9V while maintaining a high level of precision. This enables the entire signal chain solution to be powered from a single-cell battery, which removes the need for a boost converter, therefore maximising the battery life of the sensor solution.


ConCluSion


As the demand for more intelligent AI systems grows, so does the need for sensors with additional functionality, a higher degree of accuracy and longer operating life. The sensors must offer a small solution size that can be either worn by a person or networked together to determine the health of a person, production floor, building or city to enable systems to be proactive as opposed to reactive. Taking this one step further, proactivity leads to better health, lower costs, higher productivity, and greater safety for those who benefit from adopting these next- generation systems. Innovation is occurring at many different levels


in the network of sensors that enable AI systems. IC manufacturers in particular are developing lower power sensor building blocks that help engineers of today to create more intelligent and more efficient systems of tomorrow.


Analog Devices www.analog.com May 2022 Instrumentation Monthly


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