How 1V op-amp is used for low power applications for longer battery runtime.
Modern operational amplifiers (op-amps) find use in various battery-powered applications. Examples include portable medical devices, fitness trackers, mobile phones, tablets, and condition monitoring sensors. To extend battery runtime between battery replacement or charging, the system can turn off active components whilst in sleep mode. Depending upon the applications, when the system comes out of sleep via a timer or a triggering event, the system needs to be up and running quickly to allow for signal conditioning and event logging.
Consider the discharge curve of a battery illustrated in Figure 1. Discharge cycles of batteries are typically similar to this curve. The terminal voltage will gradually decrease as the battery discharges with time. As the battery comes to the end of its charge, the terminal voltage of the battery will rapidly decrease. If the op-amp circuit is designed to operate at a voltage near the nominal voltage of the battery, such as V1, the operating time of the circuit t1 will be short. However, using an op-amp capable of functioning at a slightly lower voltage, such as V2, significantly increases the operating runtime of the battery t2. The key to designing a battery-operated system that is efficient is to optimize battery runtime by minimising the current drawn by the circuit.
Figure 1: Battery discharge plot
By considering lower the quiescent current (IQ) supports longer battery runtime:
Quiescent current refers to a circuit's quiet state when it is not driving any load and its inputs are not cycling. It is typically nominal; however, it has a significant impact on battery runtime, especially in wearables, hearables, and internet of things (IoT) sensor nodes. The most straightforward strategy for lowering overall power consumption is to select an operational amplifier with a low IQ. Devices with a lower IQ often have lesser bandwidth, more noise, and maybe more difficult to stabilize. Power consumption in an op-amp circuit consists of various factors: quiescent power, op-amp output power, and load power. The quiescent power, PQuiescent, is the power needed to keep the amplifier turned on and consists of the op amp’s IQ. POutput is the power dissipated in the output stage of the op-amp to drive the load. Finally, load power, PLoad, is the power dissipated by the load itself.
These types of products are typically designed to wake periodically to perform some action. After then, they return to standby mode. Battery runtime is calculated based upon active, sleep, and hibernate currents of the central controlling unit, such as a microcontroller. Active current consumption is vital in increasing battery runtime; ultimately, the runtime is influenced by how much time is spent in each power mode. As a result, the standby current of each component becomes increasingly crucial as sleep and hibernate modes occupy longer periods of time in a device. In such instances, the power supply's quiescent current is the biggest contribution to the system's standby power consumption. An op-amp with a low quiescent current can generate considerable energy savings for a device that spends a long time in idle mode.
Use case: Revolutionise Electrochemical Sensor using a 1V op-amp
The amperometric gas sensor generates a current proportional to the volumetric fraction of the gas. It is a three-electrode device that measures ethanol at the working (or sensing) electrode (WE). The counter electrode (CE) completes the circuit, whilst the reference electrode (RE) provides a stable electrochemical potential in the electrolyte, which is not exposed to the ethanol. In the instance of the SPEC sensor, a bias voltage of +600mV is applied to RE. Figure 2 depicts a typical architecture for biasing the sensor and measuring ethanol concentration in a battery-powered sensor system.
Figure 2: Traditional architecture of battery-powered sensor system
The sensor can operate at 0.9V, however, the signal conditioning and MCU require 1.8V. This voltage is generated by a boost converter, such as the nanoPower MAX1722x series. The MCU, with its integrated ADC, is only active to make measurements in such a system, whilst the boost converter and signal conditioning (op-amp) circuits are always active since they are used to generate the bias potential required at RE.
With the Maxim Integrated MAX40108 1V op-amp, it is possible to power the signal conditioning directly from the battery as seen in Figure 3. The MAX40108 is a low-power, high-precision op-amp that operates with a power supply voltage as low as 0.9V to 3.6V. The MAX40108 features rail-to-rail CMOS inputs and outputs, a 168 kHz GBW, with low 25.5µA (typ) quiescent current and 1µV (typ) zero-drift input offset voltage over time and temperature.
Figure 3: Block diagram of battery-powered electrochemical sensing system with 1V op-amp
Since the sensor and the signal chain are always active, powering them directly from the battery reduces the output current of the nanoPower boost and thus, the current requirements of the entire system. The MCU will still be powered by the boost regulator, but it is only active to make measurements. Otherwise, it is mostly on standby. The traditional sensor circuit consumes 150.8µA of standby current and 164.4µA of average current. Replacing the signal conditioning circuit with the MAX40108 reduces the standby current down to 81.9µA, a reduction of 45% and the average current down to 95.7µA, a reduction of 41.79%. As a result, the battery runtime of the system using the MAX40108 1V op-amp is almost longer than that of the traditional system.
