Micropower Op Amps Meet Energy-Harvesting Challenges
Contributed By Electronic Products
Powered by ambient energy sources, wireless sensors lie at the forefront of industry trends such as M2M applications, the Internet of Things (IoT) movement, and automation systems in general. Underlying these tiny sensor systems, operational amplifiers provide essential buffering and conditioning of low-voltage sensor signals, yet need to remain within very strict power budgets. To design these systems, engineers can find the required balance of performance characteristics and ultra-low power consumption using available micropower op amps from manufacturers including Analog Devices, Intersil, Maxim Integrated, Microchip Technology, ROHM Semiconductor, STMicroelectronics, and Texas Instruments.
By harvesting energy from solar, vibration, temperature, or RF sources, a wireless sensor system can maintain operation for years without the replacement of batteries, or sometimes without their use at all. However, because ambient sources typically provide microwatts of sustained power, engineers face the challenge of finding availability of suitable low-power devices. Often an afterthought, signal amplifiers can represent significant power drains in these systems without due consideration of power consumption and overall device characteristics (Figure 1).
Figure 1: Micropower amplifiers play a key role in meeting application requirements within very tight power budgets available in wireless sensor systems powered from ambient energy sources (Courtesy of Texas Instruments).
Micropower op amps can meet the signal-conditioning requirements of most sensor systems. With current consumption at 1 µA and below, these devices are particularly well suited to energy-harvesting sensor applications. Their ability to operate from a single low-voltage supply simplifies design and makes them particularly well suited to low-voltage supplies in energy-harvesting applications. The Analog Devices AD8502 precision CMOS op amp operates with a supply current typically below 1 μA (Figure 2).
Figure 2: Micropower operational amplifiers such as the Analog Devices AD8502 feature very-low supply-current operation at levels typically well below 1 μA (Courtesy of Analog Devices).
Designers can find op amps in this class with even lower power requirements. The Maxim Integrated MAX4470 operates with a 750 nA supply current while the STMicroelectronics TSU101 requires 580 nA (typical).
For even more power-sensitive applications, the ROHM Semiconductor BU7265 requires only 350 nA in normal operation and features an input bias current as low as 1 pA (typical). The device features internal ESD protection and a human body model (HBM) rating of ±4,000 V.
The Intersil ISL28194 takes power requirements even lower, operating with a typical current of only 330 nA. Furthermore, the part includes a shutdown pin that brings power consumption down to 2 nA in this ultra-low-power mode.
Devices in this class typically operate with a single supply voltage down to 1.8 V, but designers will find different options. The AD8502 operates with a single-supply voltage of +1.8 to +5.5 V, as well as with a dual-supply voltage of ±0.9 to ±2.75 V. In contrast, the Microchip Technology MCP6041 operates with a single-supply voltage that can be as low as 1.4 V, while drawing 600 nA (typical).
The TS1003 op amp from Touchstone Semiconductor pushes the envelope even further with its ability to operate from a single-supply voltage as low as 0.8 V. The Touchstone device operates with a typical supply current of only 600 nA and features an input bias current as low as 2 pA.
Low-voltage supply challenges
Although their low-voltage operation matches energy-harvesting requirements, it also introduces additional challenges in designing with these sensitive devices. The use of low-voltage supplies with these devices correspondingly reduces the signal-to-noise ratio (SNR) by limiting the output-voltage swing. As a result, this class of low-voltage op amps typically features rail-to-rail input/output (RRIO). RRIO operation improves SNR by allowing full rail-to-rail output swings and an input-voltage range that includes one or both supply rails. So, for example, the AD8502, MAX4470, TSU101, and all other devices mentioned here feature RRIO to help maximize dynamic range and signal-to-noise ratio in low-voltage energy-harvesting systems.
Besides reducing signal range, low-voltage operation degrades SNR by raising the noise floor in circuits. While reducing the load on limited energy-harvesting supplies, low supply-current operation can lead to increased amplifier noise. In addition, circuits with low-power op amps typically use feedback resistors with higher values to limit supply current, adding an additional noise source. Finally, these sensitive circuits are susceptible to noise through capacitive coupling with nearby high-speed digital circuits.
To limit overall noise sources, noise contribution from micropower op amps becomes an important design parameter. In fact, micropower op amps typically feature very-low-noise operation. The Texas Instruments OPA369 op amps feature noise as low as 3.6 µVp-p (0.1 to 10 Hz) with 1.8 V RRIO operation and 800 nA supply current.
The TI OPAx369 series also addresses a key issue in low-voltage op amps. In low-voltage operation, these devices can face input crossover distortion due to changes in common-mode voltage. The OPAx369 features input offset as low as 250 µV and uses TI's "zerø-crossover" technology designed to maintain minimal distortion across the full supply range (Figure 3).
Figure 3: The OPAx369 op amp family from Texas Instruments minimizes input cross distortion due to changes in common-mode voltage often found in these types of micropower devices (Courtesy of Texas Instruments).
Micropower op amps address essential requirements for buffering and signal conditioning in sensor applications. Featuring typical current consumption below 1 µA, these devices operate from a single low-voltage supply typically found in energy-harvesting applications. Furthermore, their low-noise characteristics address increased noise sensitivity inherent in low-voltage operation. Using available micropower op amps, engineers can implement sensor designs able to operate well within the power limits typically found in energy-harvesting applications.
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