Sensing the Environment
Industrial engineering has always sought to improve the control of complex processes, which require having the optimal sensors to monitor them. Sensor designs must draw little power and be reliable to support decades-long operation. The sensing element requires further data processing, initially in the form of analog filtering and often digitally after that. From here, the data is transferred digitally to a logic or process controller via an industrial network.
Resistance Temperature Detectors (RTDs) are passive sensors with an excellent relationship between temperature and resistance. With tolerances of down to ±0.15°C (DIN Class A), they are made of pure metals, such as platinum, nickel or copper, and are increasingly replacing thermocouples in applications operating below 600°C. Such sensors come in a range of wiring options. Two-wire sensors are the simplest while three- and four-wire options use a circuit such as an unbalanced Wheatstone bridge to compensate for the impact of the lead length.
MCP6N11 Instrumentation Amplifier
Low-power instrumentation amplifiers are well suited for building the analog front-ends around RTDs. The MCP6N11 is a single-supply solution (1.8V to 5.5V) with a bandwidth of 500 kHz and nominal current consumption of just 800 µA. Five minimum gain options (1, 2, 5, 10 and 100 V/V) are easily configured using two external resistors, ensuring that input offset voltage and noise can be optimized for the application. These devices also a calibration function that corrects the input offset voltage.
Conversion to a digital output is supported by devices such as the MCP3550, a 22-bit Analog-to-Digital Converter (ADC) in a tiny 8-pin MSOP package. This device also draws a very low current from its single supply of 2.7V to 5.5V with a conversion typically requiring 100 µA at 2.7V and 120 µA at 5.0V. The delta-sigma design offers output noise as low as 2.5 µVRMS with a total unadjusted error of 10 ppm. Fully differential inputs feed a third-order delta-sigma modulator and a fourth-order modified SINC decimation filter, delivering the converted result via a three-wire SPI interface to the host MCU.
MCUs have also made significant advancements in power consumption. While some of these are in the basic active and static operation states, many creative features allow peripherals to operate in the background while portions of SRAM retain data. The 8-bit eXtreme Low Power (XLP) PIC® MCU family is designed to draw very little power in their deepest sleep mode, as low as 9 nA, while running currents are as low as 30 µA/MHz.
Low-power MCUs
Core Independent Peripherals (CIPs) are also integrated, providing autonomy for some peripherals that allows them to operate at low power independently of the CPU, only engaging it when data processing or transfer is required. XLP can be found in 8-bit MCUs, such as the PIC18F K42 and K40 families and the PIC16F191xx, and some 16-bit devices, such as the PIC24FJ128 family.
CIPs are also available in the AVR DD family of MCUs that also provides Multi-Voltage I/O (MVIO), reducing device count when interfacing with sensors and ICs of differing supply voltage. A Configurable Custom Logic (CCL) block also allows the construction of combinatorial and sequential logic for internal and external signals that can operate independently of the CPU.
AVR DD Family of AVR® MCUs
Even 32-bit performance is available with such power-saving capability. The PIC32CM Lx family uses the ultra-low-power Arm Cortex-M23 processor coupled with a SleepWalking capability. This allows the clock to select peripherals that will be temporarily engaged without having to wake the CPU from standby mode. Dynamic power domain gaiting can achieve additional power savings by leaving the power domain switched off until the peripheral is required to perform its task.
PIC32CM Low-power Features
Higher temperatures are monitored using thermocouples, and luckily, the circuitry around these devices has become much simpler.
Devices like the MCP960x are fully integrated, deliver measurements with an accuracy of up to ±0.5°C via I2C to a host MCU and include cold-junction compensation, failure detection features, ADC and filtering. With support for type K, J, T, N, S, E, B and R thermocouples (as designated by NIST ITS-90), these devices provide simple and low-power temperature sensing for all industrial sensor developers.
MCP3551 Single-channel ADC
Thermocouple ICs
Temperature Sensors
