BY MURRAY SLOVICK
Sensors are found in a wide variety of devices ranging from cell phones (there are as many as 14 in a smartphone—from accelerometers and gyroscopes to front- and rear-mounted image sensing cameras) to industrial equipment to medical devices. All are complex systems that require circuitry tuning to convert the analog signals produced by the sensor itself into a signal appropriate for a microcontroller’s (MCU) analog-to-digital converter (ADC). In the past, systems engineers were required to use discrete operational amplifiers, analog-to-digital converters, and passive components to design sensor systems, but this proved to be time-consuming, difficult to accomplish, and expensive.
Today an analog front end (AFE), an essential system building block to a sensor circuit, conditions the analog signal received from a sensor and performs the analog-to-digital conversion. Among other things, AFE circuitry boosts the voltage provided by the sensor to levels usable by the MCU, removes high-frequency and low-frequency signal noise before the analog signal is converted to digital, and, where necessary, convert signals via a transimpedance (I/V) circuit from current to voltage in cases where the sensor outputs current.
To help system designers overcome sensor integration challenges such as signal tuning and sensor drift, not only have suppliers developed AFEs but they have reference designs, online tools, and software that enable “on-the-fly” configuration and simulation of the AFE. The bottom line is this: ou no longer need to be an analog expert to design your AFE. Let’s look at two of these design helpers.
TI’s WEBENCH SensorAFE Designer (Fig. 1) lets engineers configure, evaluate and explore the capabilities of the supplier’s Sensor AFE System before implementing the design in hardware. Together with its WEBENCH Sensor Designer these tools provide complete sensor circuit design for common sensing, transmitter, and transducer applications. The development system allows engineers to evaluate the complete signal path solution (including sensors) either online or on the bench.
Fig. 1: WEBENCH Sensor AFE Designer software shows all configuration elements for easy online or on-bench design. Simply select a sensor, configure the signal path, optimize your design and load to a sensor AFE for immediate prototyping. (Courtesy of Texas Instruments)
The TI WEBENCH AFE Designer includes sensors such as resistance temperature detectors (RTDs), thermocouples, pressure sensors, load cells, and gas sensors. With the tool, the designer can start with an industry-standard sensor or specify his or her own sensor requirements to create a custom sensor.
To help simplify product development, reduce cost, and speed time to market Freescale Semiconductor has introduced a Healthcare AFE reference platform that includes hardware, schematics, and software to facilitate prototyping. An integrated hardware/software development platform, the Freescale Healthcare AFE reference platform is built around the Kinetis K53 MCU, which includes an integrated AFE featuring a complete analog measurement engine for capturing biometric sensor data: a precise voltage reference, four high-performance amplifiers, and two high-resolution ADC and DAC modules.
Six different application-specific plug-in boards allow the user to choose their application and level of integration in the reference platform. The reference platform, which is based on Freescale’s Tower System, includes the software and hardware needed to demonstrate or prototype portable electrocardiographs, heart rate monitors, pulse oximeters, glucose monitors, blood pressure monitors, spirometers, and Doppler ultrasound stethoscopes.
For applications that require higher performance and precision analog functionality, Freescale has partnered with Linear Technology to offer specific plug-in boards. The boards feature high-performance precision operational amplifiers for an external analog solution.
Mouser offers a wide variety of AFE solutions that provide a complete signal path solution between a sensor and an MCU.
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