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Connecting Sensors to the System
New IC products provide several options
Randy Frank, Contributing Editor -- Design News, January 8, 2006
Pick the sensing technique and then pick the integrated circuit (IC) to simplify connecting the sensor in a system. Factory environments frequently use a 4- to 20-mA current loop for transmitting many sensor signals. For some measurements, using a change in capacitance to measure liquid level, pressure, and other parameters provides an inexpensive sensing technique but typically requires complicated signal-conditioning circuitry. Microcontroller-based signal conditioning provides options, including the ability to interface pulse, voltage, current, inductive, capacitive, and resistive sensors. New ICs solve each of these sensor-signal-conditioning problems.
Texas Instruments 4- to 20-mA Transmitter
Price: $0.90 ea in quantities of 1,000http://rbi.ims.ca/4912-529Texas Instruments' XTR117 is a high-accuracy, 4- to 20-mA transmitter that operates over a voltage range of 7.5 to 40V with the ability to withstand power surges up to 50V. The precision current-output converter has (±500-µV max offset) and ±4 percent max span errors and provides accurate current-scaling and output-current-limit functions. The unit offers simple over-scale and noise-resistant signaling with 250-µA max power consumption. Packaged in a miniature MSOP-8 and specified for operation over the extended industrial-temperature range, -40 to +125C, the units target pressure, temperature, and humidity-transducer applications, as well as industrial-process monitors.
Analog Devices Capacitance-to-Digital Converter for Capacitive Sensors
Price: $4.60 ea in quantities of 1,000 for AD7745ARUZhttp://rbi.ims.ca/4912-530Designed for floating capacitive sensors, Analog Devices' AD7745/AD7746 are high-resolution, Ó-Ä capacitance-to-digital converters (CDCs). Each channel of the AD7745 or both of the two channels of the AD7746 can be configured as single-ended or differential. Units have 24-bit resolution with no missing codes (up to 21-bit effective resolution), ±0.01 percent linearity, and ±4 fF (factory-calibrated) accuracy. With a variable capacitance input range of ±4 pF, the CDC can accept up to 17 pF of fixed common-mode capacitance, due to compensation by a programmable on-chip, digital-to-capacitance converter. Digital communication occurs over a two-wire, I2C-compatible serial interface. Housed in a 16-lead TSSOP package, the AD7745/AD7746 are specified over the automotive-temperature range of -40 to +125C.
Sensor Platforms Sensor Signal Processor
Price: $3.65 ea in quantities of 1,000http://rbi.ims.ca/4912-531Using a high-speed 8051 processor running at over 14 MIPS, Sensor Platforms SSP1492 can process the signal from a variety of sensor elements and perform powerful vector and scalar calculations. The unit's sensor oscillator, configurable analog switches, and counter-capture unit measure a wide range of resistive, capacitive, inductive, voltage, current, or pulse-mode sensors. The processor's frequency-mode data converter has scalable dynamic range, accuracy, and speed. Digital communication includes either an SPI or I2C serial protocol. The SSP1492 is available as 4-mm-square bare die or in 80-pin land-grid-array (LGA) and micro-leadframe (MLP) packages.
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