Design Considerations

Pressure sensors convert a physical value - weight, tire pressure, level, force, and flow-into a differential signal in the mV/V range and are referred to as metal thick-film, ceramic or piezo-resistive. The majority of designers use the cost-effective piezo-sensors (25mbar - 25bar) that are nonlinear, temperature dependent and have large offset and offset drift, and require attention for electronic calibration and compensation.

Sensor Signal Conditioning - performs all necessary functions to calibrate, compensate for temperature variance, scale, and linearize the sensor signal.

Analog/Digital Processing - there are two ways to convert and linearize the sensor signal. The analog technique results in an analog solution and provides an analog output. This technique is inexpensive and fast, but limited to a maximum of 11- to 16-bit resolution. Digital is more precise, up to 24 bits, and provides a digital output at moderate speed.

There are highly integrated solution, like the PGA309, tailored for bridge pressure sensors comprising precision, low-drift programmable gain instrumentation amplifiers using auto-zero techniques and include programmable fault monitors and over/under scale limiters. The PGA309 also offers a digital temperature compensation circuit. Other recommended lowest-noise amplifier and instrumentation amplifier for pressure sensor conditioning include the OPAx227, OPAx132, INA118 and INA122, INA326.

Microcontrollers with integrated ADCs - TI's MSP430 and MSC12xx integrated microcontroller solutions family allow the definition of filters and thresholds of critical pressure levels. These are often transmitted back to a central station through the digital interface of RS-485 or CAN for recording and operator review.

Additionally, the ADS125x are precision, wide dynamic range, delta-sigma, Analog-to-Digital (A/D) converters with 18 to 24-bit resolution operating from a single +5V supply. The delta-sigma architecture supports wide dynamic range and guarantees no missing codes. They are designed for high-resolution measurement applications in cardiac diagnostics, smart transmitters, industrial process control, weigh scales.

Clocks - The CDCE9xx clock families are modular PLL-based low cost, high-performance, programmable clock synthesizers, multipliers and dividers. They generate up to 9 output clocks from a single input frequency. Each output can be programmed in-system for any clock frequency up to 230 MHz, using independent configurable PLLs.

Interface / Connectivity

Traditional analog (4 - 20mA) and RS-232/RS-485 interfaces remain popular choices for industrial control and sensor applications. Looking forward, designers are integrating mainstream wired interfaces such as Ethernet, USB, and CAN, into their products. In addition, wireless options such as ZigBee, Bluetooth, and other Low Power Wireless protocols are becoming more prevalent. TI is committed to provide solutions for both traditional and emerging industrial interfaces. For example, TI recently introduced the ISO1050, the world's first isolated CAN transceiver.

Power - LDOs are good for low noise power for amplifiers and data converters because they provide low-ripple power rails leading to better signal fidelity. The REF31xx family are precision, low power, low dropout, series voltage references available in tiny SOT23-3 package. The REF31xx does not require a load capacitor, but is stable with any capacitive load and can sink/source up to 10mA of output current.