Low-Power, portable altimeter

A low-cost, light-weight altimeter has been a target application of absolute pressure silicon sensors for more than 40 years. As the sensor technology has improved, microcontrollers (MCUs) have advanced even more dramatically. Today, an altimeter requires only a highly integrated, 16-bit microcontroller (MCU), an absolute atmospheric pressure sensor and an LCD display.

The design uses a Texas Instruments' MSP430F4270 MCU, optimized for high-precision, battery-powered portable measurement applications. The MCU's on-chip, high-resolution 16-bit sigma-delta Analog-to-Digital Converter (ADC) with an integrated Programmable Gain Amplifier (PGA) allows the device to interface directly with the absolute pressure sensor. In addition, the MCU's integrated LCD driver enables direct connection of commonly used LCD modules.

The absolute air pressure silicon sensor for this design is a MPXM2102A that outputs a differential voltage directly proportional to the ambient pressure. However, the voltage is extremely small, changing by 12 muV/mbar with the 3V battery source. Resolving pressure and, therefore, altitude with fine granularity, requires the MCU's high-precision ADC.

Using the principle that air pressure decreases with rising altitude caused by the weight of the atmosphere itself, a barometric altimeter measures the atmospheric pressure to determine the altitude of an object above sea level. At altitudes near sea-level, the pressure decreases by approximately 1 milli bar (mbar) for each 8m height increase. The exact formula to calculate the height from the absolute pressure uses the ideal gas law. This formula is also known as the "Barometric Pressure Formula".

To establish the altitude resolution of this design, it is necessary to determine the Least Significant Bit (LSB) step size at the highest PGA gain setting. The MCU's datasheet reveals that the maximum full-scale input voltage range is plus or minus 0.6V. Since the ADC outputs a 16-bit signed conversion result, the LSB step size is about 18.31 muV. Using the on-chip PGA with its highest gain setting of 32 and a full-scale input range of plus or minus 18.75 mV, reduces the LSB step size to 0.57 muV, yielding an altitude granularity of approximately 0.4m.

Because of the MCU's capability, additional features such as a real-time clock and temperature can be added without increasing the parts count. Depending on the active duty cycle for the measurements and other power saving steps that can be implemented, this type of altimeter may work for several years using a single coin cell battery.

CONTACT:

Andreas Dannenberg, MSP430 MCU Applications, Texas Instruments Tel: 972-644-5580; e-mail: [email protected]

Get more information on the MSP430F4270.

Get more information on the MPXM2102A absolute pressure sensor.