1-Wire Barometer Calibration

Posted on Feb 6, 2014

The resolution of the barometric pressure is about 0. 00417 inHg (0. 0139 kPa) for a pressure range of 31. 0 to 28. 0 inHg (105. 0 to 95. 0 kPa, or 1050 to 950 mb or hPa). Better resolutions are possible with a more restricted pressure range. The circuit requires an additional power source other than that of the 1-Wire network. The MPX4115 requires abou

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t 7 ma of current. This is more than a 1-Wire network can provide without an elaborate circuit to store parasitic power from the 1-Wire network for short burst of current for pressure measurements. For barometric pressures the MPX4115 output voltage ranges from about 4. 25 to 3. 79 volts at sea level, and about 2. 77 to 2. 45 volts at 10, 000 feet. Most of this range is above the active voltage range of a 5 volt opamp circuit. In effect the sensor voltage is referenced to the power supply, not ground as desired. Fortunately the DS2438 Smart Battery Monitor accepts inputs as high as 10 volts. Thus by powering an opamp from 10 volts the output of the MPX4115 is well in the opamp and DS2438 range. The MPX4115 output is fed through a RC filter to opamp stage, U1B, which has a fixed gain of approximately 4. This stage has an adjustable voltage input which is added to the barometric sensor output within the opamp, thereby allowing the adjustment of the output voltage offset to the A/D converter. The two 10-turn potentiometers (pots) control the gain and offset. VR2 controls the gain of U1A and VR1 controls the offset of the output voltage to the 1-Wire DS2438 A/D converter. Note that the MPX4115 feeds R2 through a jumper on J1. This allows easy change of the input voltage from a source other than the MPX4115 like the calibration tool. The design presented in the schematic above is not the only possible barometer configuration the...

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