Measuring pressure with compensated silicon sensors and precision delta-sigma ADCs

June 19, 2012

All measurements in this article were done at room temperature around +22°C, ±3°C, where water density variation is around ±0.1%. Note that this is below the targeted precision for the DAS referenced in this article. For a typical MPX2010 fullscale range of 10kPa, the water height equivalent is 1.022m.

A standardized, convenient, water height value of 1m (1000mm) was used for the fullscale calibration of the water-level measurement system. Figure 4 shows a simplified block-diagram of the pressure-measurement DAS.

Figure 4. Drawing shows the implementation of the pressure-measurement DAS with direct interface to the compensated, silicon pressure sensor utilizing a ratiometric approach. This design allows the use of the analog power supply as a reference.

The height of the liquid can be derived by the formula:

Where:
HOUT is the measured height of the liquid column (m);
HFS  is fullscale measured height (1m for water) of the liquid column (m);

Figure 4 shows that the silicon pressure-sensor measurement element represents a resistive bridge that allows use of a ratio metric approach7 to estimate the fullscale voltage span at a reduced power-supply voltage of 3.3V:

VFS = VFST × (VDD/VPST) (Eq. 5)

Where:
VFS is the fullscale system span (Figure 4) at the maximum pressure, PFS = 10kPa;
VDD is the excitation voltage with VPST as the typical excitation value.

We know that for VPST = 10V, the sensor voltage swing is VFST = 25mV. Since we are applying only a 3.3V excitation, we get:

VFS = 25mV × (3.3/10) = 8.25mV (fullscale span at 3.3V) (Eq. 6)

A simplified schematic of the precision DAS used for the liquid level-measurement system is shown in Figure 5.

Figure 5. Compensated silicon pressure sensors directly interface with MAX11206 ADC while the MAX8511 precision LDO provides 3.3V power and reference voltages. A MAXQ622 microcontroller supports data collection from the ADC and supplies the USB interface to a PC. This DAS also includes a PC-generated GUI.
The MAX11206 used in this article is a 20-bit delta-sigma ADC suitable for low-power applications that require a wide dynamic range. It has an extremely low input-referred RMS noise of 570nv at 10sps. The noise-free resolution (NFR) is around 6.6 × RMS noise and represents a value of 2.86µV. (This is called flicker-free code.)

The calculations below provide estimates for the resolution at fullscale height measurement of HFS = 1022 mm.

The estimated fullscale resolution at ±0.035% is more than sufficient to achieve the DAS’s target precision of ±1% in this article. This proves that the ADC can directly interface with a new compensated silicon pressure sensor without the need for additional instrumentation amplifiers.

Figure 6 below shows the development system of Figure 5. This system features a water-level system “calibrator” consisting of a vertical 1m long plastic water-filled tube, equipped with 1mm resolution measurement tape. The measurement tube is located inside the calibrator water tube; it connects directly to the sensor’s positive pressure port, while the reference pressure port is exposed to the atmospheric pressure.

Figure 6. The development system for Figure 5.
Hydrostatic pressure produced by the water column at the bottom of the calibrator tube is producing the same amount of pressure on the sensor through the trapped air in the measurement tube.

At its output, the MPX2010 sensor produces a pressure equivalent voltage measured and digitized by MAX11206 ADC, processed by the microcontroller MAXQ622, and finally sent to a PC though the USB cable. Table 3 below lists measurement and calculations using Equation 4 for the 1m measurement range.

Table 3. System Calibration for the ADC-Based DAS
As shown above, by using a system calibration and Equation 4, the MAX11206-based DAS achieves better than ±1% precision over a fullscale water height level of 1m (1000mm).

Conclusion
New MEMS temperature-compensated, silicon pressure sensors are dropping in price and package size. This is making them attractive for a wide variety of precision sensing applications, such as liquid level measurements or flow metering.

These applications require a low-noise delta-sigma ADC such as the MAX11206 to directly interface to the PCB-mounted silicon pressure sensors. With simple compensation schemes, this approach easily increases the absolute accuracy of these pressure sensors.

Together, the silicon pressure sensors and the ADC provide a high-performance, cost-effective measurement system that is excellent for portable sensing applications.

Sohail Mirza is strategic-application manager for the Signal-Processing and Conversion business unit at Maxim Integrated Products, where he leads application and customer-support efforts for the high-precision-ADC group. He was previously a test manager at Maxim and part of a successful startup company in semiconductor testing. Mirza holds a management-science and engineering certificate from Stanford University (Stanford, CA), a master’s degree in electrical engineering from San Jose State University (San Jose, CA), and a bachelor’s degree in electrical engineering from the University of Illinois—Urbana-Champaign.

Joseph Shtargot is a strategic-application engineer for the Signal-Processing and Conversion business unit at Maxim Integrated Products. He previously was a senior engineer in Apple’s portable-product division and held positions as a computer-aided-tomography-scanner-development engineer at General Electric and as a staff-development engineer at Metrotech Corp. Shtargot holds two patents and has co-authored several publications on instrumentation and measurement in the various industrial applications. He received a master’s degree in electrical engineering from Kiev Polytechnic Institute, (Kiev, Ukraine).

Mohammad Qazi is an application engineer for Signal-Processing and Conversion business unit at Maxim Integrated Products. He supports strategic customers for high-precision A/D conversion products and is developing application-specific demo kits. Qazi served as an R&D Engineer in a previous position. He received a Master's degree in Industrial & Systems Engineering from San Jose State University (San Jose, CA) and a bachelor's degree in electronics engineering from NED University of Technology (Pakistan).

References
1.For more information on Evangelista Torricelli, you can start here: http://en.wikipedia.org/wiki/Evangelista_Torricelli .
4. Maxim Integrated Products application note 871, “Demystifying Piezoresistive Pressure Sensors,” at www.maxim-ic.com/AN871.
5. Maxim Integrated Products application note 840, “MAX1455 Diagnostic Clip Boost Circuit,” at www.maxim-ic.com/AN840.
6. 10 kPa On-Chip Temperature Compensated and Calibrated Silicon Pressure Sensors, at http://www.freescale.com/files/sensors/doc/data_sheet/MPX2010.pdf
7. Maxim Integrated Products application note 3775, “Design Considerations for a Low-Cost Sensor and A/D Interface,” at www.maxim-ic.com/AN3775.

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