Measuring pressure with compensated silicon sensors and precision delta-sigma ADCs
Temperature-Compensated Pressure Sensors
Many modern industrial processes as well as commercial and medical applications do not require an extended temperature range. Moreover, some of these applications actually operate in an air-conditioned environment where the temperature range is quite restricted. Temperature-compensated silicon pressure sensors are well suited for these applications.
Freescale Semiconductor and OMROM temperature-compensated silicon pressure sensors are available in small sizes with prices typically in the medium, single-dollar range, depending on the package type.
These sensors reduce the cost substantially and, equally important, give designers the flexibility to place sensors on any type of printed circuit board (PCB).
One example is the popular and cost-effective MPX2010 series silicon piezoresistive pressure sensor from Freescale Semiconductor which provides temperature compensation in the range of 0°C to +85°C. Table 1 shows important operating characteristics and systematic errors of the MPX2010 at room temperature.
Table 1. MPX2010 Operating Characteristics & Error Analysis
As shown above, although this temperature-compensated pressure sensor has ±1% linearity and only ±0.4% hysteresis, the offset and fullscale errors lower its overall accuracy to 4% at a constant temperature.
At this point a new precision delta-sigma ADC becomes critically important to a design. By applying the offset and fullscale system calibration available in the ADC, the sensor’s overall measurement precision can be improved to around ±1%, or better.
Moreover, with high noise-free resolution, excellent common-mode 50Hz/60Hz rejection, and well-integrated buffers, these delta-sigma ADCs can directly interface with silicon piezoresistive pressure sensors without additional instrumentation amplifiers or dedicated current sources. Some important characteristics of the MAX11200 family of sigma-delta ADCs are listed in Table 2.
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Table 2. MAX11200 key specifications
With its direct interface to the low-noise sigma-delta ADCs, the MPX2010 pressure sensor now provides a cost-effective measurement system optimized for portable sensing applications.
Liquid Level Measurement with a Pressure Sensor
Here we will evaluate a system design using a pressure sensor and sigma-delta ADC to accurately measure water level.
The level (height) of the liquid will be determined based on the pressure produced by hydrostatic column of the liquids at the bottom of this container and measured at the end of the measurement tube.
The pressure sensor will have atmospheric pressure on one side and compressed air (by liquid) on the other. (See Figure 6.) The MPXM2010GS is used to differentially measure the pressure.
Assuming a constant density throughout the liquid and a negligible variation of the earth’s gravitational acceleration, hydrostatic pressure can be derived by a simple formula:P = D × G × H (Eq. 2)
Where:
P is hydrostatic pressure (Pa);
G is gravitational acceleration (9.8066m/s);
D is the liquid density (kg/m3);
H is the height of the liquid column (m).
Resolving Equation 2 for H: H = P/(D × G) (Eq. 3)
In general, liquid density varies with changes in temperature. For example, the density of water increases between its melting point at 0°C and +4°C, reaching a standard value of 999.972 (practically 1000) kg/m3 at +4°C. At room temperature, +22°C, the density of water is 997.774kg/m3.
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