Complementary metal oxide semiconductor (CMOS) is the most prevailing technology for integrated circuits applied for solid state memories (Flash, DRAM), Intel microprocessors, etc. By integrating the readout electronics and the sensor element, the weak sensor signals are easily amplified, digitized and conditioned. This provides a significant advantage with regard to technical performance. CMOS technology is suitable for large scale and cost-effective production.
Figure 1. Photograph of the CMOSens differential pressure sensor chip.
CMOSens represents a unique combination of the sensing element with the readout circuitry on a CMOS chip. Sensirion’s CMOSens sensors include a temperature sensor, which is utilized for accurate temperature compensation. The CMOS chip, together with an integrated self test and digital linearization, denotes a powerful microsystem, as illustrated in Figure 1. The system satisfies the needs for differential pressure measurements.
Thermal Measurement Principle
The sensor principle is based on a heater that is akin to traditional dynamic sensors. Two methods are available to combine this thermal principle on a silicon chip: traditional sensors employ a silicon bridge, which includes a temperature sensor and a heater. Although this approach is simple, dust collects at the bridge and pressurepeaks tend to destroy the small microstructure.
Figure 2. Thermal measurement principle: a heater generates a temperature profile on a membrane. The applied pressure difference DP enforces an air flow. This air flow destroys the symmetry of the temperature profile which is detected by the two temperature sensors.
In order to overcome these issues, Sensirion opted for a more robust technique. In this method, a glass passivated membrane structure is etched from the rear of the chip, as shown in Figure 2. The flat surface ensures that particles are prevented from being trapped at the sensor site. Owing to the low-noise on-chip interface circuit, the desired amount of air to sense the applied pressure variation can be reduced considerably. This results in a quasi static measurement. Field test in adverse environments such as smoke, chlorine, humid air, air with dust, etc. demonstrate how robust the CMOSens differential pressure sensors against other kinds of staining and contamination.
CMOSens Offers Excellent Technical Performance
In the center of the membrane, a heater is placed followed by placing two temperature sensors on either side (Figure 2). A temperature profile is produced by the heater. Due to the structure’s symmetry and accurate CMOS production with nanometer tolerances, the offset of such a differential sensor becomes insignificant. In fact, even the lightest movement of air results in irregular temperature distribution on the membrane. Two temperature sensors measure this temperature profile, which is amplified by a low-noise amplifier. The subsequent digital signal processing unit and analog-to-digital conversion compensate for the sensor non-linearity as well as for the temperature effects by considering the data from the on-chip temperature sensor. In this way, the sensor chip offers an output that is highly precise and compensated by temperature. Figure 3 shows a block diagram of the CMOSens chip.
Figure 3. Block diagram of the CMOSens differential pressure sensor chip; the on-chip signal conditioning includes low-noise amplification, temperature compensation and linearization.
CMOSens Sensors vs. Traditional Differential Pressure Sensors
When compared to other dynamic and static sensors for differential pressure measurement in heating, ventilation, and air conditioning (HVAC) applications, CMOSens-based sensors provide significant benefits in terms of accuracy, offset stability, and resolution. Furthermore, with CMOSens sensors the measurement range is 3 to 10 times larger than that of standard sensors. Therefore, a 0…500 Pa full scale CMOSens sensor substitutes three standard differential pressure sensors, each of which covers just a fraction of this range, for instance 0…100 Pa, 0…300 Pa, 0…500 Pa, etc. The CMOSens sensor also offers excellent performance.
Figure 4. Specified error tolerances of a traditional transmitter and the guaranteed specifications of the SDP differential pressure transmitter of Sensirion.
The drawback of traditional dynamic sensors caused by the preferred air flow are also prevented since the air flow is brought down by a factor of 10 to 50 when compared to conventional dynamic differential pressure sensors. Figure 4 shows the specified error tolerances of a conventional transmitter and the guaranteed specifications of Sensirion’s SDP differential pressure transmitter. Table 1 provides an overview of dynamic and static sensors and compares their performance with CMOSens differential pressure sensors.
Table 1. Comparison of differential pressure sensors for HVAC applications.
||Static Sensors (Membranes)
||CMOSens® Sensors (thermal)
||Large intrinsic offset. Large offset drift for low cost silicon based sensors. A periodical compensation is often required.
||Smaller than 0.5 Pa, drift below resolution
||1:10 of full scale
||1:50 up to 1:500 of full scale
|Air flow and staining
||No flow. Minimal staining effects.
||The very small air flow leads to a quasi-static measurement and reduces staining effects drastically.
||Max. 0.1 Pa if large membranes are used.
||Up to 0.008 Pa possible
||Limited to a couple of Pa due to offset drift and low sensitivity.
||Typically better than 0.1 Pa
|Sensitivity to the mounting orientation
||Mounting orientation has influence on offset and sensitivity. Needs often to be trimmed manually after installation.
||Can be mounted anywhere without restrictions.
||Has to be compensated for.
|Influence of connecting tube.
||(influence smaller than 0.04% of measurement range)
|Reproducibility below 10 Pa
||Strongly limited by offset drift, mounting orientation and limited resolution.
Transducers Sensor Components and Transducers
Sensirion supplies a comprehensive range of CMOSens-based sensor components and transducers. Most air handling tasks in HVAC applications can be covered. The discrepancy between the products is only the adaptation and calibration for the external power supply, the pressure range, and the interface for the measurement data.
CMOSens-based differential pressure sensors provide a larger measurement range, excellent reproducibility, no offset drift, and no sensitivity to the mounting direction. Due to a smaller air flow than in traditional dynamic pressure sensors, the parasitic effects and hazard for contamination are totally eliminated. The technical benefits provide excellent accuracy for measuring and controlling air volumes and pressure differences in HVAC applications.
This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.
For more information on this source, please visit Sensirion Inc.