Summary
Are you wondering how pressure sensors work in your application? Do you want to get more information about the type of pressure measurement or level of integration needed?
This knowledge article provides details of how pressure measurement is done in STMicroelectronics MEMS sensors.
Introduction
Figure 1 illustrates the physical internal structure of a pressure sensor. To help you understand how pressure sensors and pressure measurement work, it is a cross-section of a standard pressure sensor.
In figure 1, the positions of ports P1 and P2 are indicated. The importance of these ports and their positions for pressure measurement is explained in the following sections.
The die and wire bonds are protected with silicone gel. This gel can be damaged, leaving the die and wire bonds exposed to the media, if the sensor is used with media other than dry, clean air.
Type of pressure measurement
Pressure measurements are divided into three categories: absolute pressure, gage (gauge) pressure, and differential pressure, as described in the next sections.
The relationship between absolute pressure, gage pressure, and vacuum is shown in figure 2.
- Absolute pressure is the force per unit area exerted on a surface, measured relative to a perfect vacuum. In this case, pressure is applied to port P1, while port P2 is a vacuum-sealed reference.
- Gage (gauge) pressure is the difference between absolute pressure and local atmospheric pressure. In this configuration, port P2 is exposed to the local atmosphere, and pressure is applied to port P1. Local atmospheric pressure varies depending on ambient temperature, altitude, and weather conditions. By convention, gage pressure is always positive. A negative gage pressure is defined as vacuum, which is the amount by which local atmospheric pressure exceeds absolute pressure. A perfect vacuum is zero absolute pressure.
Figure 4: Gage (gauge) pressure.
- Differential pressure is the difference between two unknown pressures. Pressure is applied to both sides (port P1 and port P2) of the sensor. Since differential pressure is measured relative to another pressure, a specific reference is not required.
Level of integration
Pressure sensors are also categorized by level of integration. Three different categories can be mentioned:
- Uncompensated pressure sensors are the most basic type. They provide a differential output in the millivolt range, which requires external temperature compensation and amplification before being sent to the microcontroller unit (MCU) analog-to-digital converter (ADC).
- Compensated pressure sensors provide a differential output in the millivolt range that is already internally compensated for temperature. Only external amplification is required before sending the signal to the MCU ADC.
- Integrated pressure sensors (IPS) provide a complete solution in a single package. The output signal is internally temperature compensated and amplified (the output range is typically 0 to 3 V or 0 to 5 V, depending on the part number). Only decoupling capacitors may be required, and the signal can be sent directly to the MCU ADC.
In addition to the types mentioned above, digital output pressure sensors are available. These sensors integrate an ADC inside the signal chain, such that the pressure value is directly available in sensor registers, accessible by an MCU.
In terms of communication, different protocols are supported: SPI, I²C, PSI5, DIS3. The last two types of communication are commonly used for airbag sensors.
Conclusion
The article has shown different pressure measurements and types of integration of pressure sensors.
A product selector allows you to explore the different MEMS pressure sensors from STMicroelectronics:
Now, it is your turn, choose your pressure sensor and start designing a new breakthrough application!
