Hi Folks,
If you are planning to design an application where a pressure sensor is needed but you are not sure about which kind of Pressure Measurement and Level of Integration is needed, or you even doesn’t know what does that means (like me before start working with pressure sensors), then this document can be useful.
First, for a better understanding about what I’m talking about in this document and how pressure sensors and measurement works I’m adding below an image that shows a cross-section of a standard pressure sensor which basically shows the Physical Internal structure of a pressure sensor:
What I’m trying to show in this image is the position of the Ports P1 and P2, you will see the importance of these ports and its position for the pressure measurement.
Also in this image you can see that the die and the wire bonds are protected with a Silicone Gel, this gel can get damaged leaving the die and the wire bonds exposed to the media if it’s used with different media than dry clean air.
Pressure Measurements can be divided into three different categories: Absolute pressure, Gage (Gauge) pressure and Differential pressure. Let’s learn something about these categories:
+ Absolute pressure refers to the absolute value of the force per-unit-area exerted on a surface. Therefore the absolute pressure is the difference between the pressure at a given point and the absolute zero of pressure or a perfect vacuum. In other words: Pressure is applied on Port P1 of the sensor while the Port P2 of the sensor is a vacuum sealed reference.
+ Gage (Gauge) pressure is the measurement of the difference between the absolute pressure and the Local atmospheric pressure. In other words: Port P2 of the sensor is exposed to the local atmosphere while Port P1 is where pressure is applied.
*Local atmospheric pressure can vary depending on ambient temperature, altitude and local weather conditions.
A gage pressure by convention is always positive. A 'negative' gage pressure is defined as vacuum. Vacuum is the measurement of the amount by which the local atmospheric pressure exceeds the absolute pressure. A perfect vacuum is zero absolute pressure.
+ Differential pressure is simply the measurement of one unknown pressure with reference to another unknown pressure. The pressure measured is the difference between the two unknown pressures.
In other words: The difference in pressure between two points is measured where pressure is applied to both sides (Port P1 and Port P2) of sensor.
Since a differential pressure is a measure of one pressure referenced to another, it is not necessary to specify a pressure reference.
Figure below shows the relationship between Absolute, Gage pressure and Vacuum.
Freescale Pressure Sensors can also be divided into three different categories according to the Level of Integration: Uncompensated, Temperature Compensated and Integrated, now let’s learn the difference between these categories:
+ Uncompensated Pressure Sensors are the most basic pressure sensors according to the level of integration; this type of sensor gives a differential output in the range of millivolts, this output will need to be temperature compensated and amplified with external circuitry before sending it to the MCU’s ADC.
+ Compensated Pressure Sensors is the following step according to the level of integration; although this type of sensor also gives you a differential output in the range of millivolts, the given output it’s already internally temperature compensated, so externally you only need to add the amplification circuit before sending the sensor’s signal to the MCU’s ADC.
+ Integrated Pressure Sensor (IPS) gives you the complete solution embedded into the same package, the output signal of the integrated pressure sensors it’s already internally temperature compensated and amplified (the output range can be from 0 to ~3V or from 0 to ~5V depending on the part number), so you do not have to worry about adding external circuitry (just probably some decoupling capacitors), the sensor signal can be sent directly to the MCU’s ADC.
Additional to the mentioned types of Pressure Sensors, we also have some Digital Output Pressure Sensors which can be communicated with the MCU via SPI (MPL115A1 Digital Pressure Sensor) or I2C (MPL115A2 and MPL3115A2 Digital Pressure Sensor).
Now that you know about Pressure Measurement and Level of Integration, you can jump into my Community Guideline to select the best Freescale Pressure Sensor Part Number for your application.
If there are any questions regarding this document, please feel free to ask below.
Your feedback or suggestions are also welcome.
Regards,
Jose
