PRESSURE SENSORS

Pressure Sensor Specifications Explained Introduction This application note explains typical pressure sensor parame-ters that are used in the datasheets of SMI’s pressure sensor products. Unfortunately, there is no universally accepted standard for specifying the accuracy of pressure sensors. Therefore, it is important to understand the critical parameters and how they are defined when comparing accuracy values of different pressure sensors. Typically, the accuracy of a pressure sensor consists of several components that contribute to the overall error, i.e., pressure linearity errors, pressure and thermal hysteresis errors, short term repeatability, long term stability and zero & span offset errors. These errors and their definitions are explained in the following sections among other important parameters contained in a standard datasheet.



1. Types of Pressure Sensors





Absolute pressure sensor: Top side pressure, PTOP, results in positive change of differential output voltage of the pressure sensor.

Back side-entry absolute pressure sensor: Bottom side pressure, PBOTTOM, results in positive change of differential output voltage of the pressure sensor. The measured media only come in contact with the back side of the pressure sensor. Therefore, the more sensitive front side of the sensor is protected and reliability and media compatibility of the overall system are improved. Pref

Pbottom

Gage pressure sensor: The top side pressure, PTOP, must be higher than the gage reference, PBOTTOM, and results in positive change of differential output voltage of the pressure sensor. The gage reference is identical to the local atmospheric pressure level.

Ptop

Pbottom

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PRESSURE SENSORS



Differential pressure sensor: A top side pressure, PTOP, higher than the bottom side pressure, PBOTTOM, results in positive change of differential output voltage of the pressure sensor. Bottom side pressure being higher than top side pressure results in negative change of differential output voltage of the pressure sensor.

Ptop

Pbottom

7. Supply Voltage or Supply Current

The constant supply voltage or constant supply current required to drive a pressure sensor.

8. Output Span

The span for a certain temperature is defined as the difference between the bridge output at full scale pressure and the bridge output at minimum pressure at the respective temperature and a certain supply voltage or supply current. The output span also called full-scale output (FSO).

9. Zero Offset

2. Media Compatibility

The pressure media are the gases or liquids that are in direct contact with the pressure sensor. It is important that the media are not corroding the materials of the pressure sensor element and its package to maintain long-term stability and reliability of the pressure sensor.

3. Rated Pressure

The maximum pressure value to which the specifications of the pressure sensor are guaranteed. This pressure value defines the full-scale output (FSO) of the sensor.

4. Proof Pressure



Zero offset is the output of a pressure sensor when no pressure is applied. Zero offset is either expressed as percentages of full-scale output or in electrical units such as mV, mA or digital bits. Typically, this parameter listed as a separate line item on a pressure sensor datasheet. Zero offset can be easily eliminated during a calibration step. However, replacing a pressure sensor without calibrating the zero offset may affect the overall accuracy of the system.

10. Linearity

Ideally, the transfer function (bridge output voltage vs. applied pressure) of the respective pressure sensor is a straight line. In reality, the transfer function shows more or less non-linear behavior. The non-linearity is used to describe this behavior. The non-linearity of the respec- tive sensor is calculated using the Best-Fit Straight Line (BFSL) method.

Proof pressure is the maximum pressure that may be applied to the sensor without causing any changes in performance to the specification. It still meets the specifi- cation after exposure to proof pressure. Proof pressure is not a regular operating condition.

5. Burst Pressure

Burst pressure is the maximum pressure that may be applied to the sensor without causing the sensor catastrophic failure.

6. Temperature Compensation Range

The temperature range across which the specification values of the pressure sensor are guaranteed, i.e., the measurement error of the pressure senor will be within a certain band.

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PRESSURE SENSORS

11. Thermal Hysteresis

The Thermal hysteresis of the zero offset is the maximum deviation of the zero offset at any tempera- ture within the operating temperature range after the temperature is cycled between the minimum and maximum operating temperature points.



In other words: Tharmal hysteresis describes a phenomenon whereby the same applied temperature results in different output signals depending upon whether the temperature is approached from a lower or higher temperature.



The temperature hysteresis strongly depends on the measurement conditions, e.g. dwell times, and the chosen temperature range.



compensation lower limit temperature and com- pensation upper limit temperature are obtained, and the span output temperature coefficient is expressed as the ratio of the larger of these two differences in % per degree Celsius or Kelvin.

14. Bridge resistance

Refers to the resistance value of the Wheatstone bridge of four resistors formed on a monolithic silicon sub- strate. For example, the values of the resistances R1 to R4 in the bridge are typically 5.0 kΩ each. When the resistances of the resistive elements R1 to R4 that comprise the bridge are 5.0 kΩ each, the equivalent composite resistance of the bridge is 5.0 kΩ. The figure below shows a closed bridge. Other bridge configura- tions, e.g., open bridge are possible as well.

+Vsupply / +Isupply

R2

R3 +Vout -Vout

-Vsupply / -Isupply

R1

R4 Vsub

15. Overall accuracys 12. Temperature Coefficient of Zero Offset

The zero offset of piezoresistive pressure sensors changes over temperature. The temperature coefficient of zero, TCZ, is used to describe this behavior. The difference between the zero offset output at the standard temperature and the offset output at the compensation lower limit temperature and compen- sation upper limit temperature are obtained, and the zero offset output temperature coefficient is expressed as the ratio of the larger of these two differences (absolute) with respect to the full scale output (FS) per degree Celsius or Kelvin.

13. Temperature Coefficient of Span



The calculation of total output accuracy applies to com- pensated and calibrated parts or signal-conditioned parts. The combination of all error components can then be used to describe the sensor’s error contribution to the system performance or, in other words, the sensor’s accuracy. The temperature errors are normally specified over a minimum and maximum temperature which is called the compensated temperature range and does not necessarily refer to the operating temperature range which is often wider than the compensated temperature range for a pressure sensor.



The most probable output error is defined by the root of the sum of the squares (RSS) of the individual components contributing to the total error. The main contributors are TCS, TCZ, TCR, Temperature Hysteresis, Pressure Hysteresis, Pressure Non-Linearity. For constant voltage parts the TCR does not have to be taken into account.

The span of piezoresistive pressure sensors changes over temperature. The temperature coefficient of span, TCS, is used to describe this behavior. The difference between the full scale output at the standard temperature and the full scale output at the

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16. Long-term Stability

17. Repeatability



Long term stability is a measure of how much the output signal will drift over time under normal operating conditions.



Typically, long-term stability values are determined using a lifetime test such as High-Temperature Operating Life with Pulsed Pressure (HTOL+PP).





A 1000 hour HTOL+PP could simulate 10 years of device operation. Long term drift is a figure for comparing one technology with another and cannot be relied on for a particular application. Actual lifetime drift is heavily dependent on the system, application, and types of stress, to which the sensor is exposed in the field.



To provide typical values for stability the average change of tested parts in %FS units at the 1000 hours test point is divided by 10 years.

Repeatability is the variation in measurements taken by a single person or instrument on the same item and under the same conditions. A measurement may be said to be repeatable, when this variation is smaller than some agreed limit. It does not include hysteresis. The precision of a pressure sensor sometimes includes short term repeatability errors.

  

         

  

    


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