Download Pressure Sensor Specifications Explained

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
© Silicon Microstructures, Inc. 2001-2013. All rights reserved.
+1 (408) 577-0100 | [email protected] |
40AN1302.00
1
Application Note An13-02
www.si-micro.com
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.
© Silicon Microstructures, Inc. 2001-2013. All rights reserved.
+1 (408) 577-0100 | [email protected] |
40AN1302.00
2
Application Note An13-02
www.si-micro.com
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
© Silicon Microstructures, Inc. 2001-2013. All rights reserved.
+1 (408) 577-0100 | [email protected] |
40AN1302.00
3
Application Note An13-02
www.si-micro.com
PRESSURE SENSORS
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.
© Silicon Microstructures, Inc. 2001-2013. All rights reserved.
+1 (408) 577-0100 | [email protected] |
40AN1302.00
4
Application Note An13-02
www.si-micro.com
PRESSURE SENSORS
Warranty and Disclaimer
Information in this document is provided solely to enable software and system implementers to use Silicon Microstructures,
Inc. (SMI) products and/or services. No express or implied
copyright licenses are granted hereunder to design or fabricate any silicon-based microstructures based on the information in this document.
SMI makes no warranty, representation, or guarantee regarding the suitability of its products for any particular purpose,
nor does SMI assume any liability arising out of the application or use of any product or silicon-based microstructure,
and specifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical”
parameters which may be provided in SMI’s datasheets and/
or specifications can and do vary in different applications
and actual performance may vary over time. All operating
parameters, including “Typicals”, must be validated for each
customer application by customer’s technical experts. SMI does
not convey any license under its patent rights nor the rights
of others. SMI makes no representation that the circuits are
free of patent infringement. SMI products are not designed,
intended, or authorized for use as components in systems
intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other
application in which the failure of the SMI product could
create a situation where personal injury or death may
occur. Should Buyer purchase or use SMI products for any such
unintended or unauthorized application, Buyer shall indemnify and hold SMI and its officers, employees, subsidiaries,
affiliates, and distributors harmless against all claims, costs,
damages, and expenses, and reasonable attorney fees arising
out of, directly or indirectly, any claim of personal injury or
death associated with such unintended or unauthorized use,
even if such claim alleges that SMI was negligent regarding
the design or manufacture of the products.
© Silicon Microstructures, Inc. 2001-2013. All rights reserved.
SMI warrants goods of its manufacture as being free of defective materials and faulty workmanship. SMI standard product
warranty applies unless agreed to otherwise by SMI in writing. Please refer to your order acknowledgement or contact
SMI directly for specific warranty details. If warranted goods
are returned to SMI during the period of coverage, SMI will
repair or replace, at its option, without charge those items it
finds defective. The foregoing is buyer’s sole remedy and is in
lieu of all warranties, expressed or implied, including those
of merchantability and fitness for a particular purpose. In no
event shall SMI be liable for consequential, special, or indirect
damages.
While SMI may provide application assistance to aid its customers' design process, it is up to each customer to determine the suitability of the product for its specific application.
The information supplied by SMI is believed to be accurate
and reliable as of this printing. However, SMI assumes no
responsibility for its use. SMI assumes no responsibility for any
inaccuracies and/or errors in this publication and reserves the
right to make changes to any products or specifications herein
without further notice.
Silicon Microstructures, Inc. TM and the Silicon Microstructures, Inc. logo are trademarks of Silicon
Microstructures, Inc. All other service or product names are
the property of their respective owners.
© Silicon Microstructures, Inc. 2001-2013. All rights reserved.
+1 (408) 577-0100 | [email protected] |
40AN1302.00
5