SX Series
Pressure sensors
FEATURES
· 0...1 to 0...150 psi
· Absolute, differential
and gage devices
· High impedance bridge
· Low power consumption
for battery operation
GENERAL DESCRIPTION
The SX series of pressure sensors
provides the most cost effective
method of measuring pressures up
to 150 psi. These sensors were
specifically designed to be used with
non-corrosive and non-ionic media,
such as air and dry gases. Convenient
pressure ranges are available to
measure differential, gage and
absolute pressures from 0 to 1 psi up
to 0 to 150 psi.
The absolute (A) devices have an in-
ternal vacuum reference and an output
voltage proportional to absolute
pressure. The differential (D) devices
allow application of pressure to either
side of the diaphragm and can be
used for gage or differential pressure
measurements.
This product is packaged either in
SenSym's standard low cost chip
carrier "button" package, a plastic
ported "N" package or a dual inline
package (DIP). All packages are
designed for applications where the
sensing element is to be integral to
the OEM equipment. These packages
APPLICATIONS
· Industrial controls
· Pneumatic controls
· Medical instrumentation
· Barometry
can be o-ring sealed, epoxied, and/or
clamped onto a pressure fitting.
A closed bridge 4-pin SIP configuration
is provided for electrical connection to
the button or "N" package.
Because of its high-impedance
bridge, the SX series is ideal for
portable and low power or battery
opera-ted systems. Due to its low
noise, the SX is an excellent choice
for medical and low pressure
measurements.
EQUIVALENT CIRCUIT
Vs
ELECTRICAL CONNECTION
+
Output
Button sensor
GND
out +
+VS
out -
1
2
3
4
-
Button sensor or "N" package
P1
Vs
+Vs
P2
4
1
out + out -
GND GND
1
P1
4
+Vs out +
GND
+Vs out -
1
P1
P2
+Vs
+Vs
out -
vent hole
out +
4 +Vs
-
Output
SXxxxGD2 DIP
SXxxxAD2
+
DIP package
The polarity indicated is for pressure applied to:
SX...
: P1 (forward gage)
SX...AD2
: P1 (forward gage)
SX...GD2
: P2 (backward gage)
SX...DD4
: P2 (backward gage)
SXxxxD4 DIP
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SX Series
Pressure sensors
PRESSURE SENSOR CHARACTERISTICS
(V
s
= 5.0 ± 0.01 V, t
amb
= 25 °C, common-mode pressure = 0 psig, pressure applied to
P1
for Button, N and A2
housings, pressure applied to
P2
for G2 and D4 housings)
Maximum ratings
Suppy voltage, V
S
Temperature ranges
Operating
Storage
+12 V
DC
-40°C to +85°C
-55°C to +125°C
Maximum pressure at any port
11
Lead temperature (soldering 4 sec.)
150 psig
250 °C
Part number
SX01...
SX05...
SX15...
SX30...
SX100...
SX150...
Operating pressure
0...1 psi
0...5 psi
0...15 psi
0...30 psi
0...100 psi
0...150 psi
Proof pressure
8
2 0 p si
2 0 p si
3 0 p si
6 0 p si
1 5 0 p si
2 0 0 p si
Full-scale span
1
Min.
Typ.
Max.
15 mV
50 mV
75 mV
75 mV
100 mV
75 mV
20 mV
75 mV
110 mV
110 mV
150 mV
110 mV
25 mV
100 mV
150 mV
150 mV
200 mV
150 mV
PERFORMANCE CHARACTERISTICS
(V
s
= 5.0 ± 0.01 V, t
amb
= 25 °C, common-mode pressure = 0 psig, pressure applied to
P1
for Button, N and A2
housings, pressure applied to
P2
for G2 and D4 housings)
Characteristics
Zero pressure offset
9
Temperature effects
(0 to 70 °C)
4,7
Offset
Span
Bridge impedance
Min.
-35
-2550
+690
Typ.
-20
±4
-2150
+750
±0.2
±0.5
±0.1
4.1
4.1
0.1
Max.
0
-1900
+810
±0.5
Unit
mV
µV/V/°C
ppm/°C
%FSS
mV
kΩ
ms
Combined linearity and hysteresis
2
Repeatability
3
Long term stability of offset and span
6
Input impedance
Output impedance
Response time
5
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SX Series
Pressure sensors
TYPICAL PERFORMANCE CHARACTERISTICS
Specification notes:
1.
Span is the algebraic difference between the output voltage at full-scale pressure and the output at zero pressure.
2.
Hysteresis is the maximum output difference at any point within the operating pressure range for increasing and decreasing
pressure. Linearity is the maximum deviation of measure output at constant temperature (25°C) from "Best Straight Line"
determined by three points, offset, full scale pressure and half full scale pressure.
3.
Maximum difference in output at any pressure with the operating pressure range and temperature within 0°C to +70°C after:
a) 100 temperature cycles, 0°C to +70°C
b) 1.0 million pressure cycles, 0 psi to full scale span
4.
Slope of the best straight line from 0°C and 70°C. For operation outside this temperature, contact Sensortechnics for more
specific applications information.
5.
Response time for a 0 to full-scale span pressure step change.
6.
Long term stability over a one year period .
7.
This parameter is not 100 % tested. It is guaranteed by process design and tested on a sample basis only.
8.
If the proof pressure is exceeded, even momentarily, the package may leak or burst, or the pressure sensing die may fracture.
Note: The proof pressure for the forward gage of all devices in the D4-package is the specified value or 100 psi, whatever is less.
9.
The zero pressure offset is 0 mV Min, 20 mV Typ and 35 mV Max for part nos. SX...G2 and SX...D4.
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SX Series
Pressure sensors
MECHANICAL AND MOUNTING CONSIDERATIONS
Button sensor element
The button sensor element was designed
to allow easy interface with additional
cases and housings which then allow
pressure connection. The device can be
mounted with an o-ring, gasket, or RTV
seals on one or both sides of the device.
The device can then be glued or clamped
into a variety of fixtures and the leads can
be bent as necessary to allow for ease of
electrical connection. However, caution is
advised as repeated bending of the leads
will cause eventual breakage.
For most gage applications, pressure
should be applied to the top side of the
device (see Physical Construction
Drawing). For differential applications, the
top side of the device (P1) should be used
as the high pressure port and the bottom
(P2) as the low pressure port.
The button SX package has a very small
internal volume of 0.06 cubic centimeters
for P1 and 0.001 cubic centimeters for P2.
“N” packaged sensor
The "N” packaged sensor is designed for
convenient pressure connection and easy
PC board mounting. To mount the device
horizontally to a PC board, the leads can be
bent downward and the package attached
to the board using either tie wraps or
mounting
screws.
For
pressure
attachment, tygon or silicon tubing is
recommended.
The “N” package version of the sensor has
two (2) tubes available for pressure
connec-tion. For gage devices, pressure
should be applied to port P1. For differential
pressure applications, port P1 should be
used as the high pressure port and P2
should be used as the low pressure port.
Vacuum
reference
(absolute
GENERAL DISCUSSION
Output characteristics
The SX series devices give a voltage output
which is directly proportional to applied
pressure. The devices will give an increase
in positive going output when increasing
pressure is applied to pressure port P1 of
the device. If the devices are operated in
the backward gage mode, the output will
increase with decreases in pressure. The
devices are ratiometric to the supply
voltage. Changes in supply voltage will
cause proportional changes in the offset
voltage and full-scale span.
devices)
Absolute sensors have a hermetically
sealed vacuum reference chamber. The
offset voltage on these units is therefore
measured at vacuum, 0 psia. Since all
pressure is measured relative to a vacuum
reference, all changes in barometric
pressure or changes in altitude will cause
changes in the device output.
Media compatibility
SX devices are compatible with most non-
corrosive gases. Because the circuitry is
coated with a protective silicon gel, some
otherwise corrosive environments can be
compatible with the sensors. As shown in
the physical construction diagram below
for the button sensor element and ,,N”
package, fluids must generally be
compatible with silicon gel, RTV, plastic, and
aluminum for forward gage use and RTV,
silicon, glass and aluminum for backward
gage or differential applications. For
questions concerning media compatibility,
contact the factory.
User calibration
SX series devices feature the button IC
pressure sensor element. This will keep
overall system costs down by allowing the
user to select calibration and temperature
compensation circuits which specifically
match individual application needs. In most
cases, the primary signal conditioning
elements to be added to the SX by the user
are: offset and span calibration and
temperature compensation.
Some typical circuits are shown in the
application section.
PHYSICAL CONSTRUCTION
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SX Series
Pressure sensors
APPLICATION INFORMATION
General
The SX family of pressure sensors functions
as a Wheatstone bridge. When pressure is
applied to the device (see Figure I) the
resistors in the arms of the bridge change
by an amount
∆.
it can be seen that the sensitivity change
with temperature is slightly non-linear and
can be correlated very well with an equation
of the form:
S = SO[(1 - ßTD) +
ρT
D2]
(3)
where TD is the temperature difference
between 25°C and the temperature of inte-
rest, SO is the sensitivity at 25°C, and beta
(ß) and rho (ρ) are correlation constants.
Fortunately, between 0°C and 70°C the
change in sensitivity with temperature is
quite linear, and excellent results can be
obtained over this temperature range by
ignoring the second-order temperature
dependent term. Operating outside the 0°C
and 70°C temperature range will require a
more rigorous mathematical approach and
the use of non-linear compensating cir-
cuitry, if accuracy of better than ±1 % is re-
quired. Because the majority of SX appli-
cations fall within the 0°C to 70°C operating
temperature range, the discussion and
circuit designs given here will ignore the
non-linear effects.
Thus:
S = SO (1 - ßTD)
(4)
This term enters into several compensation
circuit equations, particularly when the
bridge excitation is from a constant current
source.
To summarize, the following list indicates
how the sensor variables can be accommo-
dated
• Full-scale span from device to device.
Make the gain adjustment in the op amp
circuitry
• Temperature coefficient of span:
1) temperature compensate the bridge or
2) temperature compensate the op amp
gain
• Offset voltage:
Adjustment in op amp circuitry
• Offset voltage temperature coefficient:
Usually can be ignored. For more precise
design requirements, contact the factory
for information on how to compensate for
this term.
Figure I. Button sensor bridge
schematic
The resulting differential output voltage V0
is easily shown to be VO= VB x
∆.
Since
the change in resistance is directly pro-
portional to pressure, VO can be written as:
VO = S x P x VB ± VOS
(1)
Where: VO is the output voltage in mV
S is the sensitivity in mV/V per psi
P is the pressure in psi
VB is the bridge voltage in volts.
VOS is the offset error (the differential output
voltage when the applied pressure is zero).
The offset voltage presents little problem in
most applications, since it can easily be
corrected for in the amplifier circuitry, or
corrected digitally if a microprocessor is
used in the system.
Bridge compensation circuits
Although thermistors can be used to tempe-
rature compensate the bridge (and in fact
will be required for extended temperature
operation), they are inherently non-linear,
difficult to use in volume production, and
more expensive than the circuit approaches
shown here, which use inexpensive semi-
conductor devices The circuits shown have
been designed to incorporate a minimum
number of adjustments and allow inter-
changeability of devices with little variation
from device to device. In general, equations
for the bridge voltage and its change with
temperature are given to enable the user to
modify or adjust the circuitry as required.
1. Diode string
(Figure II)
For systems using 6 V supplies, this method
of compensating for the effects of span over
temperature is the lowest cost solution The
diodes are small signal silicon diodes, such
as 1N914 or 1N4148, and do not have to
be matched.
Substituting equation (4) into equation (1)
and ignoring VOS, it can be shown that the
necessary bridge voltage, VB, will be of the
form:
2
VB = VBO = VBO [(1 - ßTD + (ßTD) +...)]
(1-ßTD)
where VBO is the bridge voltage at 25°C.
This equation is again non-linear.
However, for the temperature range of
interest, and since ß is small (0.215%/°C
from the electrical tables), the above
expression can be approximated by:
VB=VBO [1 +ßTD]
with less than 1 % error. Thus to compen-
sate for a negative 2150 ppm/°C sensitivity
change with temperature, the bridge voltage
should increase with temperature at a rate
of +2150 ppm/°C.
The above value of bridge voltage change
will be used in the circuit discussions that
follow. That is to say, the required change
in terms of ppm/°C is:
•
VB
= +2050 ppm/°C
VB
Temperature effects
In this discussion, for simplicity of notation,
the change of a variable with temperature
will be designated with a dot (•) over the
variable. For example,
•
in sensitivity
=
change in temperature
=
δS
change
δT
S
From equation (1), and ignoring the VOS
term, it in seen that for a given constant
pressure, the output voltage change, as a
function of temperature*, is:
•
•
VO = SPVB
(2)
Thus, in order for output voltage to be inde-
pendent of temperature, the voltage across
the bridge, VB, must change with tempera-
ture in the "opposite direction” from the
sensitivity change with temperature. From
the typical curves for the temperature
dependence of span (span = S x P x VB),
July 2008 / 052
( )
The bridge input resistance*, R B also
changes with temperature and is quite linear
in the temperature range of interest. The
bridge resistance has a temperature
coefficient of typically:
•
RB
= +750 ppm/°C
RB
( )
Figure II. Diode String Span
Compensation
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