SCX Series
Precision compensated pressure sensors
FEATURES
· 0...1 to 0...150 psi
· Precision temperature
compensation
· Calibrated zero and span
· Small size
· Low noise
· High accuracy
· High impedance for
low power applications
GENERAL DESCRIPTION
The SCX series sensors will provide a very
cost effective solution for pressure
applications that require high accuracy over
a wide temperature range. These internally
calibrated and temperature compensated
sensors were specifically designed to
provide an accurate and stable output over
a 0°C to 70°C temperature range. This series
is intended for use with non-corrosive, non-
ionic working fluids such as air, dry gases,
and the like.
Devices are available to measure absolute,
differential, and gage pressures from 1 psi
(SCX01) up to 150 psi (SCX150). The abso-
lute (A) devices have an internal vacuum
reference and an output voltage propor-
tional to absolute pressure. The differential
(D) devices allow application of pressure
to either side of the pressure sensing
diaphragm and can be used for gage or
differential pressure measurements.
The SCX devices feature an integrated cir-
cuit sensor element and laser trimmed thick
film ceramic housed in a compact nylon
case. This package provides excellent
corrosion resistance and provides isolation
to external package stresses. The package
has convenient mounting holes and pres-
sure ports for ease of use with standard
plastic tubing for pressure connection.
All SCX devices are calibrated for span to
within ±1 % and provide a very low zero
pressure output of ±300 microvolts
maximum. Thus, for many applications no
trimming networks are required in the signal
Scale:
1 cm
½ inch
APPLICATIONS
· Medical equipment
· Barometry
· Computer peripherals
· Pneumatic control
· HVAC
conditioning circuitry. If the application
requires extended temperature range
operation, beyond 0 to 70°C, two pins which
provide an output voltage proportional to
temperature are available for use with
external circuitry.
The output of the bridge is ratiometric to the
supply voltage and operation from any D.C.
supply voltage up to +20 V is acceptable.
Because these devices have very low
noise and excellent temperature compen-
sation, they are ideal for medical and other
high performance applications. The 100
microsecond response time also makes this
series an excellent choice for computer peri-
pherals and pneumatic control applications.
EQUIVALENT CIRCUIT
2
Vs
ELECTRICAL CONNECTION
1
1 2 3 4 5 6
3
5
Bottom view
Pin 1)
Pin 2)
Pin 3)
Pin 4)
Pin 5)
Pin 6)
Temperature output (+)
V
S
Output (+)
Ground
Output (-)
Temperature output (-)
6
Note:
The polarity indicated is for pressure applied to port B
(for absolute devices pressure is applied to port A
and the output polarity is reversed.)
4
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SCX Series
Precision compensated pressure sensors
PRESSURE SENSOR CHARACTERISTICS
1
STANDARD PRESSURE RANGES
Full-scale span
1,3
Part Number
SCX01DN
SCX05DN
SCX15AN
SCX15DN
SCX30AN
SCX30DN
SCX100AN
SCX100DN
SCX150DN
Operating pressure
0 - 1 psid
0 - 5 psid
0 - 15 psia
0 - 15 psid
0 - 30 psia
0 - 30 psid
0 - 100 psia
0 - 100 psid
0 - 150 psid
Proof pressure
20 psid
20 psid
30 psia
30 psid
60 psia
60 psid
150 psia
150 psid
150 psid
2
Min.
17.82 mV
59.4 mV
89.1 mV
89.1 mV
89.1 mV
89.1 mV
99.0 mV
99.0 mV
89.0 mV
Typ.
18 mV
60 mV
90 mV
90 mV
90 mV
90 mV
100 mV
100 mV
90 mV
Max.
18.18 mV
60.6 mV
90.9 mV
90.9 mV
90.9 mV
90.9 mV
101.0 mV
101.0 mV
91.0 mV
Maximum ratings
(for all devices)
Supply voltage V
S
Common-mode pressure
Lead temperature (soldering, 4 seconds)
+20 V
DC
50 psig
250°C
Environmental specifications
(for all devices)
Temperature range
Compensated
Operating
Storage
Humidity limits (no condensation)
0 to 70°C
-40°C to +85°C
-55°C to +125°C
0 to 99 %RH
COMMON PERFORMANCE CHARACTERISTICS
1
Characteristic
Zero pressure offset
12
Combined linearity and hysteresis
4
Temperature effect on span (0 - 70°C)
5
Temperature effect on offset (0 - 70°C)
5
Repeatability
6
Input impedance
7
Output impedance
8
Common-mode voltage
9
Response time
10
Long term stability of offset and span
11
Min.
-300
---
---
---
---
---
---
5.8
---
---
Typ.
0
±0.1
±0.2
±100
±0.2
4.0
4.0
6.0
100
±0.1
Max.
+300
±0.5
±1.0
±500
±0.5
---
---
6.2
---
---
Unit
µV
%FSO
%FSO
µV
%FSO
kΩ
kΩ
V
DC
µsec
mV
Specification notes:
1.
2.
3.
4.
5.
6.
Reference conditions: unless otherwise noted: supply voltage, V
S
= 12 V, T
A
= 25°C, common-mode line pressure = 0 psig,
pressure applied to Port B. For absolute devices only, pressure is applied to Port A and the output polarity is reversed.
Maximum pressure above which causes permanent sensor failure.
Span is the algebraic difference between the output voltage at full-scale pressure and the output at zero pressure. Span
is ratiometric to the supply voltage.
See Definition of Terms. Hysteresis - the maximum output difference at any point within the operating pressure range for
increasing and decreasing pressure.
Maximum error band of the offset voltage and the error band of the span, relative to the 25°C reading.
Maximum difference in output at any pressure with the operating pressure range and temperature within 0°C to +50°C after:
a) 1,000 temperature cycles, 0°C to +70°C
b) 1.5 million pressure cycles, 0 psi to full-scale span
Input impedance is the impedance between pins 2 and 4.
Output impedance is the impedance between pins 3 and 5.
This is the common-mode voltage of the output arms (pins 3 and 5) for V
S
= 12 V
DC
.
Response time for a 0 psi to full-scale span pressure step change, 10 % to 90 % rise time.
Long term stability over a one year period.
Maximum zero pressure offset for absolute devices is 0 ±500 µV.
7.
8.
9.
10.
11.
12.
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SCX Series
Precision compensated pressure sensors
TYPICAL PERFORMANCE CHARACTERISTICS
GENERAL DISCUSSION
The SCX series devices give a voltage output which is directly pro-
portional to applied pressure. The devices will give an increasing
positiv going output when increasing pressure is applied to pressure
port P
B
of the device. If the input pressure connections are reversed,
the output will increase with decreases in pressure. The devices
are ratiometric to the supply voltage and changes in the supply
voltage will cause proportional changes in the offset voltage and
full-scale span. Since for absolute devices pressure is applied to
port P
A
, output polarity will be reversed.
Media compatibility
SCX devices are compatible with most non-corrosive gases.
Because the circuit is coated with a protective silicon gel, many
otherwise corrosive environments can be compatible with the
sensors. As shown in the physical construction diagram below, fluids
must generally be compatible with silicon gel, plastic, aluminium,
RTV, silicon, and glass for use with Port B. For questions concerning
media compatibility, contact the factory.
User calibration
The SCX devices are fully calibrated for offset and span and should
therefore require little if any user adjustment in most applications.
For precise span and offset adjustments, refer to the applications
section herein.
MECHANICAL AND MOUNTING
CONSIDERATIONS
The SCX nylon housing is designed for convenient pressure con-
nection and easy PC board mounting. To mount the device horizon-
tally 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.
All versions of the SCX sensors have two (2) tubes available for
pressure connection. For absolute devices, only port P
A
is active.
Applying pressure through the other port will result in pressure dead
ending into the backside of the silicon sensor and the device will
not give an output signal with pressure.
For gage applications, pressure should be applied the port P
B
. Port
P
A
is then the vent port which is left open to the atmosphere. For
differential pressure applications, to get proper output signal polarity,
port P
B
should be used as the high pressure port and P
A
should be
used as the low pressure port.
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Vacuum reference (absolute device)
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.
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SCX Series
Precision compensated pressure sensors
Physical construction (cutaway diagram)
(not drawn to scale)
APPLICATION INFORMATION
The following circuits show some typical designs using the SCX
series sensors. For specific applications information or assistance,
please contact your nearest Sensym sales office or the Sensym
factory.
Adjustment procedure
1. With zero-pressure applied, adjust the offset adjust R
3
,
until V
OUT
= 2.000 V
2. Apply full-scale pressure (10 in. W.C) to port B
1
and adjust the
full-scale adjust R
2
, so that V
OUT
= 5.000 V.
3. Repeat procedure if necessary.
Low pressure applications
For sensing pressures below 1 psi, the circuit shown in Figure A
uses the SCX01DN to provide a 2 to 5 V output for a 0 to 10 inch of
water column input pressure. This output signal is compatible with
many A/D converters and hence can be used to interface to a
microprocessor system. This low-cost circuit is easily adaptable to
lower full-scale pressures down to 5 inches of water column.
Medical applications
For blood pressure monitoring applications, the circuit shown in
Figure B provides a 0.5 V to 3.5 V output for a 0 to 300 mm Hg input
pressure. The circuit is easily calibrated and is not affected by
changes in the voltage supply. Because 300 mm Hg is approximately
5.8 psi, an SCX05DN is used.
Circuit description
The LM10 is used to provide a voltage reference for the excitation
voltage (V
E
), and for the voltage node V
REF
. With this configuration,
V
E
and V
REF
are not affected by noise or voltage variations in the 12
V power supply. R
3
is used to adjust V
REF
to set the initial offset
voltage at the output, V
OUT
.
The pressure signal, V
IN
, is amplified by amplifiers B
1
, and B
2
(see
Sensym Application Note SSAN-17A for details on this amplifier)
R
2
is used to adjust the signal gain of the circuit. The output equation
is given below.
V
OUT
= V
IN
[ 2 (1+
R
/
R1
)]+V
REF
For the best circuit performance, a careful selection of components
in necessary. Use wirebound pots of insure low temperature
coefficients and low longterm drift. A five-element resistor array
(10kΩ) SIP should be used for the resistors in the amplifier stage in
order to obtain closely matched values and temperature coefficients.
All other resistors should be 1% metal film. Amplifiers B
1
, and B
2
should have low offset voltage and low noise. Signal lines should
be as short as possible and the power supply should be capacitively
bypassed on the PC board.
Circuit description
The circuit shown here in Figure B is very similar to that shown in
Figure A. The internal 200 mV reference voltage of the LM10 is
amplified to provide power to the sensor and to provide a voltage
reference, V
REF
. This allows the circuit to operate at a supply voltage
between 5 and 20 volts without affecting performance of the circuit.
By adjusting R
3
, V
REF
is used to set the initial zero-pressure voltage
at V
OUT
. The pressure signal, V
IN
, is amplified by amplifiers B
1
and
B
2
. These amplifiers should be precision op amps with low offset
voltages and high common-mode rejection. The signal gain is
adjusted by R
2
, and the overall equation for the output voltage is
given by ,
V
OUT
= V
IN
[ 2 (1+
R
/
R1
)]+V
REF
Adjustment procedure
1. With zero-pressure applied, adjust the offset adjust
R
3
, until V
OUT
= 0.500 V
2. Apply full-scale pressure (300 mm Hg) to port B
1
and adjust
R
2
, until V
OUT
= 3.500 V.
3. Repeat procedure if necessary.
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SCX Series
Precision compensated pressure sensors
APPLICATION INFORMATION
(cont.)
Parallel A/D conversion
The SCX sensor can be easily interfaced to a microprocessor bus.
Using an A/D converter, for a 0 to 1 psig input, the circuit in Figure
C will provide an eight-bit parallel output which is proportional to
applied pressure. The circuit allows for easy calibration and uses a
single 5 V supply.
Serial A/D conversion
The circuit shown in Figure D is similar to that shown in Figure C,
except the output is bit serial. Also shown (under the dashed line)
is a complimentary circuit for converting the serial output to a par-
allel output for simplified testing.
Circuit description
The output signal of the sensor is amplified by A
1
, and A
2
. The pot,
in resistor R
1
, can be adjusted to calibrate the gain of the circuit as
shown in the following gain equation:
V
OUT
= V
IN
2 [1+
R
/
R1
]
Circuit description
The three op amp configuration allows V
OUT
to be at the same
common-mode voltage as V
IN
, and takes advantage of the excellent
CMRR of the ADC0831. R
2
is used to adjust the gain of the amplifier
such that
V
OUT
= V
IN
2 [
3
/
2
+
2R
/
R1
]
The A/D converter will output the maximum digital code when V
OUT
is equal to the zener voltage minus 1
1
/
2
LSB. the initial offset of the
circuit can be nulled out by adjusting pot R
3
. The converter circuit
requires only a clock and a chip select (CS) line in order to operate.
As shown in Figure E, when CS goes low, the A/D converter will
start a new conversion on the next rising edge of the clock. On the
next falling edge of the clock, D
O
will have a zero start bit. Then,
starting with the MSB, the data out line (D
O
) will provide the converted
digital output during the next eight consecutive falling edges of the
clock. The serial output can be read by using an oscilloscope, a
microprocessor, or a simple serial-to-parallel converter as shown
in Figure D.
By adjusting R
3
, V
IN
(-) on the A/D converter is used to adjust the
initial offset voltage. A zener diode (LT1004) sets the initial input
voltage and provides the reference voltage for the converter. The
converter will output the maximum digital code when the A/D
converter´s input voltage, V
OUT
, is twice the zener voltage, minus 1
1
/
2
LSB. The A/D converter, as shown, is a free-running configuration
where the binary output is updated continously*. The only
requirement is that the WR and INTR must be momentarily grounded
after power-up to ensure proper operation.
Adjustment procedure
1. With no pressure applied, adjust the offset pot R
3
until all bits
are zero except the LSB, which should be switching between
one and zero.
2. Apply full-scale pressure (1 psig) to port B, and adjust the full-
scale pot R
2
until all bits are ones except the LSB which should
be flickering between one and zero.
3. Repeat procedure if necessary.
* For timing specifications and bus interface, see the ADC0804
Datasheet from National Semiconductor.
Adjustment procedure
1. With zero-pressure, adjust R
3
, until the output of the
A/D converter is alternating between 00 and 01 (HEX).
2. Apply full-scale pressure (1 psig) to port B, and adjust R
4
unti
l
the digital output alternates between the FE to FF transition.
3. Repeat procedure if necessary.
Figure A. Low pressure circuits provide a 2 to 5 V output for a 0-10 in. W.C. pressure input
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