March 1998/053
SCX - Series
Precision Compensated Pressure Sensors
Aubinger Weg 27, 82178 Puchheim, Germany
Phone 0049 - (0) 89 80 08 30, Fax 0049 - (0) 89 8 00 83 33
http://www.sensortechnics.com
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.
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.
Circuit Description
The LM10 is used to provide a voltage reference for the excitation
voltage (V
E), and for the voltage node VREF. With this configuration,
V
E and VREF are not affected by noise or voltage variations in the
12 V power supply. R
3 is used to adjust VREF 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
(10k
Ω) 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 B2 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.
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.
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 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, VIN , is amplified by
amplifiers B
1 and B2. 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 = VIN
[ 2 (1+ R/
R1)]+VREF
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 VOUT = 3.500 V.
3. Repeat procedure if necessary.
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