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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
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 proportio-
nal to applied pressure. The circuit allows for easy calibration and
uses a single 5V supply.
Circuit Description
The output signal of the sensor is amplified by A
, and A
. The pot,
in resistor R
, 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
]
By adjusting R
, V
(-) 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
, is twice the zener voltage, minus
1
1
/
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
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
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.
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
parallel output for simplified testing.
Circuit Description
The three op amp configuration allows V
to be at the same
common-mode voltage as V
, 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
is equal to the zener voltage minus 1
1
/
LSB. the initial offset of the
circuit can be nulled out by adjusting pot R
. 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
will have a zero start bit. Then,
starting with the MSB, the data out line (D
) 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.
Adjustment Procedure
1. With zero-pressure, adjust R
, 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
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