SCX Series
Precision compensated pressure sensors
5/10
August 2005 / 053
www.sensortechnics.com
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.
Circuit description
The output signal of the sensor is amplified by A
1, and A2. 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 = VIN
2 [1+ R/
R1]
By adjusting R
3, VIN (-) 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.
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 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 = VIN
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.
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
APPLICATION INFORMATION (cont.)
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