SCX01DN Datasheet, PDF (5/10 Pages) List of Unclassifed Manufacturers – Precision Compensated Pressure Sensors

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 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 A₁, and A₂. The pot,
in resistor R , can be adjusted to calibrate the gain of the circuit as
1
shown in the following gain equation:
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_OUT to be at the same
common-mode voltage as V_IN, and takes advantage of the excellent
CMRR of the ADC0831. R is used to adjust the gain of the amplifier
2
such that
V_OUT = V_IN 2 [1+ ^R/_R1]
By adjusting R , V (-) on the A/D converter is used to adjust the
3 IN
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/ LSB. The A/D converter, as shown, is a free-running
2
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 R₃ 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 R₂ 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.
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 R₃. 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 R₃, 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 R₄ 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
March 1998/053
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Phone 0049 - (0) 89 80 08 30, Fax 0049 - (0) 89 8 00 83 33
http://www.sensortechnics.com