Theory of Operation T400 Ozone Analyzer Operator’s Manual
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13.3.3. MOTHERBOARD
This printed circuit assembly provides a multitude of functions including, A/D
conversion, digital input/output, PC-104 to I
2
C translation, temperature sensor signal
processing and is a pass through for the RS-232 and RS-485 signals.
13.3.3.1. A to D Conversion
Analog signals, such as the voltages received from the analyzers various sensors, are
converted into digital signals that the CPU can understand and manipulate by the analog
to digital converter (A/D). Under the control of the CPU, this functional block selects a
particular signal input and then coverts the selected voltage into a digital word.
The A/D consists of a voltage-to-frequency (V-F) converter, a programmable logic
device (PLD), three multiplexers, several amplifiers and some other associated devices.
The V-F converter produces a frequency proportional to its input voltage. The PLD
counts the output of the V-F during a specified time, and sends the result of that count,
in the form of a binary number, to the CPU.
The A/D can be configured for several different input modes and ranges but in the T400
is used in uni-polar mode with a +5V full scale. The converter includes a 1% over and
under-range. This allows signals from –0.05V to +5.05V to be fully converted.
For calibration purposes, two reference voltages are supplied to the A/D converter:
Reference ground and +4.096 VDC. During calibration, the device measures these two
voltages, outputs their digital equivalent to the CPU. The CPU uses these values to
compute the converter’s offset and slope and uses these factors for subsequent
conversions. See Section 5.10.2 for instructions on performing this calibration.
13.3.3.2. Sensor Inputs
The key analog sensor signals are coupled to the A/D through the master multiplexer
from two connectors on the motherboard. 100K terminating resistors on each of the
inputs prevent cross talk from appearing on the sensor signals.
O
3
DETECTOR OUTPUT: This is the primary signal used in the computation of the
O
3
concentration.
GAS PRESSURE SENSOR: This sensor measures the gas pressure in the sample
chamber upstream of the critical flow orifice (see Figure 3-16). The sample
pres
sure is used by the CPU to calculate O
3
Concentration.
GAS FLOW SENSOR: This sensor measures the flow rate of the sample gas
through the instrument. This information is used as a diagnostic tool for determining
gas flow problems
13.3.3.3. Thermistor Interface
This circuit provides excitation, termination and signal selection for several negative-
coefficient, thermistor temperature sensors located inside the analyzer. They are:
SAMPLE TEMPERATURE SENSOR: The source of this signal is a thermistor
attached to the absorption tube inside the optical bench assembly. It measures the
temperature of the sample gas in the chamber. This data is used to during the
calculation of the O
3
concentration value.
