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Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 247
11.5.6. 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.
11.5.6.1. A to D Conversion
Analog signals, such as the voltages received from the analyzer’s 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 (e.g. BOX TEMP, CO MEAS, CO REF, etc.)
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 period, 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 GFC 7001E/EM is used in uni-
polar mode with a +5 V full scale. The converter includes a 1% over and under-range. This allows signals from
–0.05 V to +5.05 V 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 7.4.3 for instructions on performing this calibration.
11.5.6.2. 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.
CO MEASURE AND REFERENCE
These are the primary signals that are used in the computation of the CO concentration. They are the
demodulated IR-sensor signals from the sync demodulator board.
SAMPLE PRESSURE AND FLOW
These are analog signals from two sensors that measure the pressure and flow rate of the gas stream at the
outlet of the sample chamber. This information is used in two ways. First, the sample pressure is used by the
CPU to calculate CO concentration. Second, the pressure and flow rate are monitored as a test function to
assist the user in predicting and troubleshooting failures.
11.5.6.3. Thermistor Interface
This circuit provides excitation, termination and signal selection for several negative-coefficient, thermistor
temperature sensors located inside the analyzer. They are as follows:
SAMPLE TEMPERATURE SENSOR
The source of this signal is a thermistor located inside the sample chamber of the Optical Bench. It measures
the temperature of the sample gas in the chamber. This data is used to during the calculation of the CO
concentration value.