Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 226)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 208
The A/D can be configured for several different input modes and ranges but in the GFC 7000TA 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 6.13.4 for instructions on performing this calibration.
9.3.6.2. 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
2
Measure and Reference
These are the primary signals that are used in the computation of the CO
2
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
2
Concentration. Second, the pressure and flow rate are monitored as a
test function to assist the user in predicting and troubleshooting failures.
9.3.6.3. Thermistor Interface
This circuit provides excitation, termination and signal selection for several negative-coefficient, thermistor
temperature sensors located inside the analyzer. They are:
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
2
concentration value.
Bench Temperature Sensor
This thermistor, attached to the sample chamber housing, reports the current temperature of the chamber
housing to the CPU as part of the bench heater control loop.
Wheel Temperature Sensor
This thermistor (attached to the heat-sync on the GFC wheel motor assembly) reports the current
temperature of the wheel/motor assembly to the CPU as part of the Wheel Heater control loop.