Teledyne GFC-7001T - Trace CO Analyzer - Manual (Page 308)

Theory of Operation Teledyne API – Model T300/T300M CO Analyzer
Teledyne Analytical Instruments 308
The A/D can be configured for several different input modes and ranges but in the
GFC7001T/GFC7001TM 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 5.9.3.2 for instructions on performing this calibration.
13.4.5.2. 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.
C O M EASURE A ND R EFERENCE
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.
S AMPLE P RESSURE A ND F LOW
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.
13.4.5.3. 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:
S AMPLE T EMPERATURE S ENSOR
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.
B ENCH T EMPERATURE S ENSOR
This thermistor is attached to the sample chamber housing. It reports the current
temperature of the chamber housing to the CPU as part of the bench heater control loop.
W HEEL T EMPERATURE S ENSOR
This thermistor is attached to the heatsink on the GFC Wheel motor assembly. It reports
the current temperature of the wheel/motor assembly to the CPU as part of the Wheel
Heater control loop.