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Teledyne API Model T360/T360M Operation Manual Principles of Operation
215
7.4.6. Motherboard
This printed circuit assembly provides a multitude of functions including, A/D
conversion, digital input/output, PC-104 to I2C translation, temperature sensor
signal processing and is a pass through for the RS-232 and RS-485 signals.
7.4.6.1. 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, CO2
MEAS, CO2 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 T360 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 4.13.4 for instructions on performing this calibration.
7.4.6.2. 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.
07272B DCN6552