Mother Board - Teledyne 400E Instruction Manual

Model 400E Ozone Analyzer
Instruction Manual

10.3.6. Mother Board

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.
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 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
M400E 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 6.7.3 for instructions on performing this calibration.
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 DETECTOR OUTPUT: This is the primary signal used in the computation of the O
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concentration.
GAS PRESSURE SENSOR: This sensor measures the gas pressure in the Sample
Chamber upstream of the Critical Flow Orifice (see Figure 10-8). The Sample
Pressure is used by the CPU to calculate O
P/N 04316 Rev: B
THEORY OF OPERATION
Concentration.
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