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Teledyne API - Model 200EH/EM Operation Manual Theory of Operation
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10.4.9. MOTHERBOARD
This is the largest electronic assembly in the analyzer and is mounted to the rear panel as the base for the CPU
board and all I/O connectors. 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.
10.4.9.1. 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 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 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 6.13.5.4 for instructions on performing this calibration.
10.4.9.2. Sensor Inputs
The key analog sensor signals are coupled to the A/D converter through the master multiplexer from two
connectors on the motherboard. Terminating resistors (100 kΩ ) on each of the inputs prevent cross-talk
between the sensor signals.
PMT DETECTOR OUTPUT: This signal, output by the PMT preamp PCA, is used in the computation of the NO,
NO
2
and NO
x
concentrations displayed at the top right hand corner of the front panel display and output through
the instruments analog outputs and
COMM ports.
PMT HIGH VOLTAGE POWER SUPPLY LEVEL: This input is based on the drive voltage output by the PMT
pram board to the PMT’s high voltage power supply (HVPS). It is digitized and sent to the CPU where it is used
to calculate the voltage setting of the HVPS and stored in the instruments memory as the test function
HVPS.
HVPS
is viewable as a test function (see Section 6.2.1) through the analyzer’s front panel.
PMT TEMPERATURE: This signal is the output of the thermistor attached to the PMT cold block amplified by the
PMT temperature feedback circuit on the PMT preamp board. It is digitized and sent to the CPU where it is used
to calculate the current temperature of the PMT.
This measurement is stored in the analyzer. Memory as the test function
PMT TEMP and is viewable as a test
function (see Section 6.2.1) through the analyzer’s front panel.
04521C (DCN5731)