Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 197)

Model 9110EH Instruction Manual Theory of Operation
for 30 seconds, the watchdog circuit will assume lost connection and automatically turn of
all valves and heaters and may cause the analyzer to reboot or the I 2 Z bus to hang up.
10.3.5. 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.
10.3.5.1. 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 6.7.8 for instructions on performing this calibration.
10.3.5.2. 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.
• The PMT detector output from the PMT preamplifier is used in the computation of the
NO, NO
X
and NO
2
concentrations displayed on the front panel display and reported
through the instrument’s analog outputs and COM ports.
• The PMT high voltage is based on the drive voltage from the preamplifier board. It is
digitized and sent to the CPU where it is used to calculate the voltage setting of the
HVPS and stored in the instrument’s memory as the test function HVPS.
• The PMT temperature is measured with a thermistor inside the PMT cold block. Its
signal is amplified by the PMT temperature feedback circuit on the preamplifier board
and 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 iDAS and displayed as
test function PMT TEMP.
M9110EH Rev 0 183