Teledyne T200, Operation Manual, Page: 307 / 425

Teledyne API
– T200 NO/NO
2
/NO
X
Analyzer Operation Manual Principles of Operation
305
13.3.3. MOTHERBOARD
This PCA 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.
13.3.3.1. 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, 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 T200
it is used in unipolar 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 5.9.3.10 for instructions on performing this calibration.
13.3.3.2. 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.
PMT DETECTOR OUTPUT: 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.
This information is available in several forms:

As a raw voltage signal via the test function
PMTDET
and the SIGNAL I/O function
PMT_SIGNAL
, or;

Normalized for temperature, pressure and auto-zero offset via the front panel test
function
NORM PMT
.

It is recorded by the DAS system in the following parameters:

PMTDET
– The same as the test function
PMT DET
.

RAWNOX
– The raw PMT output when the instrument is measuring NO
X
.

RAW NO
– The raw PMT output when the instrument is measuring NO.
06858E DCN7057