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Teledyne API T703 Calibrator Operation Manual Theory of Operation
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9.2.4. MOTHERBOARD
This is the largest electronic assembly in the calibrator 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.
9.2.4.1. A to D Conversion
Analog signals, such as the voltages received from the calibrator’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 converter 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 T703 it 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 also uses these factors for
subsequent conversions.
9.2.4.2. 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.
9.2.4.3. Thermistor Interface
This circuit provides excitation, termination and signal selection for several negative-coefficient, thermistors
(temperature sensors) located inside the calibrator.
9.2.4.4. Analog Outputs
The T703 calibrator comes equipped with one analog output. It can be set by the user to carry the current signal
level of any one of the parameters (see Table 7-4) and will output an analog VDC signal that rises and falls in
relationship with the value of the parameter.
07223B DCN6378