Teledyne API T703/T703U Calibrator Operation Manual Principles of Operation
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MOTHERBOARD 9.2.4.
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.
A to D Conversion 9.2.4.1.
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
this calibrator 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.
Sensor Inputs 9.2.4.2.
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.
Thermistor Interface 9.2.4.3.
This circuit provides excitation, termination and signal selection for several
negative-coefficient, thermistors (temperature sensors) located inside the
calibrator.
Analog Outputs 9.2.4.4.
The 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.
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