Control Section
2-3
Isolated Input Section
The isolated input section consists of a 16-channel input multiplexer, a
CJC sensor input, calibration inputs, a programmable gain amplifier, and
a V/F (voltage-to-frequency) converter. During operation, the DAS-TC/B
constantly monitors and uses the CJC input to maintain the accuracy of
the board. At the same time, the board switches in a precision 9.9V input
to measure gain error at a gain of 1 and to measure offset errors for all
four gain ranges. The board then stores these measurements in onboard
memory.
The four ranges of the programmable gain amplifier are selectable to
match the thermocouple input ranges and to accommodate the voltage
input range of
−
2.5V to 10V (using a gain of 1).
The V/F converter provides excellent noise rejection and high resolution,
while generating a pulsed output whose frequency is proportional to the
voltage input. The pulsed output passes first through a stage of optical
isolation and then into a stage of counting, where a count of the square
waves over a specified period determines a value for the corresponding
voltage input. The period for each count of square waves is set by the
value entered in the configuration file for Normal Mode Rejection
Frequency (50, 60, or 400 samples/s); the actual period is one-half this
selected value. The longer the count time, the higher the resolution and
better the noise rejection.
Control Section
The control section consists of the microprocessor and its memory. The
microprocessor performs all control functions and mathematical calcula-
tions, precluding the need for any computer processing. Working from the
configuration file, the microprocessor sets up the board for the desired
configuration. Board parameters include interrupt level and Normal Mode
Rejection Frequency. Channel parameters include thermocouple type,
number of samples to average, and type of engineering units. During the
acquisition process, the microprocessor also handles scan order, thermo-
couple linearization, and calculations for CJC.
