Teledyne 7120 - NDIR analyzer - Manual (Page 29)

3 Start-up and Theory of Operation Model 7120
3-6
Teledyne Analytical Instruments
ALC amplifier.
The output of the ALC is the video signal superimposed on a DC level. This
signal is clamped to ground level by means of the zero restorer, U4A and
U4B. The reference ID pulse and measurement ID pulse are used to separate
their respective signals by FET switches Q2 and Q3, and the measurement
peak is inverted by U4D.
The measurement pulse is fed to the auto-zero circuit which uses a digital
successive-approximation technique to equalize the measurement and
reference peaks while the cell contains zero gas. The auto-zero circuit holds
its gain during the measurement period. As a result, any imbalance in the two
peaks is corrected, and the analyzer is effectively zeroed.
The output of the auto zero circuit, i.e., the inverted measurement peak and
the reference peak, is fed to a summing amplifier, U6A. The summing action
adds the two waveforms to produce a single waveform with excursions
above the zero level (from the reference peak), and below it (from the
measurement peak). The av
erage then corresponds to the difference between the peaks, and, properly
filtered, becomes the output signal. The summed signal is fed to the low pass
filter U6B, whose output is a DC signal fed to an amplifier, U6C, which
produces the preamplifier output.
3.5 Digital Signal Processing & Electronics
The Model 7120 uses an 8031 microcontroller (Central Processing
Unit—CPU) with 32 kB of RAM and 128 kB of ROM to control all signal
processing, input/output, and display functions for the analyzer. System
power is supplied from a universal power supply module designed (C65507)
to be compatible with any international power source. (See Major Internal
Components in chapter Maintenance for the location of the power supply
and the main electronic PC boards.)
The Temperature Control board (C69535A) is set to a single voltage
(110 or 220 VAC) and set the temperature of the sampling system at 45 o C.
The signal processing electronics including the microprocessor, analog to
digital, and digital to analog converters are located on the Motherboard
(C67435B) on side of the case.
The Auto-Zero amplifier PCB board is mounted on top of the
Motherboard. These boards are accessible by sliding the system out of the
case.
The Temperature Control Board keeps the temperature of the measur-
ing cell regulated to within 0.1 degree C. A thermistor is used to measure the