Teledyne 3000TA Operating Instructions Manual page 21

Trace Oxygen Analyzer
In the presence of oxygen the cell generates a current. A current to
voltage amplifier converts this current to a voltage, which is amplified in
the second stage amplifier.
The second stage amplifier also supplies temperature compensation
for the oxygen sensor output. This amplifier circuit incorporates a ther-
mistor, which is physically located in the cell block. The thermistor is a
temperature dependent resistance that changes the gain of the amplifier in
proportion to the temperature changes in the block. This change is in-
versely proportional to the change in the cell output due to the same tem-
perature changes. The result is a signal that is temperature independent.
The output from the second stage amplifier is sent to an 18 bit analog to
digital converter controlled by the microprocessor.
The digital concentration signal along with input from the control
panel is processed by the microprocessor, and appropriate control signals
are directed to the display, alarms and communications port. The same
digital information is also sent to a 12 bit digital to analog converter that
produces the 4-20 mA dc and the 0-1 V dc analog concentration signal
outputs, and the analog range ID outputs.
Signals from the power supply are also monitored, and through the
microprocessor, the system failure alarm is activated if a malfunction is
detected.
Teledyne Analytical Instruments
Operational Theory 2
2-9