Teledyne 3000ZA - Trace oxygen analyzer - Manual (Page 23)

Trace Oxygen Analyzer Operational Theory
Teledyne Analytical Instruments 11
The signal processing electronics including the microprocessor,
analog to digital, and digital to analog converters are located on the
motherboard at the bottom of the case. The preamplifier board is
mounted on top of the motherboard as shown in the figure. These boards
are accessible after removing the back panel. Figure 2-4 is a block
diagram of the analyzer electronics.
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 output from the second stage amplifier is
sent to an 18 bit analog to digital converter controlled by the
microprocessor.
The high operating temperature of the ZrO
2
sensor is required for
proper operation of the sensor. The amplifier PCB contains a circuit that
drives the heater embedded in the sensor. The voltage across the heater
rises slowly to avoid damaging the heater/sensor assembly. After a few
minutes, the sensor will be at the proper operating temperature.
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 VDC 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.