Teledyne 2750 - Portable turbine generator gas analzyer - Manual (Page 17)

Turbine Generator Gas Analyzer Theory of Operation
Teledyne Analytical Instruments 17
2.2 Electronics
The Model 2750 uses a sophisticated miniature solid-state thermal
conductivity sensor. This cell uses internal temperature compensation
circuitry. It has both a reference and a measurement leg. The reference
leg is driven by a constant current source and is used to heat the sensor’s
substrate and to thermally compensate for ambient fluctuations. The
sensor membrane then operates at a constant temperature around 70°C.
Refer to Figure 3-1 for the block diagram of the electronics used in
the Model 2750.
Figure 3-1: Electronics Block Diagram
Current generated in the measuring leg of the sensor is sent to a
buffer amplifier. The output voltage from the amplifier then goes to 3
individual zero offset gain amplifiers, one for each analysis range. Each
offset amplifier has its own span attenuator. Both the zero offset
amplifiers and the span attenuators are used in setting the zero and span
for each range. The three output
s from the offset amplifiers are delivered
to the PIC microcontroller where the signals are digitized and then
linearized. The linearization process uses separate linearization tables
for each instrument range. Each table includes a 10 point correction
curve which was pre-programmed in at the factory to provide linearity
across the specific range. The output from the microcontroller is a pulse