Teledyne 238 - Thermal conductivity analyzer - Manual (Page 19)

4-1 TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
Linearizer
4.1 Theory of Operation
The need for an electronics linearizer circuit arises in those applications
where the output of an instrument is not linearly related to the parameter the
instrument tries to measure. Often, this is the concentration of a chemical of
interest, color values, absorbance, or transmittance. When the calibration
curve, which is a plot of concentration versus instrument signal output, is
not a straight line, the linearizer can correct the curve and make it approach
a straight line. The linearizer does this by dividing the curve into eight
sections. Each section is amplified and added to the previously corrected
section.
Each section has a “breakpoint”, which connects it to the next section
away from zero; zero is the starting point of the curve. (Refer to Figures
4-1, 4-2, and 4-3.) The error left after linearization is due to the curvature of
each individual section. This error can be made quite small by correct selec-
tion of the breakpoints. The output of the linearizer is 0-1 Volt. See Figure
4-4 to visualize the linearization process.
Figure 4-5 shows how the linearizer works in actuality. It is exagger-
ated for clarity. For segment 1, the output will be some number (or fraction)
times the input voltage:
V
out
= 0.8 x V
in
.
where V
out
is the output voltage and V
in
is the input voltage.
Here, for this example,, the gain of the circuit is 0.8 for an input
voltage between 0 and 0.125 Volts.
When the input voltage exceeds 0.125 Volts, the second amplifier, as
well as the first, is working; it is adding or subtracting its output in propor-
tion according to the setting of trimpot P2. In this case, its output is added
to the output of the first amp. The total gain (the slope of the line segment)
for the combined segment is now about 1.9.