Teledyne 238 - Thermal conductivity analyzer - Manual (Page 30)

TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-12
4 Linearizer Model 235
The negative supply voltage is -15 Volts. A similar expression for the
output is V
out
= -4(V
in
- V
cutoff
).
Notice that V
in
is actually 2.5 times the voltage at the linearizer input,
due to the gain of AlA. This gain acts to minimize the effect of offset errors.
The cutoff voltage (V
cutoff
) is set by the choice of R6; the “breakpoint”
referenced to the input is approximately V
cutoff
/2.5. Thus:
R6 = 15xR7x 12.5 x V
bkpt
or (299.4/ V
bkpt
) K ohms
Note: Each amplifier amplifies everything above its cutoff voltage, and not
just a segment between two cutoff voltages.
The output of each amplifier other than A1D is brought to the slider of
a trimpot. One end of the pot goes through a resistor (e.g., R28) to the
summing node of the output amp, A3B. The other end of the pot goes
through another resistor (e.g. R29) to the summing node of the inverter
A3C.
The output from the inverter is then also brought into the summing
node of the output amp A3B. Clearly the position of the slider on pot P2
will determine how much, signal goes directly into the summing input of
A3B, and now much goes through the inverter. If the slider is up at the top,
almost all of the output of AlC will add to the output of A1D. If the slider is
down at the bottom, then the output of A1C will be subtracted from that of
AlD. If the slider is in the middle, the output will be added and subtracted in
the same amount and thus will have no effect.
So, we see that the gain of the first section of the curve from 0 to the
first breakpoint is set at some value, (A), with P1. The gain of the second
section of the curve is the value (A) plus a value (B), which is set by P2.
The gain of the third breakpoint then, would be the sum, (A+B), plus a third
value, (C), set by P3. In other words, each pot affects the gain of all the
sections above where it starts working.
The gain of A3C is kept low by the small value of R43 (2K). This is to
stop it from saturating if it gets too much input from all the amplifiers. The
resistor that sums its output into A3B, (R44), is selected at 2K to compen-
sate for this.
A3B has a gain of about 0.3 to compensate for the gain of AlA and to
reduce zero errors. It also sums all the positive contributions via R44 and
A3C. Finally, it provides a low impedance output for the circuit.