Page 174 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 167

0
-0.01 -100 -50 0 50 100 150
-0.02
-0.03
Alpha (/°C) -0.04

-0.05
-0.06
-0.07
-0.08
-0.09
Temperature (°C) (a)
1.0
0.9
0.8
0.7 1600 Ohm
Ratio 0.6 300
0.5
610
1600
0.4
300 Ohm
0.3
0.2
0.1
0.0
-100 -50 0 50 100 150
Temperature (°C) (b)
Figure 8.3 (a) At room temperature the tempco of many thermistors a is
about -3.5 percent/°C. (b) The most linear region of a resistor/thermistor
potential divider can be shifted though choice of the load resistor.

In use the thermistor can make up one arm of a voltage divider powered by
the signal. By choice of the division ratio the region of best linearity can be
shifted in temperature. Analog Devices “Transducer Interfacing Handbook”
(1981) suggests a value for the series resistor of 61 percent of the thermistor
nominal resistance for best linearity. Figure 8.3b plots the division ratio with
three values for this thermistor with the series resistor connected to ground.
In the 0 to 50°C range the 61 percent value does indeed look about right. It is
possible to further linearize this characteristic through a combination of series
and parallel resistances, at the cost of reduced temperature coefficient, or by
using two or more different thermistors in a resistor ladder network. Good lin-
earity makes compensation for the photodiode characteristic easier.
Tiny glass-encapsulated or polymer-coated thermistors are available which

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