Chapter 4 Temperature Controller Module Operation 55
4.5.1.3 Temperature Resolution
You must also consider measurement resolution since the resolution decreases as the
thermistor temperature increases. The 8000 uses an A/D converter that has a
maximum resolution of about 76 µV. The microprocessor converts this digital
number to resistance, stores this resistance, then converts it to a temperature using the
Steinhart-Hart equation, and stores this temperature. A temperature change of one
degree centigrade will be represented by a greater resistance increase (and therefore
more A/D counts) at a lower temperature than at a higher temperature because of the
non-linear resistance of the thermistor. Resolution figures for a typical 10 k Ohm
thermistor are given below.
Temperature
Voltage at 10 µA Resolution
-20 °C 56.0 mV/°C 0.018 °C/mV
25 °C 4.4 mV/°C 0.230 °C/mV
50 °C 1.4 mV/°C 0.700 °C/mV
For this thermistor, a temperature change from -20°C to -19°C will be represented by
737 A/D counts (if supplied with 10µA). The same thermistor will only change
about 18 A/D counts from 49°C to 50°C.
4.5.1.4 Selecting Thermistor Current
To select the current setting for a typical 10 K thermistor, determine the lowest
temperature you will need to sample and select the current according to the range
limits given above. If the temperature you want to sample is below -10°C you should
use the 10µA setting.
With the current set to 10µA the best resolution you will see will be a 1.0°C
temperature change. If, for example, the lower limit is 0°C you can choose either
setting, but there is a tradeoff in terms of resolution. If you need better than 0.1°C
measurement resolution you will have to change to 100µA.
If you need high resolution over a narrow range, for a very accurate measurement,
you can set the current setting for the maximum resolution. For example, at a high
temperature of 15°C, you require a measurement resolution of at least 0.05°C. This
resolution is within the range of either setting, but at the 10µA setting the resolution
is only 0.2°C while at the 100 µA setting the resolution is better than .05 °C.
Generally, it is best to use the 100µA setting for all measurements of -10°C or greater
with a 10 K thermistor.
4.5.1.5 Selecting Thermistors
The type of thermistor you choose will depend primarily on the operating
temperature range. These guidelines for selecting the range and resolution will apply
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