Book Analog Interfacing to Embedded Microprocessors
Repeating the above cal-
culation for this resistance results in a minimum pullup resistance of 2725
ohms. The original 1210 ohm value would cause too much dissipation at high
temperatures.
Scaling
Sometimes it is necessary to shift an analog signal so that it is in the right
range for an A/D converter to use it. Figure 3.3 shows such a situation. Here
we have a thermistor that is interfaced to an 8-bit, 0-to-5v A/D converter, such
as that found on the Microchip 16C7x parts. We’ll use the same thermistor
we’ve been using. The formula for the voltage V1 is:
2.5 ¥ Rth
V1 =
Rth + R1
In Figure 3.3, R1 = 10K. Using this equation and the resistance/tempera-
ture table for the thermistor, we can calculate the value of V1 for the tem-
perature range we are interested in:
Temp °C Rth V1
annaabi.ee 
