Datasheet

Chapter 9 Analog-to-Digital Converter (S08ADC10V1)
MC9S08SG32 Data Sheet, Rev. 8
Freescale Semiconductor 125
— By using the calculated value of V
DD
, convert the digital value of AD26 into a voltage, V
TEMP
Equation 9-1 provides an approximate transfer function of the temperature sensor.
Temp = 25 - ((V
TEMP
-V
TEMP25
) ÷ m) Eqn. 9-1
where:
—V
TEMP
is the voltage of the temperature sensor channel at the ambient temperature.
—V
TEMP25
is the voltage of the temperature sensor channel at 25°C.
— m is the hot or cold voltage versus temperature slope in V/°C.
For temperature calculations, use the V
TEMP25
and m values from the ADC Electricals table.
In application code, the user reads the temperature sensor channel, calculates V
TEMP
, and compares to
V
TEMP25
. If V
TEMP
is greater than V
TEMP25
the cold slope value is applied in Equation 9-1. If V
TEMP
is
less than V
TEMP25
the hot slope value is applied in Equation 9-1. To improve accuracy the user should
calibrate the bandgap voltage reference and temperature sensor.
Calibrating at 25°C will improve accuracy to
± 4.5°C.
Calibration at three points, -40°C, 25°C, and 125°C will improve accuracy to ± 2.5°C. Once calibration
has been completed, the user will need to calculate the slope for both hot and cold. In application code, the
user would then calculate the temperature using Equation 9-1 as detailed above and then determine if the
temperature is above or below 25°C. Once determined if the temperature is above or below 25°C, the user
can recalculate the temperature using the hot or cold slope value obtained during calibration.