AN2623 Freescale Semiconductor / Motorola, AN2623 Datasheet - Page 2

• Manufacturer Part Number: AN2623
• Description: LIN Temperature Sensor Using the MC68HC908QY/QY MCU
• Manufacturer: Freescale Semiconductor / Motorola

AN2623/D
Temperature Sensor
A/D Conversion
Power Supply
Dependency
2
V
N
ref
ref
[V]
[-]
LIN Temperature Sensor Using the MC68HC908QT/QY MCU
When MCUs are used without having the analog-to-digital (A/D) converter
reference-voltage bonded out, the main challenge with measuring temperature
is calibrating the temperature-to-digital conversion. Many temperature sensors
have 1°C or less accuracy (which is sufficient for such car applications), but the
power supply voltage dependency of A/D conversion on the MCU affects the
temperature-to-digital conversion. This measurement error can range ±11%
from the nominal value.
To show how the challenge of the A/D conversion power supply dependency
can be resolved, a self-calibration procedure has been implemented in this
application. As an example, the Maxim
used. This sensor provides a voltage reference, which is almost independent
of the power supply voltage. Because the value of this reference is known, the
voltage output can be measured by the second A/D channel and calibrate the
temperature value using the following equation:
where N
temperature value (measured in the first A/D channel); N
to the voltage reference (TEMP_REFERENCE symbolic constant used in the
source code, e.g., 0xD1 for MAX6611); and N
reference measured in the second A/D channel.
Examples of N
provided by the temperature sensor):
If V
supply voltage, the appropriate formula to determine the N
Maxim
National Semiconductor
Table 1. Temperature Sensor Reference Output
Freescale Semiconductor, Inc.
temp
MAX6610
For More Information On This Product,
®
Maxim
2.560
0x83
is a registered trademark of Maxim Integrated Products, Inc.
is the voltage corresponding to the temperature, and V
cor
is a corrected temperature value; N
Go to: www.freescale.com
ref
values are shown in
®
is a registered trademark of National Semiconductor Corporation.
Temperature Sensor Type
N
N
MAX6611
unc =
Maxim
cor =
4.096
0xD1
V
Table 1
®
(N
temp
MAX6611 temperature sensor will be
V
unc
N
DD
mref
× 0xFF
× N
mref
(V
unc
ref
ref
is the sensor voltage
)
is an uncorrected
is the voltage reference
National Semiconductor
ref
LM66, LM56
unc
value corresponds
1.250
0x40
value is:
DD
is the power
MOTOROLA
(EQ 1)
(EQ 2)
®