AN897 Datasheet, PDF (2/16 Pages) Microchip Technology – Thermistor Temperature Sensing with MCP6SX2 PGAs

AN897
V_DD = 5.0V
R_A
R_B
1%
V_DIV ^{100 kΩ}
R_TH
10 kΩ
C_B
1 µF
V_DD
MCP6SX2
PGA
CH0
CH1
V_REF
V_OUT
V_DD
PIC16F684
V_IN
10-bit ^VREF
ADC
Firmware
Other Input
FIGURE 3:
Thermistor PGA Circuit.
The ADC’s voltage reference is powered from the
same voltage as the voltage divider, giving a ratiometric
circuit; errors in V_DD will be automatically corrected at
the ADC.
FIRST DESIGN
This design emphasizes simplicity and uses a standard
approach to designing the thermistor circuit. The
traditional op amp is replaced with a PGA so that it can
multiplex multiple inputs.
Analog Design
The first design keeps the PGA at a gain of +1 V/V for
design simplicity. The resistor R_A is set to its nominal
+25°C value (10.0 kΩ) for best performance at room
temperature; this is a very common design choice.
While this is a simpler design, its accuracy is relatively
low, as will be seen. Notice that Figure 4 shows a much
more linear response than Figure 1.
3
SPI™ Bus
The thermistor power dissipation causes a self-heating
temperature error. Calculating the thermistor’s power
dissipation across temperature, and then dividing by
the specified 2.2 mW/°C thermal dissipation factor,
gives the self-heating temperature error shown in
Figure 5. This is a small, consistent error. It is simple to
adjust for this error using the piece-wise linear
interpolation table in firmware.
0.30
Design # 1
0.25
G = +1
0.20
R_A = 10 k:
0.15
0.10
0.05
0.00
-50
-25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-50
Design # 1
G = +1
R_A = 10 k:
-25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 4:
Outputs.
Voltage Divider and PGA
Temperatures between +125°C and +150°C can be
included in the design for overtemperature indication
where accuracy is not as important.
FIGURE 5:
Error.
Thermistor Self-heating
Analog Error Analysis
Figure 6 displays the ADC’s temperature resolution,
while Figure 7 shows the expected worst-case analog
circuit errors. Both plots are based on these
assumptions:
• ADC’s DC Error ≤ ±3.5 LSb
• PGA’s gain error ≤ ±0.1% (G = +1)
• PGA’s input offset error ≤ ±1 mV (including PSRR
and temperature drift)
• Specified thermistor accuracy
This design achieves an ADC temperature resolution of
0.25°C over the -25°C to +73°C temperature range.
The analog circuit accuracy is better than 1.2°C over
the same range. Other temperature ranges will have
different resolutions and accuracies.
DS00897B-page 2
 2004 Microchip Technology Inc.