AN897 Datasheet, PDF (8/16 Pages) Microchip Technology – Thermistor Temperature Sensing with MCP6SX2 PGAs

AN897
0.0
-0.5
-1.0
2^nd Design
1^st Design
-1.5
-2.0
-2.5
-50
10-bit ADC
DC Error ≤ 3.5 LSb
-25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 19:
Resolution.
ADC Temperature
Figure 20 compares the analog circuit errors between
the designs. The second design’s error is better at high
temperatures because the ADC’s temperature resolu-
tion is better. It is also better at low temperatures
because R_A has been selected to linearize the
temperature-to-voltage conversion there.
11
10
9
8
7
6
5
4
3
2
1
0
-50
Design # 1
Design # 2
-25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 20:
Comparison.
Analog Circuit Error
The digital piece-wise linear interpolation errors are
compared in Figure 21. The second design has much
better performance because the linear interpolation
table segments cover smaller changes in temperature.
1.0
0.8
Design # 1
0.6
Design # 2
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 21:
Comparison.
Digital Interpolation Error
DS00897B-page 8
Figure 22 compares the total errors (thermistor plus
circuit plus piece-wise linear interpolation) of the first
and second designs. The digital roundoff error has
been excluded for simplicity.
12
11
10
9
8
7
6
5
4
3
2
1
0
-50
1^st Design
2^nd Design
-25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 22:
Total Error Comparison.
Other trade-offs between the two designs are
summarized in Table 4.
TABLE 4: TRADE-OFFS.
Criteria
First Second
Design Design
Temperature Range
Temperature Accuracy
Discontinuity at gain changes
Firmware Size
Medium
Low
—
Low
High
High
±0.3°C
Medium
MEASURED RESULTS
Both designs were measured on the bench. The
thermistor was emulated with the variable resistor Rvar
on the board shown in Appendix A.1 “Thermistor
PGA PICtail Daughter Board”. The ADC outputs were
converted to estimated thermistor temperatures using
nominal resistor values. Figure 23 shows the first
design’s measured error, while Figure 24 shows the
second design’s measured error.
4
Design # 1
3 _{Thermistor Emulator, Rvar}
2
1
0
-1
-2
-3
-4
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 23:
Design # 1.
Measured Errors,
 2004 Microchip Technology Inc.