AN897 Datasheet, PDF (4/16 Pages) Microchip Technology – Thermistor Temperature Sensing with MCP6SX2 PGAs

AN897
Digital Error Analysis
Figure 9 shows the estimated interpolation error for the
interpolation table. This design suffers from poor ADC
resolution at temperature extremes. The accuracy of
this piece-wise linear interpolation table is 0.05°C over
the -25°C to +73°C temperature range. Over the -40°C
to +150°C temperature range, the accuracy degrades
to 1.0°C.
1.0
0.8 ^{Design # 1}
0.6
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 9:
Piece-wise Linear
Interpolation Error, Design # 1.
The digital roundoff error will be roughly proportional to
the ADC temperature resolution curve’s envelope (see
Figure 6). If the roundoff error is much less than the
ADC resolution, this error will have little impact.
The total digital error includes both the piece-wise
linear interpolation error and the round-off error.
SECOND DESIGN
This design emphasizes accuracy and resolution. It
uses the PGA’s capability to change gain to overcome
the limitations of the first design. The PGA can
multiplex multiple inputs if needed.
Analog Design
The second design changes the PGA’s gain from +1 to
+8 to +32 V/V. The resistor R_A is set to 28.0 kΩ so that
the voltage vs. temperature response is reasonably
linear at low temperatures; see Figure 10 (compare to
Figure 4). The response is nearly flat at higher temper-
atures, so the PGA’s gain will be increased to compen-
sate. Though this is a more complex design, its
resolution and accuracy are greater than the first
design’s.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-50
Design # 2
R_A = 28.0 k:
-25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 10:
Voltage Divider Output.
Temperatures between +125°C and +150°C can be
included in the design for overtemperature indication
when accuracy is not as important.
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 11. This is a small, consistent error. It is simple
to adjust for this error using the piece-wise linear
interpolation table in firmware.
0.12
Design # 2
0.10
R_A = 28.0 k:
0.08
0.06
0.04
0.02
0.00
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 11:
Error.
Thermistor Self-heating
PGA Gain
The sensitivity that V_DIV shows to temperature
(Figure 10) is poor at higher temperatures. It is inten-
tionally designed this way so that the PGA can be set
at higher gains as temperature increases (Figure 12).
DS00897B-page 4
 2004 Microchip Technology Inc.