AN897
Thermistor Temperature Sensing with MCP6SX2 PGAs
Key specifications include [1, 2]:
Author:
Kumen Blake and Steven Bible
Microchip Technology Inc.
• Resistance at +25°C: 10 kΩ ± 1%
• B25/85 tolerance: ±0.75%
INTRODUCTION
• Operating temperature range: -40°C to +125°C
(to +150°C for short periods)
This application note shows two designs that use a
precise, negative temperature coefficient (NTC)
thermistor for temperature measurement. The
thermistor is placed in a resistive divider to linearize the
temperature-to-voltage conversion. The voltage is
processed in the analog domain by the MCP6SX2
(MCP6S22 or MCP6S92) Programmable Gain Amplifier
(PGA) before conversion to the digital domain.
• Maximum power
- 100 mW, TTH = 0°C to +55°C
- 100% de-rated at TTH = -40°C and +85°C
• Thermal dissipation factor: 2.2 mW/°C
• Response time: 1.7 s (in oil)
2.0
The first design is simpler and has a smaller tempera-
ture range. The second design changes the PGA’s gain
to achieve a greater temperature range. Both designs
use a piece-wise linear interpolation table to correct the
remaining non-linearity and convert voltage into
degrees Celsius. The design trade-offs between these
approaches will be discussed.
BC Components®
1.8
# 2322 640 55103
1.6
10 kΩ @ +25°C
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
These circuits take advantage of the MCP6SX2’s input
multiplexer (MUX). The PGA is used to process
multiple signals and/or temperatures and digitally sets
the most appropriate gain for each input. This reduces
overall design complexity and allows for temperature
correction of other sensors.
-50 -25
0
25
50
75 100 125 150
Thermistor Temperature (°C)
FIGURE 2:
Thermistor Accuracy.
Thermistors with different price and accuracy trade-offs
may also be used in this application. It is simple to
modify the circuits to match the desired accuracy.
THERMISTOR
The thermistor used in the application note is part
number 2322 640 55103 from BC Components®; see
Figure 1 and Figure 2. This part is selected for its
accuracy and cost. The thermistor’s temperature is
TTH, while the rest of the circuit is at ambient
temperature TA.
CIRCUIT
The circuit shown in Figure 3 is used for both designs
described later. It is implemented on the Thermistor
PGA PICtail™ Daughter Board (see Appendix A.1
“Thermistor PGA PICtail Daughter Board”).
1000000
1M
BC Components®
# 2322 640 55103
10 kΩ @ +25˚C
The resistor RA makes the voltage vs. temperature
response reasonably linear. RB and CB reduce the
noise and act as an anti-aliasing filter for the ADC. The
MCP6SX2 PGA (MCP6S22 [5] or MCP6S92 [6])
buffers the voltage VDIV. The PGA can be digitally
controlled to change its gain or channel (input).
100000
100k
10000
10k
1000
1k
The PIC16F684 [8] is on the Signal Analysis PICtail™
Daughter Board (see Appendix A.2.5 “Signal Analy-
sis PICtail Daughter Board”). It has an internal 10-bit
ADC that converts VOUT to the digital domain. It can fur-
ther process VOUT (e.g., averaging) and convert it to
temperature. It communicates with the PGA via the
SPI™ serial bus.
100
100
-50 -25
0
25
50
75 100 125 150
Thermistor Temperature (˚C)
FIGURE 1:
Thermistor Response.
2004 Microchip Technology Inc.
DS00897B-page 1
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