AN897 Datasheet, PDF (1/16 Pages) Microchip Technology – Thermistor Temperature Sensing with MCP6SX2 PGAs

AN897
Thermistor Temperature Sensing with MCP6SX2 PGAs
Author: Kumen Blake and Steven Bible
Microchip Technology Inc.
INTRODUCTION
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.
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.
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.
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
T_TH, while the rest of the circuit is at ambient
temperature T_A.
1M^1000000
100k¹⁰⁰⁰⁰⁰
BC Components^®
# 2322 640 55103
10 kΩ @ +25˚C
10k¹⁰⁰⁰⁰
1k¹⁰⁰⁰
100100
-50
-25 0 25 50 75 100 125 150
Thermistor Temperature (˚C)
FIGURE 1:
Thermistor Response.
Key specifications include [1, 2]:
• Resistance at +25°C: 10 kΩ ± 1%
• B_25/85 tolerance: ±0.75%
• Operating temperature range: -40°C to +125°C
(to +150°C for short periods)
• Maximum power
- 100 mW, T_TH = 0°C to +55°C
- 100% de-rated at T_TH = -40°C and +85°C
• Thermal dissipation factor: 2.2 mW/°C
• Response time: 1.7 s (in oil)
2.0
1.8 ^{BC Components®}
1.6 ^{# 2322 640 55103}
1.4 ^{10 kΩ @ +25°C}
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-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.
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”).
The resistor R_A makes the voltage vs. temperature
response reasonably linear. R_B and C_B reduce the
noise and act as an anti-aliasing filter for the ADC. The
MCP6SX2 PGA (MCP6S22 [5] or MCP6S92 [6])
buffers the voltage V_DIV. The PGA can be digitally
controlled to change its gain or channel (input).
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 V_OUT to the digital domain. It can fur-
ther process V_OUT (e.g., averaging) and convert it to
temperature. It communicates with the PGA via the
SPI™ serial bus.
 2004 Microchip Technology Inc.
DS00897B-page 1