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參數(shù)資料
| 型號(hào)： | AN897 |
| 廠商： | Microchip Technology Inc. |
| 元件分類： | 熱敏電阻 |
| 英文描述： | Thermistor Temperature Sensing with MCP6SX2 PGAs |
| 中文描述： | 熱敏電阻溫度傳感器與MCP6SX2 PGA的 |
| 文件頁(yè)數(shù)： | 1/16頁(yè) |
| 文件大?。?/td> | 356K |
| 代理商： | AN897 |
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2004 Microchip Technology Inc.
DS00897B-page 1
AN897
INTRODUCTION
This application note shows two designs that use a
precise, negative temperature coefficient (NTC)
thermistor
for
temperature
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
measurement.
The
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
.
FIGURE 1:
Thermistor Response.
Key specifications include [1, 2]:
Resistance at +25°C: 10 k
± 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)
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.
Author:
Kumen Blake and Steven Bible
Microchip Technology Inc.
100
1000
10000
100000
1000000
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (C)
T
)
100
1k
10k
100k
1M
BC Components
# 2322 640 55103
10 k
@ +25C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (°C)
T
(
BC Components
# 2322 640 55103
10 k
@ +25°C
Thermistor Temperature Sensing with MCP6SX2 PGAs
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