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AN897

更新时间： 2024-11-13 03:18:51

品牌	Logo	应用领域
美国微芯 - MICROCHIP		温度传感
页数	文件大小	规格书
16页	430K
描述
Thermistor Temperature Sensing with MCP6SX2 PGAs

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AN897 数据手册

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%

• B_25/85tolerance: ±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, 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

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

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

T_TH, while the rest of the circuit is at ambient

temperature T_A.

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

BC Components^®

# 2322 640 55103

10 kΩ @ +25˚C

The resistor R_Amakes the voltage vs. temperature

response reasonably linear. R_Band C_Breduce 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).

100000

100k

10000

10k

1000

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_OUTto 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.

100

-50 -25

75 100 125 150

Thermistor Temperature (˚C)

FIGURE 1:

Thermistor Response.

 2004 Microchip Technology Inc.

DS00897B-page 1

12 3 4 5 6 7...16

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