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AN929

更新时间： 2024-09-17 03:18:51

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美国微芯 - MICROCHIP		/
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Temperature Measurement Circuits for Embedded Applications

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AN929 数据手册

AN929

Temperature Measurement Circuits for

Embedded Applications

• Local sensors are located on the same PCB as

the microcontroller

Author:

Jim Lepkowski

Microchip Technology Inc.

• Precision is the ability to measure a small

temperature gradient and determines the

interchangeability of the sensor

INTRODUCTION

• Remote sensors are located at a distance from

the microcontroller’s PCB

This application note shows how to select a tempera-

ture sensor and conditioning circuit to maximize the

measurement accuracy and simplify the interface to the

microcontroller. Practical circuits and interface

techniques will be provided for embedded applications

with thermocouples, Resistive Temperature Detectors

(RTDs), thermistors and silicon integrated circuit

temperature sensors. The attributes of each tempera-

ture sensor and the advantages of analog, frequency,

ramp rate, duty cycle, serial and logic output solutions

will be discussed. An analog output thermocouple

circuit will be compared with a frequency output RTD

oscillator circuit, along with design examples using

serial and analog output silicon Integrated Circuit (IC)

sensors. In addition, a Programmable Gain Amplifier

(PGA) circuit will be shown that can increase the

effectiveness of the Analog-to-Digital Converter (ADC)

bit resolution of a non-linear thermistor sensor.

• Repeatability is the sensor’s ability to reproduce

previously measured values

• Stability is defined as the long-term drift of the

sensor over a period of time

SENSOR-TO-MICROCONTROLLER

INTERFACE OPTIONS

The sensor interface option is selected by evaluating

the complexity of the sensor circuitry, in addition to the

required hardware and software trade-offs of the micro-

controller. The available sensor interface options that

are proportional to temperature include:

• Analog

• Frequency

• Ramp Rate

• Duty Cycle

• Serial Output

• Logic Output

DEFINITIONS

The following terms are used in this application note:

• Accuracy is the difference between the true and

measured temperature

Analog Output

• Common Mode Rejection Ratio (CMRR) is

defined as the ability of the amplifier to reject a

signal which is common to both inputs

Thermocouples, RTDs and thermistors can be inter-

faced to an amplifier circuit to provide a voltage which

is proportional to temperature. Analog output silicon

sensors are available that integrate the sensor and the

signal conditioning circuit. Figure 1 shows a block

diagram of a typical analog interface circuit.

• Input Offset Voltage (V_OS) is the voltage that must

be applied to an amplifier to produce a zero volt

output

Anti-

Aliasing

Filter

EMI / ESD

Filter*

Temperature

Sensor

PICmicro®

MCU

Amplifier

ADC

*Required only for remote sensors

FIGURE 1: Block Diagram of an Analog Output Sensor.

 2004 Microchip Technology Inc.

DS00929A-page 1

12 3 4 5 6 7...22

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