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 2004 Microchip Technology Inc. DS00895A-page 1

AN895

INTRODUCTION

This application note shows how to design a

temperature sensor oscillator circuit using Microchip’s

low-cost MCP6001 operational amplifier (op amp) and

the MCP6541 comparator. Oscillator circuits can be

used to provide an accurate temperature measurement

with a Resistive Temperature Detector (RTD) sensor.

Oscillators provide a frequency output that is propor-

tional to temperature and are easily integrated into a

microcontroller system.

RC oscillators offer several advantages in precision

sensing applications. Oscillators do not require an

Analog-to-Digital Converter (ADC). The accuracy of the

frequency measurement is directly related to the quality

of the microcontroller’s clock signal and high-frequency

oscillators are available with accuracies of better than

10 ppm.

RTDs serve as the standard for precision temperature

measurements because of their excellent repeatability

and stability characteristics. A RTD can be character-

ized over it’s temperature measurement range to

obtain a table of coefficients that can be added to the

measured temperature in order to obtain an accuracy

better than 0.05°C. In addition, RTDs have a very fast

thermal response time.

Two oscillator circuits are shown in Figures 1 and 2 that

can be used with RTDs. The circuit shown in Figure 1

is a state variable RC oscillator that provides an output

frequency that is proportional to the square root of the

product of two temperature-sensing resistors. The

circuit shown in Figure 2, which is referred to as an

astable multi-vibrator or relaxation oscillator, provides a

square wave output with a single comparator. The state

variable oscillator is a good circuit for precision

applications, while the relaxation oscillator is a good

alternative for cost-sensitive applications.

FIGURE 1: State Variable Oscillator.

FIGURE 2: Relaxation Oscillator.

Author: Ezana Haile and Jim Lepkowski

Microchip Technology Inc.

Attributes:

• Precision dual Element RTD

Sensor Circuit

• Reliable Oscillation Startup

• Freq. ∝ (R1 x R2)1/2

VDD

VDD/2

R1 = RTDA

VDD/2

R2 = RTDBR4

VDD/2

VOUT

A2A3A5

VDD

Attributes:

• Low Cost Solution

• Single Comparator Circuit

• Square Wave Output

• Freq. = 1/ (1.386 x R1 x C1)

OUT

C1VDD

1 = RTD

VDD

Oscillator Circuits For RTD Temperature Sensors

AN895

DS00895A-page 2  2004 Microchip Technology Inc.

WHY USE A RTD?

RTDs are based on the principle that the resistance of

a metal changes with temperature. RTDs are available

in two basic designs: wire wound and thin film. Wire

wound RTDs are built by winding the sensing wire

around a core to form a coil, while thin film RTDs are

manufactured by depositing a very thin layer of

platinum on a ceramic substrate.

Table 1 provides a comparison of the attributes of

RTDs, thermocouples, thermistors and silicon IC

sensors. RTDs are the standard sensor chosen for

precision sensing applications because of their

excellent repeatability and stability characteristics.

Also, RTDs can be calibrated to an accuracy that is

only limited by the accuracy of the reference

temperature.

TABLE 1: ATTRIBUTES OF RTDS, THERMOCOUPLES, THERMISTORS AND SILICON IC

SENSORS

WHY USE AN OSCILLATOR?

There are several different circuit methods available to

accurately measure the resistance of a RTD sensor.

Figure 3 provides simplified block diagrams of three

common RTD-sensing circuits. A constant current,

voltage divider or oscillator circuit can be used to

provide an accurate temperature measurement.

The constant current circuit uses a current source to

create a voltage that is sensed with an ADC. A constant

current circuit offers the advantage that the accuracy of

the amplifier is not affected by the resistance of the

wires that connect to the sensor. This circuit is

especially useful with a small resistance sensor, such

as an RTD with a nominal resistance of 100Ω, where

the resistance of the sensor leads can be significant in

proportion to the sensor’s resistance. In remote

sensing applications, the sensor is connected to the

circuit via a long wire and multiple connectors. Thus,

the connection resistance can be significant. The

resistance of 18 gauge copper wire is 6.5 mΩ/ft. at

25°C. Therefore, the wire resistance can typically be

neglected in most applications.

The constant current approach is often used in

laboratory-grade precision equipment with a 4-lead

RTD. The 4-lead RTD circuits can be used to provide a

Kelvin resistance measurement that nulls out the

resistance of the sensor leads. Kelvin circuits are

relatively complex and are typically used in only very

precise applications that require a measurement

accuracy of better than 0.1°C.

Another advantage of the constant current approach is

that the voltage output is linear. While linearity is

important in analog systems, it is not usually a critical

parameter in a digital system. A table look-up method

that provides linear interpolation of temperature steps

of 5°C is adequate for most applications and can be

easily implemented with a microcontroller.

The voltage divider circuit uses a constant voltage to

create a voltage that is proportional to the RTD’s

resistance. This method is simple to implement and

also offers the advantage that precision IC voltage

references are readily available. The main

disadvantage of both the voltage divider and constant

current approach is that an ADC is required. The

Attribute RTD Thermocouple Thermistor Silicon IC

Temperature Range -200 to 850°C -184 to 1260°C -55 to +150 C -55 to +125° °C

Temperature (t)

Accuracy

Class B = ±[0.012 +

(0.0019t) -6x10 -7t2]

Greater of ± °2.2 C

or ±0.75%

Various,

± °0.5 to 5 C

Various,

± °0.5 to 3 C

Output Signal ≈ 0.00385 Ω/Ω/°C Voltage (40 µ/°∆C) ≈ 4% ∆R/ t for

0 t C°C ≤ ≤ 70°

Analog, Serial, Logic,

Duty Cycle

Linerarity Excellent Fair Poor Good

Precision Excellent Fair Poor Fair

Durability Good, Wire wound

prone to open-circuit

vibration failures

Good at lower temps.,

poor at high temps.,

open-circuit vibration

failures

Good, Power

Specification is

derated with

temperature

Excellent

Thermal Response

Time

Fast (function of

probe material)

Fast (function of

probe material)

Moderate Slow

Cost Wire wound - High,

Thin film - Moderate

Low Low Moderate

Package Options Many Many Many Limited, IC packages

Interface Issues Small ∆ ∆R/ t Cold junction com-

pensation, Small ∆V

Non-linear resistance Sensor is located on

PCB

 2004 Microchip Technology Inc. DS00895A-page 3

AN895

accuracy of the voltage-to-temperature conversion is

limited by the resolution of the ADC and the noise level

on the PCB.

Oscillators offer several advantages over the constant

current and voltage RTD sensing circuits. The main

advantage of the oscillator is that an ADC is not

required. Another key attribute of oscillators is that

these circuits can produce an accuracy and resolution

that is much better than an analog output voltage

circuit. The accuracy of the frequency-to-temperature

conversion is limited only by the accuracy of the

counter or microcontroller time processing unit’s high

frequency clock signal. High frequency clock signals

are available with an accuracy better than 10 ppm over

an operating temperature range of -40°C to +125°C. In

addition, the temperature sensitivity of the reference

clock signal can usually be compensated with a simple

calibration procedure.

Designers are often reluctant to use oscillators due to

their lack of familiarity with these circuits. A negative

feature with oscillators is that they can be difficult to

troubleshoot and may not oscillate under all conditions.

However, the state variable and relaxation oscillators

provide very robust start-up oscillation characteristics.

FIGURE 3: Common RTD Sensor Signal Conditioning Circuits.

VREF

RRTD

VOUT Amplifier Anti-Aliasing

Filter

RC Oscillator PICmicro®

Microcontroller

PICmicro®

Microcontroller

RRTD

VOUT Amplier Anti-Aliasing

Filter ADC PICmicro®

Microcontroller

Precision

Current

Source Attributes:

• Insensitive to resistance of

leads with Kelvin connection

• Temperature proportional to

resistance (Temp. ∝

RRTD)

• Constant current source

circuits typically require a VREF

and several op amps

Attributes:

• Most popular method

• Temperature proportional to

resistance (Temp. ∝ 1 / RRTD)

• Precision VREF ICs are readily

available

Attributes:

• Does not require ADC or VREF

• Excellent noise immunity

• Accuracy proportional to quality

of microcontroller clock

Clock

RC Oscillator

Voltage Divider Circuit

Constant Current Circuit

IREF

VOUT = IREF x RRTD

VOUT = [RRTD / (R +RRTD)] x VREF

freq.∝ RRTD

ADC

RRTD

Termékspecifikációk

Márka:	Microchip
Kategória:	nincs kategorizálva
Modell:	MCP6543

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