Microchip MCP6042 Manual de Usario

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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
C1
VDD
VDD/2
R1 = RTDA
C2
VDD/2
R2 = RTDBR4
VDD/2
R3
R8
VDD/2
R7
VDD/2
R5
R6
VOUT
C5
C4
A2A3A5
A4
A1
VDD
R3
Attributes:
• Low Cost Solution
• Single Comparator Circuit
• Square Wave Output
• Freq. = 1/ (1.386 x R1 x C1)
V
OUT
C1VDD
R
1 = RTD
R4
R2
VDD
A1
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 +
(0.0019t) -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
R
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
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

Especificaciones del producto

Marca: Microchip
Categoría: No categorizado
Modelo: MCP6042

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