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
