AN929 Datasheet, PDF (1/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
Temperature Measurement Circuits for
Embedded Applications
Author: Jim Lepkowski
Microchip Technology Inc.
INTRODUCTION
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.
DEFINITIONS
The following terms are used in this application note:
• Accuracy is the difference between the true and
measured temperature
• Common Mode Rejection Ratio (CMRR) is
defined as the ability of the amplifier to reject a
signal which is common to both inputs
• Input Offset Voltage (V_OS) is the voltage that must
be applied to an amplifier to produce a zero volt
output
• Local sensors are located on the same PCB as
the microcontroller
• Precision is the ability to measure a small
temperature gradient and determines the
interchangeability of the sensor
• Remote sensors are located at a distance from
the microcontroller’s PCB
• 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
Analog Output
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.
Temperature
Sensor
EMI / ESD
Filter*
Amplifier
Anti-
Aliasing
Filter
*Required only for remote sensors
FIGURE 1:
Block Diagram of an Analog Output Sensor.
 2004 Microchip Technology Inc.
ADC
PICmicro®
MCU
DS00929A-page 1