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AN929 Datasheet(PDF) 1 Page - Microchip Technology |
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AN929 Datasheet(HTML) 1 Page - Microchip Technology |
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1 / 22 page  2004 Microchip Technology Inc. DS00929A-page 1 AN929 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 (VOS) 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. FIGURE 1: Block Diagram of an Analog Output Sensor. Author: Jim Lepkowski Microchip Technology Inc. Temperature Sensor EMI / ESD Filter* Amplifier Anti- ADC Aliasing Filter PICmicro® MCU *Required only for remote sensors Temperature Measurement Circuits for Embedded Applications |
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