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AN897 Datasheet(PDF) 1 Page - Microchip Technology |
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AN897 Datasheet(HTML) 1 Page - Microchip Technology |
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1 / 16 page  2004 Microchip Technology Inc. DS00897B-page 1 AN897 INTRODUCTION This application note shows two designs that use a precise, negative temperature coefficient (NTC) thermistor for temperature measurement. The thermistor is placed in a resistive divider to linearize the temperature-to-voltage conversion. The voltage is processed in the analog domain by the MCP6SX2 (MCP6S22 or MCP6S92) Programmable Gain Amplifier (PGA) before conversion to the digital domain. The first design is simpler and has a smaller tempera- ture range. The second design changes the PGA’s gain to achieve a greater temperature range. Both designs use a piece-wise linear interpolation table to correct the remaining non-linearity and convert voltage into degrees Celsius. The design trade-offs between these approaches will be discussed. These circuits take advantage of the MCP6SX2’s input multiplexer (MUX). The PGA is used to process multiple signals and/or temperatures and digitally sets the most appropriate gain for each input. This reduces overall design complexity and allows for temperature correction of other sensors. THERMISTOR The thermistor used in the application note is part number 2322 640 55103 from BC Components®; see Figure 1 and Figure 2. This part is selected for its accuracy and cost. The thermistor’s temperature is TTH, while the rest of the circuit is at ambient temperature TA. FIGURE 1: Thermistor Response. Key specifications include [1, 2]: • Resistance at +25°C: 10 k Ω ± 1% •B25/85 tolerance: ±0.75% • Operating temperature range: -40°C to +125°C (to +150°C for short periods) • Maximum power - 100 mW, TTH = 0°C to +55°C - 100% de-rated at TTH = -40°C and +85°C • Thermal dissipation factor: 2.2 mW/°C • Response time: 1.7 s (in oil) FIGURE 2: Thermistor Accuracy. Thermistors with different price and accuracy trade-offs may also be used in this application. It is simple to modify the circuits to match the desired accuracy. CIRCUIT The circuit shown in Figure 3 is used for both designs described later. It is implemented on the Thermistor PGA PICtail™ Daughter Board (see Appendix A.1 “Thermistor PGA PICtail Daughter Board”). The resistor RA makes the voltage vs. temperature response reasonably linear. RB and CB reduce the noise and act as an anti-aliasing filter for the ADC. The MCP6SX2 PGA (MCP6S22 [5] or MCP6S92 [6]) buffers the voltage VDIV. The PGA can be digitally controlled to change its gain or channel (input). The PIC16F684 [8] is on the Signal Analysis PICtail™ Daughter Board (see Appendix A.2.5 “Signal Analy- sis PICtail Daughter Board”). It has an internal 10-bit ADC that converts VOUT to the digital domain. It can fur- ther process VOUT (e.g., averaging) and convert it to temperature. It communicates with the PGA via the SPI™ serial bus. Author: Kumen Blake and Steven Bible Microchip Technology Inc. 100 1000 10000 100000 1000000 -50 -25 0 25 50 75 100 125 150 Thermistor Temperature (˚C) 100 1k 10k 100k 1M BC Components ® # 2322 640 55103 10 k Ω @ +25˚C 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -50 -25 0 25 50 75 100 125 150 Thermistor Temperature (°C) BC Components ® # 2322 640 55103 10 k Ω @ +25°C Thermistor Temperature Sensing with MCP6SX2 PGAs |
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