AN897 Datasheet | Microchip Technology

Thermistor Temperature Sensing with MCP6SX2 PGAs Thermistor Resistance (:) Thermistor Error Magnitude (°C) Author: Kumen Blake and Steven Bible Microchip Technology Inc. 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 temperature 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 plexity and allows for temperature correction of other sensors. THERMISTOR The thermistor used in the application note is part number 2322 640 55103 from BC ponents®; 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. 1M1000000 100k100000 BC ponents® # 2322 640 55103 10 k: @ +25°C 10k10000 1k1000 100100 -50 -25 0 25 50 75 100 125 150 Thermistor Temperature (°C) FIGURE 1: Thermistor Response. Key specifications include [1, 2]: - Resistance at +25°C: 10 kΩ ± 1% - B25/85 tolerance: ±0.75% - Operating temperature range: -40°C to +125°C (to +150°C for short periods) - Maximum power - 100 m W, TTH = 0°C to +55°C - 100% de-rated at TTH = -40°C and +85°C - Thermal dissipation factor: 2.2 m W/°C -...