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Temperature Sensing System Design

Almost all processes, measurements, and control systems vary as a function of temperature. Today's engineers are required to design systems to measure temperature in order to accurately control other functions in their system. If the rate of change of temperature is important to a system designer, then more accurate temperature sensors with finer resolution are required. For example, cell phone designers can leverage higher accuracy temperature sensors in transmit voltage-controlled oscillator applications (TVCO). The improved temperature sensor results in fewer dropped calls, improves user call quality, and reduces unnecessary power drain from the battery. In another example, inkjet and bubble jet printers use higher accuracy print-head temperature sensing to improve ink flow characteristics. Needless to say, temperature sensor resolution requirements of better than 0.1°F are driving the system requirements of high performance electronic equipment in the market today.

There are many different types of temperature sensors used today for a very wide range of temperatures. This paper focuses on describing three temperature sensors used in systems for measurements ranging from about -50°C (or lower) to +125°C, and discusses the tradeoffs to each system. Most systems today desire a digital interface, so requirements for the total system will be analyzed. The three types of thermal sensing solutions addressed in this paper are ones using Thermistors, CMOS Analog Output Temperature Sensors, and CMOS Digital Temperature Sensors .

Thermistor-based Temperature Sensor System
One of the oldest, most popular temperature sensing devices is the thermistor. This device is extremely non-linear, dropping in resistance as temperature increases. Often thermistors are specified by what the resistance is at 25°C (ambient), frequently 10K Ohms. The non-linearity provides large changes at low temperatures, but small changes at high temperatures. This phenomenon requires a very accurate Analog to Digital Converter and other components in the system to make fine measurements over a large temperature range.

Figure 1 below shows how a Thermistor (RT can be used in a system to accurately measure with fine resolution. A 4.096 Volt reference with a 16-bit ADC provides a 62.5 milliVolt (mV) per Least Significant Bit (LSB). If a 5 volt supply is used, and a 100K pull up, with a 47K at 25°C Thermistor, then the 1°C change will correspond to a 2.9 mV change, and a 0.1°C change will result in a 290 mV change. However, typically, passive based thermistor accuracy variation can range from part to part as high as ±6%, requiring an additional calibration step to ensure accurate temperature measurements. The non-linear, non-calibrated thermistor may be inexpensive, however the rest of the system — a 16 bit ADC, 4.096V reference, non-volatile memory, and software overhead in addition to the microcontroller, coupled with the system level calibration ” add significantly to overall system cost.

Thermistor Solution
Figure 1. Thermistor Temperature Measurement System

CMOS Analog Output Temperature Sensor System

Another method to design a temperature measurement system is to use a linear output silicon temperature sensor such as an aTS50 or aTS10 as shown in Figure 2 below.

The Temperature System shown using a linear output temperature sensor with a +10mV/°C output slope and a 500 mV offset (at 0°C) allows the user to use a relaxed ADC because the output and subsequently the ADC conversion , will be linear. Decisions on bit resolution and the need for a separate voltage reference device will be up to the user, and can be quickly calculated based on the accuracy and resolution requirement. If a 3.0 volt supply is used with an 8 bit ADC then a 12mV LSB (1.2°C LSB) resolution can be achieved, hence a 10 bit ADC will provide a 3mV LSB (or 0.3°C LSB), and a 12 bit ADC will provide a 0.7mV LSB (or 0.07°C LSB). All these resolutions will be for the entire temperature range. If absolute accuracy is required for example better than 1°C, then a voltage reference and a single point calibration may still be required.

CMOS Analog Sensor
Figure 2. CMOS Analog Temperature Measurement System

CMOS Digital Temperature Sensor System
A digital temperature sensor is, in effect, an integrated thermal management system device (see Figure 3 ) digitally converting direct temperature measurements within the chip itself. The digital device includes the analog temp sensor, voltage reference, the ADC, user-programmable alarms, and a serial port. Most digital temperature sensors include a 9-bit ADC. Newly marketed digital temp sensors achieve ±1°C accuracy and offer programmable 9- to 12-bit resolution. With 12 bit programmable temperature sensors the system can achieve a resolution of 0.0625°C LSB, which is pretty close to the frequently desired 0.1°F resolution.

CMOS Analog Sensor
Figure 3. CMOS Digital Temperature Measurement System

Summary
There are multiple ways to design an effective system to sense temperature, with varying levels of complexity and cost. Considering the system impact, system designers must think through their resolution needs before implementation. An easy way to quickly measure temperature, with very good resolution is to use a digital temperature sensor as described in the Figure 3. Its simple interface and direct temperature-to-digital-conversion functionality provide temperature sensing resolutions as fine as 0.0625°C — a good starting point for implementing the measuring and monitoring needs of your high performance systems.

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