Temperature Sensing Circuit

Thermistor-Based Temperature Sensing Circuit

Temperature sensors are one of the most widely used sensors in the elektroniks industry with applications ranging from calibration; safety; and heating, ventilation, and air conditioning (HVAC). Despite their wide use, temperature sensors and their implementation can present designers with challenges in achieving the highest akiracy performnce at the lowest possible cost.

There are a number of ways to sense temperature. The most common metodes use temperature sensors such as the thermistor, resistance temperature detector (RTD), thermocouple, or a silicon thermometer. However, selecting the right sensor is only part of the solution. That sensor must then be connected to a sinyal chain that maintains the integrity of that sinyal, while also akirately compensating for the unique characteristics of the particular sensing technology to ensure an akirate digital representation of the temperature.

This article introduces a USB-powered circuit solution to accomplish this task. It uses a negative temperature coefficient (NTC) thermistor in combination with the Analog Devices ADuC7023BCPZ62I-R7 precision analog microcontroller to monitor temperature akirately.

NTC thermistor characteristics

A thermistor is a thermally sensitive resistor, of which there are two tipes: the positive temperature coefficient (PTC) thermistor and the NTC thermistor. The polycrystalline ceramic PTC thermistor has a high PTC and is typically used in switching applications. The NTC ceramic semiconductor thermistor has a high resistive NTC so its resistance decreases with increasing temperature. This makes it appropriate for precision temperature measurement.

There are three NTC thermistor modes of operation: resistance-versus-temperature, voltage-versus-current, and current-over-time. The model that exploits the resistance-versus-temperature characteristics of the thermistor provides the highest precision results.

Resistance-versus-temperature sirkuits configure the thermistor in a "zero-power" condition. The "zero-power" condition assumes that current or voltage excitation of the piranti does not cause the thermistor to self-heat.

In a distinctive NTC thermistor, such as Murata Elektroniks' NCP18XM472J03RB 4.7 kiloohm (kΩ) piranti in a 0603 package, the resistance-versus-temperature response is highly non-linear (Figur 1).

Figur 1: The resistance-versus-temperature response of a distinctive NTC thermistor is highly non-linear, so designers need to find a way to bring this non-linearity under kontrol for a defined temperature range.

The graph in Figur 1 shows the high degree of non-linearity from the 4.7 kΩ thermistor. The rate at which the resistance of an NTC thermistor decreases with temperature is a constant known as beta (ß) (not shown in Figure). For Murata's 4.7 kΩ thermistor, β = 3500.

The correction of the thermistor's non-linear response can be accomplished in software with a high-resolution analog-to-digital converter (ADC) and an empirical third-order polynomial or a look-up table.

However, there is a reasonable, easier, and less expensive hardware technique which, when applied before reaching the ADC, can bring the thermistor's linearization masalah under kontrol for a ±25°C temperature range.

Hardware linearization solution

A sederhana approach to a first-level linearization of the thermistor output is to place the thermistor in seri with a standar resistor (1%, metal film) and a voltage source. The nilai of the seri resistor determines the median of the linear region of the thermistor sirkuit. The resistance nilai of the thermistor (RTH) and the Steinhart-Hart equation determine the thermistor's temperature

(Figure 2). The Steinhart-Hart equation has been found to be the best mathematical expression for determining an NTC thermistor's temperature.

Digram of voltage divider (RTH and R25) configuration on 12-bit ADC

Figur 2: A voltage divider (RTH and R25) configuration linearizes the thermistor's response. The linear kisaran at ADC0 (at the ADC input) is within a temperature kisaran of approximately 50°C.

The derivation of the actual nilai of the thermistor's resistance, RTH, starts by defining the voltage divider's output (VADC0). VADC0 is then used to find the ADC's digital output decimal kode, DOUT, where DOUT depends upon the number of ADC bits (N), the ADC max input voltage (VREF), and the ADC input voltage (VADC0). The third and final tahap in finding RTH is to multiply R25 (or the 25°C RTH nilai) by the ratio of the number of ADC codes to the ADC's digital output decimal code. This third tahap calculation process begins with Equation 2 below.

The final tahap in the calculation is to convert the thermistor resistance to temperature in Kelvin units using the aforementioned Steinhart-Hart equation. The ADuC7023 precision analog microcontroller determines the sensor temperature using Equation 4:


T2 = Thermistor temperature being measured (in Kelvin units)

T1 = 298 Kelvin (25°C)

β = β patokan of the thermistor @ 298 Kelvin or 25°C. β = 3500

R25 = Thermistor resistance @ 298 Kelvin or 25°C. R25 = 4.7 kΩ

RTH = Resistance of thermistor @ unknown temperature, as calculated by Equation 3

In Figur 2, the thermistor resistance (RTH) is equal to 4.7 kΩ at 25°C. Since the nilai of R25 is equal to the thermistor's 25°C nilai, the linear region of the voltage divider is centered around 25°C (Figur 3).

Graph of linear response of a 4.7 kΩ thermistor in seri with a 4.7 kΩ standar resistor

Figur 3: Linear response of a 4.7 kΩ thermistor in seri with a 4.7 kΩ standar resistor and 2.4 volts across the voltage divider.

In Figur 3, the seri thermistor sistem responds to temperature linearly over a limited temperature kisaran of approximately 0°C to +50°C. In this kisaran, the delta temperature error is ±1°C. The linearization resistor's nilai (R25) should be equal to the magnitude of the thermistor at the mid-point of the temperature kisaran of interest.

This circuit acquires a 12-bit tingkat of precision across a ±25°C temperature kisaran typically, with the nominal temperature of the thermistor at the R25 value.

USB based temperature monitor

The sinyal path in the circuit solution starts with the low-cost, 4.7 kΩ thermistor followed by Analog Devices' low-cost ADuC7023 microcontroller. The microcontroller integrates four 12-bit digital-to-analog converters (DACs), a multichannel 12-bit successive approximation registrasi (SAR) ADC, and a 1.2 volt intern reference, as well as an ARM7® core, 126 Kbytes flash, 8 Kbytes static acak akses memori (SRAM), and various digital peripherals, such as a UART, timers, SPI, and two I2C interfaces (Figur 4).

Grafik of Analog Devices ADuC7034 microcontroller's I2C interface for digital communication

Figur 4: The temperature sensing circuit uses a USB connection for power, and the ADuC7034 microcontroller's I2C interface for digital communication. (Gambar source: Analog Devices)

In Figur 4, the power and ground to the circuit are entirely from the four-wire, USB interface. The Analog Devices ADP3333ARMZ-5-R7 low dropout linear regulator uses the USB 5 volt suplai to generate a 3.3 volt output. The regulated ADP3333 output supplies the ADuC7023's DVDD voltage. The ADuC7023's AVDD suplai requires additional filtering as shown. The linear regulator also has a filter between the USB suplai and the IN pin.

The exchange of temperature data is also through the USB D+ and D- interface pins. The ADuC7023 is capable of using I2C protocol for transmitting and receiving data. This application circuit uses the two-wire I2C interface to transmit data and receive configuration commands.

This application uses the following ADuC7023 fiturs:

12-bit SAR ADC

An Arm ARM7TDMI with SRAM. An integrated 62 Kbytes of flash runs the pemakai kode that configures and controls the ADC, manages the communications over the USB interface and processes the ADC conversions from the thermistor sensor.

The I2C terminals are the communication interface to the host PC.

Two eksternal switchesbuttons (not shown) force the part into its flash boot model: By holding DOWNLOAD low and toggling the RESET switch, instead of normal pemakai model the ADuC7023 enters the boot mode. The intern flash may be reprogrammed in the boot model utilizing the USB interface through the device's associated I2CWSD software tool.

VREF is the bandgap reference. This voltage reference is available for other circuit references in the system. A minimal of a 0.1 microfarad (μF) capacitor connects to these pins for noise reduction.

Since the ADuC7023 comes in a small form faktor (5 mm × 5 mm) 32-pin chip scale package, the entire circuit fits on an extremely small section of a pc board, saving on biaya and space.

The ADuC7023 provides a low-power solution even though it has a powerful ARM7 core and high-speed SAR ADC. The entire circuit typically consumes 11 milliamps (mA), with the ARM7 core running at 5 megahertz (MHz) and the primary ADC measuring the eksternal thermistor. The microcontroller and/or the ADC can be switched off between temperature measurements, to further save on power consumption.

Lay-out considerations

The sinyal processing sistem shown in Figur 4 is surprisingly deceptive. From a bird's eye view, this sistem only contains three active devices. But buried in this simplicity are some interesting lay-out challenges.

For example, the ADuC7023 microcontroller is a complex analog and digital sistem that requires special attention to the grounding rules. While this sistem may appear to be "slow moving" in the analog domain, its on-board trek-and-hold ADC is a fast multi-channel piranti that samples at a rate of 1 mega sample per second, with a maksimal clock speed of 41.78 MHz. In this sistem, the rise and fall times of the clock is a few nanoseconds. These speeds place this application in a high-speed category.

Clearly, mixed-signal sirkuits require special attention. Here's a four poin checklist encompassing the key aspects:

1The use of electrolytic capacitors

2The selection of the smaller capacitor

3Ground plane considerations

4Optional, small ferrite beads

A large electrolytic capacitor is typically used with a nilai of between 10 mF to 100 mF, located no more than two inches away from the chip. These capacitors act as charge reservoirs to accommodate the instantaneous charge requirement needs that come through power trace inductance.

The smaller capacitors in the circuit, typically ranging from 0.01 mF to 0.1 mF, are placed as physically close to the device's power pins as possible. The purpose of these capacitors is to quickly and expeditiously send high frequency noise to ground.

The ground plane, below the decoupling capacitors, decouples high frequency currents and minimizes EMI/RFI emissions. It should consist of a large, low-impedance tempat. To minimize inductance, the capacitor connection to ground is through a lewat or a short trace.

In addition to Figur 4's decoupling capacitors, EMI/RFI protection to the USB cable requires the use of ferrite. The ferrite beads in this circuit are Taiyo Yuden's BK2125HS102-T which have an impedance of 1000 Ω at 100 MHz.