An overview of a technology that uses two different temperature coefficient materials to implement a temperature sensor

Yang Xiaoqi, Wan Jianjun, Liu Wenjiang

Abstract: Traditional embedded temperature sensors are implemented using triodes and ADCs. This paper presents a technique that uses two different temperature coefficient materials as sensors and uses a shared-capacitor dual-circuit ring oscillator to implement a temperature sensor. It features low power consumption, small area, and high accuracy.

1 Introduction

In electronic products, the characteristics of many electronic components are closely related to temperature. Therefore, in order to eliminate the drift of electronic components at different temperatures, temperature sensors are embedded in various electronic products. For example, temperature-induced crystal frequency compensation [1], temperature-based MEMS systems [2], etc. Individual temperature sensors are also embedded in various applications, such as health care [3] and near field communication [4]. For the temperature sensor technology itself, the use of energy harvesting (Energy Harvesting) to complete low power [5], the use of digital and adaptive compensation [6-11] has become a research direction. Therefore, how to design a temperature sensor with low power consumption, small chip area, and high accuracy becomes the driving force for continuous research on this topic. Traditional CMOS temperature sensors are designed using the temperature characteristics of a transistor or a thermistor. In this paper, we propose a temperature sensor using two different temperature coefficient materials and a shared dual-ring ring oscillator to achieve a temperature sensor. technology.

2 temperature sensor principle

The design of the temperature sensor requires a temperature-sensitive electronic component to implement the thermistor is a commonly used electronic components. In common thermistors, their resistance is a function of temperature, and the temperature-resistance relationship is generally similar to Equation 1.

(1)

Among them, R0 is the resistance value when T0 temperature is, B is the temperature parameter of thermistor, this coefficient is related to the material of thermistor. The thermistor's temperature-resistance curve is approximately linear in its normal operating temperature range, as shown in Figure 1. In order to calculate conveniently, the curve is usually approximated by a linear equation. After the fitting, the formula is usually in the form of formula 2 [12,13].

(2)

Among them, A1, A2 are temperature parameters. For different thermistor materials, the temperature coefficient is usually different. Table 1 shows the temperature coefficient information of a thermistor made of N+ type polysilicon, a P+ type polysilicon, and a metal material [14]. This paper uses the different temperature coefficient of polysilicon and metal to design a temperature sensor.

3 temperature sensor circuit

In the actual circuit, the resistance value is not convenient for direct measurement, so it is usually through a certain circuit to convert the resistance value into a current, voltage, or frequency value that is in a certain functional relationship with it, so as to facilitate the measurement and processing of the circuit. Among these, the most common method is to use a constant current source to convert the resistance value into a voltage value, and then convert the analog voltage value into a digital value through the ADC and provide it to the back-end circuit. This method generates a constant current by adjusting the constant current source circuit, which generates a suitable voltage offset when flowing through the resistor. The post-stage filter amplifying circuit processes this signal and sends the filtered amplified voltage to the ADC. After the ADC converts, the resulting digital voltage value is provided to the subsequent circuit for processing. Because the current ADC can provide high conversion accuracy, and each circuit module has a very mature solution, it is widely used in various types of temperature sensor products and solutions. However, ADC circuits generally require higher power at the time of conversion, and their cost is higher. Therefore, for low-power, low-cost applications, this program has yet to be improved. In this paper, a shared-capacitor dual-channel ring oscillator is presented in Fig. 2. A scheme for calculating the temperature using the difference of two oscillator frequencies [15-22] has small area, low power consumption, and high accuracy. The advantages.

The temperature sensor circuit proposed in this paper is a Ring oscillator, in which RC determines the frequency of the Ring oscillator. In this circuit, R is optional. Metal resistors or poly resistors can be used. Different resistors can be connected in series to obtain different oscillation frequencies.

The relationship between the oscillator frequency and RC is shown in Equation 3.

(3)

In this circuit, when SEL=1, the circuit selects PolyResistor. When SEL=0, the circuit selects MetalResistor. According to formula (1-4), the relationship between the ratio of the frequency and the resistance ratio can be obtained. The equation can be seen that the frequency ratio eliminates the influence of the capacitor C. Even if the chip and the chip capacitor have poor consistency, it can ensure that the proportional relationship of the frequency reflects the proportional relationship between the two resistors. In this way, the capacitor C can use the double internal chip. Polysilicon/Bimetal/Layered Metal (PIP/MIM/MOM) capacitors are implemented. No need to add an accurate capacitor.

Fmetal / Fpoly = C×Rpoly / C×Rmetal

= Rpoly / Rmetal

(4)

4 Temperature sensor implementation

According to this principle, a temperature sensor has been designed in the SMIC CMOS process. According to the PCM specification of this process, we get the temperature curves of Poly Resistor and Metal Resistor as shown in Figure 3. Since the metal resistance is a positive temperature coefficient and the polysilicon resistance is a negative temperature coefficient, one of the resistances in the graph rises with temperature and one decreases with temperature. Although the resistance is a quadratic function of temperature, the coefficient of the quadratic term is small, which is 3 to 4 orders of magnitude smaller than the coefficient of the first term. Therefore, it can be approximated as a linear function of temperature within the working range of the industrial chip. This is convenient for calculation.

Since the size of the metal square resistor is much smaller than that of the polysilicon resistor, it can be seen from Fig. 3 that the ratio of the two is up to 1,000 times. The same square resistance, the area of ​​the metal resistor is much larger than the polysilicon resistance. Figure 4 is a top view of the temperature sensor under the microscope. The figure shows the approximate proportional relationship of the capacitance resistance. The capacitance is the MIM capacitor.

The test data of this temperature sensor chip is shown in Table 2. It can be seen from the table that the cycle of the ring oscillator rises with the rise of temperature. This is mainly due to the temperature increase causing the MOS tube current to decrease. The absolute value of the material's oscillator period increases with temperature, but the oscillator period ratio decreases with increasing temperature. This just reflects the positive temperature coefficient of the metal resistor and the negative resistance of the polysilicon. Temperature Coefficient.

Using Matlab's least-squares fit from the data in Table 2, we can derive the relationship between oscillator period ratio and temperature in Figure 5. The minimum quadratic coefficient for this fit, R2 = 0.9999, gives a very good fit. . In actual products, you can select two or more points to fit the entire curve. We use the two points of 20°C and 50°C in the product test to fit the entire curve. The temperature is calculated based on the fitted curve and the frequency of the actual test. The temperature accuracy in the temperature range of 20 °C to 50 °C reaches 0.1. °C, and the entire sensor consumes less than 1μA. The actual test found that the more points fitted, the higher the accuracy. After the temperature sensor has passed the life test of 1 000 hours, the accuracy can still reach 0.4 °C.

5 Conclusion

In this paper, a temperature sensor with a dual ring oscillator structure is designed. The use of shared capacitors solves the problem of inconsistent components during circuit processing. The measured data show that the accuracy of the circuit can reach ±0.1°C. The sensor consumes less than 1μA.

The temperature sensor designed in this paper has better performance and performance and can meet the needs of most applications.