Upgrade of temperature sensor for a two-way ring oscillator structure
1 Introduction
The characteristics of many electronic components in electronic products 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, crystal frequency compensation due to temperature [1], temperature control based MEMS system [2] and so on. Separate temperature sensors are also embedded in a variety of applications, such as medical health [3], near field communication [4]. For the temperature sensor technology itself, energy harvesting technology (Energy Harvest) is used to complete low power consumption [5], and digitalization and adaptive compensation [6-11] have become some research directions. Therefore, how to design a temperature sensor with low power consumption, small chip area and high precision has become the driving force for continuous research on this subject. The traditional CMOS temperature sensor is designed by the temperature characteristics of the triode or the thermistor. In this paper, a two-way ring oscillator with shared capacitance is used to realize the temperature sensor. technology.
2 Temperature sensor principle
The design of the temperature sensor requires a temperature-sensitive electronic component, which is a commonly used electronic component. A common thermistor whose resistance is a function of temperature, and the temperature-resistance relationship is usually approximated as shown in Equation 1.
(1) where R0 is the resistance value at the temperature of T0, and B is the temperature parameter of the thermistor, which is related to the material of the thermistor. Typically, the temperature-resistance curve of the thermistor is approximately linear over its normal operating temperature range, as shown in Figure 1. For engineering convenience, the curve is usually approximated by a linear equation. The fitted formula is usually in the form of Equation 2 [12, 13].
(2) where A1 and A2 are temperature parameters. For different thermistor materials, the temperature coefficient is usually different. Table 1 shows the temperature coefficient information of a N+ type polysilicon, a P+ type polysilicon, and a thermistor made of a metal material [14]. This paper uses a 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. Therefore, the resistance value is usually converted into a current, voltage or frequency value by a certain circuit to facilitate the measurement and processing of the circuit. Among them, the scheme of converting the resistance value into a voltage value by using a constant current source and converting the analog voltage value into a digital value through the ADC is most commonly used for the back-end circuit. This method produces a constant current by adjusting the constant current source circuit that produces a suitable voltage bias as it flows through the resistor. The post-stage filter amplifying circuit processes this signal, and sends the filtered amplified voltage to the ADC, and the digital voltage value obtained by the ADC conversion 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 temperature sensor products and solutions. However, ADC circuits typically require higher energy during conversion and are more expensive. Therefore, for low-power, low-cost applications, this solution needs to be improved. This paper proposes a two-way ring oscillator with shared capacitors. Figure 2 shows the scheme for calculating temperature using two oscillator frequencies [15-22]. The oscillator has small area, low power consumption and high accuracy. The advantages.
The temperature sensor circuit proposed in this paper is a ring oscillator, where RC determines the frequency of the Ring oscillator. R is optional in this circuit. Metal Resistor or Poly Resistor can be used. Different resistors can be connected in series to obtain different oscillation frequencies.
The relationship between the oscillator frequency and RC is as 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 ratio of frequency and resistance ratio can be obtained. Relationship, it can be seen from the equation that the frequency ratio eliminates the influence of the capacitor C. Even if the consistency of the chip and the chip capacitor is poor, the ratio of the frequency can be ensured to reflect the proportional relationship between the two resistors, so that the capacitor C can utilize the chip. Internal dual poly/bimetallic/stacked metal (PIP/MIM/MOM) capacitors are implemented. No external precision capacitors are required.
Fmetal / Fpoly = C&TImes;Rpoly / C&TImes;Rmetal
= Rpoly / Rmetal
4 temperature sensor implementation
According to this principle, a temperature sensor is designed in the CMOS process of SMIC. According to the PCM specification of the process, we get the temperature profiles 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, the resistance in the figure rises as the temperature rises, and one decreases as the temperature rises. 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 primary term. Therefore, it can be approximated as a linear function of temperature in the working range of the industrial chip. This is convenient for calculation.
Since the metal block resistance is much smaller than the polysilicon resistance, it can be seen from Figure 3 that the ratio of the two is 1000 times. The same sheet resistance, the area of ​​the metal resistor is much larger than the polysilicon resistor. Figure 4 is a top view of the temperature sensor under the microscope. The figure shows the approximate proportional relationship of the capacitor resistance. The capacitor is the MIM capacitor.
The test data of the temperature sensor chip is shown in Table 2. As can be seen from the table, the period of the ring oscillator rises with the rise of temperature, which is mainly caused by the temperature rise and the MOS tube current is reduced, so two types The absolute value of the oscillator period of the material increases with increasing temperature, but the ratio of the oscillator period decreases as the temperature increases, which reflects the positive temperature coefficient of the metal resistor and the negative resistance of the polysilicon resistor. Temperature Coefficient.
According to the data of Table 2, using Matlab's least squares fitting, we can get the relationship between the oscillator period ratio and temperature of Figure 5. The minimum quadratic coefficient of the fit is R2 = 0.9999, which can get a high degree of fitness. . In the actual product, you can select two or more points to fit the entire curve. In the product test, we use the two points of 20 °C and 50 °C to fit the whole curve. The temperature is calculated according to the fitting curve and the frequency obtained by the actual test. The temperature accuracy in the temperature range of 20 °C to 50 °C reaches 0.1. °C, while the entire sensor consumes less than 1μA. The actual test found that the more points are fitted and the higher the accuracy, the accuracy of the temperature sensor after the 1000-hour life test can still reach 0.4 °C.
5 Conclusion
In this paper, a temperature sensor with two-way ring oscillator structure is designed. The problem of inconsistent components during circuit processing is solved by using shared capacitors. The measured data shows 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 good performance and performance, which can meet the needs of most application fields.
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