A TEMPERATURE COMPENSATED CMOS RING OSCILLATOR FOR WIRELESS SENSING APPLICATIONS

Journal of Electrical and Electronics Engineering (JEEE)) ISSN 2250-2424 Vol.2, Issue 1 Sep 2012 1-10 TJPRC Pvt. Ltd., A TEMPERATURE COMPENSATED CMOS RING OSCILLATOR FOR WIRELESS SENSING APPLICATIONS JAMEL NEBHEN 1,2, STÉPHANE MEILLÈRE 1, MOHAMED MASMOUDI 2, JEAN-LUC SEGUIN 1, HERVÉ BARTHELEMY 3, KHALIFA AGUIR 1 1 Aix-Marseille Université, IM2NP-CNRS-UMR 7334 Avenue Escadrille Normandie Niemen - Case 152, 13397 Marseille Cedex 20, France 2 EMC Research Group-National Engineering, school of Sfax, Electrical Engineering Department, Route de Soukra Km 2.5, BP. 1173 3038, Sfax, Tunisia 3 Université du Sud-Toulon Var, IM2NP-CNRS-UMR 7334 Avenue de l'université - BP20132 83957 La Garde Cedex - France ABSTRACT This paper presents a CMOS voltage controlled ring oscillator (VCO) with temperature compensation circuit suitable for low-cost and low-power wireless sensing applications. To operate at low frequency, a control voltage generated by a CMOS bandgap reference (BGR) is described and the measurement results of the fabricated chips are presented. The output voltage of the reference is set by resistive subdivision. In order to achieve small area and low power consumption, n-well resistors are used. This design features a reference voltage of 1V. The chip is fabricated in AMS 0.35 µm CMOS process with an area of 0.032mm 2. Operating at 1.25V, the output frequency is within 200±l0kHz over the temperature range of -25 C to 80 C with power consumption of 810µW. KEYWORDS: Voltage-controlled oscillator, CMOS, bandgap, temperature compensation, low-power. INTRODUCTION Recent advances in silicon sensors and circuit technology have enabled various wireless sensing applications. The deployed wireless sensors are highly integrated with limited power sources to reduce cost. Systems relying on batteries or energy scavenging from the environment operate at low frequencies to save power. Nevertheless, most of functions are scheduled with a low duty-cycle to maintain longlifetime operation. Power is then drawn by an oscillator to generate the reference frequency. As a critical part of the system, a low-power oscillator is required. In recent years, LC oscillators [1] have been known with good phase noise performance, but their tuning range is relatively small (around 10-20%) and on-chip spiral inductors occupy a lot of chip area. On the other hand, ring oscillators usually have a wide tuning range, occupy less on chip integration area, which makes them being more widely used than LC oscillators. The well known ring oscillator is shown in Fig.1. High integration capability of ring oscillators has made them an integral part of many digital and analog systems [2]. These building blocks are used as beating heart of applications such as disk drive read

2 Jamel Nebhen, Stéphane Meillère, Mohamed Masmoudi, Jean-Luc Seguin, Hervé Barthelemy, Khalifa Aguir channels [3], clock recovery circuits for serial data communications [4]. Stable frequency is imperative in order to minimize interference in adjacent frequency bands, as far as many communications applications are concerned [5]. A popular method for realizing digital-output VCOs in CMOS technology is a subclass of the CMOS relaxation oscillator family, such as the structure of ring oscillator. One of the most important considerations when designing an IC oscillator is its frequency stability with temperature. In addition, the required temperature dependence assumes one of following forms in most application such as proportional to absolute temperature (PTAT) or constant-gm behavior like the transconductance of certain transistors remains constant or temperature independent. On the other hand, the frequency steadiness depends basically on the following; first, the internal current sources variation with temperature, second, the voltage reference drift across the temperature range, third, the device offset voltage and bias current deviations due to temperature changes and forth, the capacitor variation with process, voltage and temperature [6]. Figure 1. Fig Single ended ring VCO This work deals with the design of a MOS temperature compensated voltage controlled ring oscillator for wireless sensing applications. Section II presents the CMOS ring VCO architecture. The circuit design of the BGR is presented in section III. Measurements and discussions are presented in section IV and finally, the conclusion is given in section V. CMOS RING VCO ARCHITECTURE A conventional VCO, as shown in Fig. 2, is realized by N stages of inverters (N is an odd number), with a control mechanism of the current passing through these inverters. Usually we use one PMOS transistor to control the upper side current and an NMOS transistor to control the lower side one. Assume that the gate parasitic capacitances C g of the NMOS and PMOS transistors are equal, the frequency of the oscillator can be found as: f osc 1 = (1) 2Nτ

A Temperature Compensated Cmos Ring Oscillator 3 for Wireless Sensing Applications Figure 2. Fig Conventional ring VCO Where τ is the delay for one stage, which could be given by: V C osc g τ = (2) I ctrl Where V osc is the oscillation amplitude and I ctrl is the control current. From the above two equations, we can get: f osc = Ictrl 2NV C (3) osc g It is obvious to see the oscillation frequency can be controlled by varying the control current, if assuming the number of stages N and C g are fixed. The advantage for this configuration is that the oscillation frequency can be tuned for a wide range by changing the value of control current. However, when I ctrl is very small, it is difficult to keep matching between its upper and lower limits. In addition, the small current will make the voltage swing slow and sometimes we will get a non-full swing signal at the output. Figure 3. Fig Delay cell with variable current Fig. 3 shows one stage of the proposed VCO. It consists of one inverter with current controlled by a cascading NMOS transistor M1. By changing only one control voltage, the oscillation frequency can be varied according to equation (3).

4 Jamel Nebhen, Stéphane Meillère, Mohamed Masmoudi, Jean-Luc Seguin, Hervé Barthelemy, Khalifa Aguir THE BANDGAP CIRCUIT A conventional bandgap reference is a circuit that subtracts the voltage of a forward-biased diode having a negative temperature coefficient from a voltage proportional to absolute temperature (PTAT). A PTAT can be realized by amplifying the voltage difference of two forward-biased base-emitter junctions. As a consequence, a temperature compensated voltage close to the material bandgap of silicon (~ 1.22 V) results. For low supply voltage operation this voltage is too high to be realized. Some ways to overcome this limitation have been proposed in [7] [8] [9] and [10]. References [7][8] and [10] are based on resistive subdivision of the bandgap voltage and reference [9] is using dynamic-threshold MOS transistors (DTMOST s) resulting in lower apparent bandgap voltage. In [7] the resistive voltage division is made at the output of the BGR whereas in [8] and [10] the additional resistors are connected at the input of the amplifier in the BGR. Theoretically, the latter topologies will lead to larger variation of the reference voltage than the first one since the voltage error present at the input (caused by resistor mismatch) will be amplified to the output, i.e. added to the reference voltage of the BGR. Also, the noise performance will be degraded. In this work, a BGR based on the topology presented in [7] has been designed and optimized for low voltage and low power operation. The circuit topology of the designed BGR is presented in Fig. 4. The operation of the circuit is similar to a conventional CMOS topology except that there is an additional resistor R 3 at the output of the BGR in parallel with components R 2 and B 3. The voltage difference V EB (Eq. 4) between the emitterbase junctions of B 1 and B 2 is obtained using an emitter area ratio of 8 and setting the currents through both components equal. kt j 1 VEB = VEB 1 VEB 2 = ln q j2 (4) Where k is Boltzmann s constant, q is the electron charge, T is the absolute temperature and J 1 and J 2 are the current densities of the forward biased diodes. Figure 4. Circuit topology of the BGR.

A Temperature Compensated Cmos Ring Oscillator 5 for Wireless Sensing Applications An opamp (consisting of transistors M 7 M 11 ) sets the emitter currents of B 1 and B 2 equal (M 1 = M 2 ) and then the voltage across R 1 becomes V EB. Therefore, the current flowing through R 1 and M 2 is proportional to absolute temperature (PTAT). This causes the currents through M 1 and M 3 to be also PTAT. The output voltage of the BGR is formed by the opamp, which adds the emitter-base voltage V EB3, which has a negative temperature coefficient, to K V EB, which has a positive temperature coefficient, resulting in a temperature independent output voltage V REF at a reference temperature (Eq. 5). This is the first order temperature compensation technique for the BGR circuit. Where K is [11]: V = V + K V (5) REF EB3 EB kt0 V + ( m + η 1) V q K = kt 0 j 1 ln q j G0 EB0 1 2 In Equation (6) V G0 is the bandgap voltage of silicon extrapolated to 0 o K, m is a temperature constant approximately 2.3, η = 2.2 is a correction term due to temperature dependency of n-well resistors used, T 0 is the reference temperature, and V EB0-1 is the junction voltage of B 1 at reference temperature. In practice, K sets the resistor ratio R 2 /R 1. If a resistor R 3 is added to the output of the BGR, the output voltage of the BGR can be written as follows: V = R V + I R ( ) 3 REF EB3 M 3 2 R2 + R3 Now, the reference voltage can be freely chosen by the resistor ratio to a more suitable level for low voltage operation. In order to achieve low voltage and low power operation, all MOS transistors are designed to operate in weak/moderate inversion. Threshold voltages of the PMOS and NMOS transistors in the process used are 0.65 V and 0.5 V, respectively. Components B 1 B 2 are diode connected vertical PNP transistors, readily available in standard CMOS processes. The numbers below the PNP s are the relative areas of the devices compared with each other. Resistors R 1 R 3 are implemented using n-well resistors since there are no high- ohmic poly-resistors available in this process. Large resistance values are needed in order to realize the desired voltage drops across the resistors with low power consumption. (6) (7)

6 Jamel Nebhen, Stéphane Meillère, Mohamed Masmoudi, Jean-Luc Seguin, Hervé Barthelemy, Khalifa Aguir TABLE I. COMPONENT VALUES FOR THE BGR The opamp is a single stage amplifier consisting of a differential pair with a current mirror load. The input stage is realized with NMOS transistors since the input voltage is closer to V DD than ground. The output of the opamp is driving the gates of M 1 M 3 and M 12. The area of transistors M 1 M 3 has been designed fairly large in order to minimize the 1/f-noise of the circuit. The W/L-ratio of M 3 has been designed two times larger than the W/L of M 1 and M 2. This reduces the resistor sizes of R 2 and R 3 by 50 %. This is a compromise made between current consumption and area. A start-up circuit has been added to the design to ensure correct operation of the circuit. The start-up circuit consists of transistors M 5 M 7, which have been designed weak in order to minimize their effect on the reference voltage when the circuit has settled. Capacitor C 1 in the bias line is needed to stabilize the circuit. C 1 also reduces the bandwidth of the opamp resulting in lower noise. The values of the components are presented in Table 1. With these values the simulated output voltage of the bandgap reference is 1V at a temperature of 20 o C with a 1.25-V supply. MEASUREMENT RESULTS The voltage-controlled ring oscillator with bandgap reference circuit is fabricated in AMS 0.35- µm CMOS technology where the active area is 0.032mm 2. The die photograph is shown in Fig.5. The waveform of the output ring VCO is shown in Fig. 6. Current consumption is 810µA at 25 C. The output frequency is 200kHz. Operating at 1.25, the output frequency of the compensated Ring VCO is compensated between 191kHz and 212kHz over a range of -25 C to 80 C. Measurement results show the output frequencies as a function of temperature is presented in Figures 7. They are compensated between 191kHz and 212kHz and yield a temperature stability less than a range of -4.5% to +6% over a range of - 25 C to 80 C. The reference voltage of the BGR as a function of temperature is presented in Figures 8.

A Temperature Compensated Cmos Ring Oscillator 7 for Wireless Sensing Applications Measurement results show the output voltages are compensated between 0.989V and 1.021V and yield a temperature stability less than a range of -1.1% to +2.1% over a range of -25 C to 80 C. Figure 5. Die photo of the compensated Ring VCO with the bandgap fabricated in AMS 0.35µm CMOS process. Finally, we summarize the measurement results of our work and [12] in Table II. In [12], the frequency variation and the frequency variation ratio are a range of 118MHz to 486MHz and ±3.5% across temperature varied from 0 C to 70 C respectively. In our work, the output frequency and the frequency variation ratio are 200kHz and a range of -4.5% to +6% across temperature varied from -25 C to 80 C respectively. Our compensated ring VCO possesses the lowest operation voltage and power consumption. The low power consumption and low cost make it suitable for wireless sensing applications. Figure 6. Waveform of the output compensated ring VCO

8 Jamel Nebhen, Stéphane Meillère, Mohamed Masmoudi, Jean-Luc Seguin, Hervé Barthelemy, Khalifa Aguir Figure 7. Temperature variation of the output frequency of the compensated ring VCO Figure 8. Temperature variation of the output voltage of the BGR

A Temperature Compensated Cmos Ring Oscillator 9 for Wireless Sensing Applications Table II. Comparison of Measurement And Reference Result of Compensated Ring Vco CONCLUTIONS A design of a CMOS voltage controlled ring oscillator (VCO) with temperature compensation circuit suitable for low-cost and low-power wireless sensing applications is described and the results of fabricated chip are presented. The circuit is fabricated in AMS 0.35µm CMOS technology with 1.25V power supply. Furthermore, this compensated ring VCO circuit occupies an area of 0.032mm 2 and the compensated frequency yields an accuracy less than a range of 191kHz to 212kHz and yield a temperature stability less than a range of -4.5% to +6% over a range of -25 C to 80 C with 810µA of current consumption. In the same way, the reference voltage of the BGR is compensated between 0.989V and 1.021V and yield a temperature stability less than a range of -1.1% to +2.1% over a range of -25 C to 80 C. It can be fully integrated in wireless sensing applications to provide a low-cost and low-power reference frequency without any external reference. ACKNOWLEDGMENT This work was done thanks to financial support of the Franco-Tunisian Integrated Action of the French Ministry of Foreign and European Affairs and the Ministry of Higher Education, Scientific Research and Technology of Tunisia (project grant 09G1126). REFERENCES 1. Graninckx and M Steyaert (1997), A 1.8GHz Low-phase noise CMOS VCO using optimized hollow spiral inductors, IEEE J.Solid-State Circuit, Volume: 32, pp. 736-744. 2. A. Hajimiri, S. Limotyrakis, T.H. Lee (1999), Jitter and Phase Noise in ring oscillator, IEEE J. Solid State Circuit, Volume 34, No 6, pp. 790-804. 3. M. Negahban, R. Behrasi, G. Tsang, H.Abouhossein and G. Bouchaya (1993), A Tow-chip CMOS read channel for hard-disk drive, IEEE ISSCC Dig. Tech. Papers, pp. 216-217.

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