International Journal of Scientific & Technology Research Volume 1, Issue 4, May 2012 ISSN 2277-8616 76 IJSTR©2012 www.ijstr.org Ring Oscillator Based CMOS Temperature Sensor Design Shruti Suman, Prof. B.P. Singh Abstract- This paper presents the ring oscillator based temperature sensor. The method is highly area efficient, simple and easy for IC implementation as compared to traditional temperature sensors. The proposed temperature sensor was fabricated using 0.35 μm technology, which occupies extremely small silicon area. It exploits the frequency of the ring oscillator that is proportional to temperature, which is displayed in the form of a digital output. The proposed temperature sensor comprises a ring oscillator, a voltage level shifter, a 10-bit counter, and a 10-bit register. The designed ring oscillator is frequency-tunable and the voltage level shifter provides the output to full-scale to make sure that the number of its rising edge is counted by the counter. The register saves the counted output. Index Terms— Charge Recycling, Noise Shaping, Ring Oscillator, Temperature Sensor. ———————————————————— 1. INTRODUCTION THE simplified block diagram of ring oscillator based CMOS temperature sensor is shown in Fig. 1. Proposed ring oscillator generates a clock signal which is proportional to the change in temperature. A current-starved inverter with 4-digit tunable inputs makes the ring oscillator frequency-tunable. The voltage level shifter makes sure that the number of its rising edge is counted by the counter. The register saves the counter output at every positive edge of the external clock. The difference between two successive outputs of the register indicates the temperature [1][2][3]. 2. EXISTING RING VCO A ring oscillator consists of number of gain stages in a loop with the output of the last stage fed back to the input of the first. The ring must satisfy the Barkhausen Criteria according to which it should provide a phase shift of 2π and must have unity voltage gain to achieve oscillation. The total delay is equally divided among all stages. So each delay provides a phase shift of π /N, where N is the number of delay stages. The remaining π phase shift is provided by a DC inversion. The most basic ring oscillator is simply a chain of single ended digital inverters. This circuit is shown in Fig. 2. To provide the DC inversion, an odd number of stages must be used. To see why this circuit will oscillate, assume that the output of the first inverter is a ‗0‘. Therefore, the output of the Nth inverter, where N is odd, must also be ‗0‘. However, this output is also the input to the first inverter, so the first inverter‘s output must switch to a ‗1‘. By the same logic, the output of the last inverter will eventually switch to a ‗1‘, switching the output of the first inverter back to ‗0‘. This process will repeat indefinitely, resulting in the voltage at each node oscillating. Fig.1. Block diagram of proposed temperature sensor To determine the frequency at which this circuit will oscillate, the delay provided by each inverter cell is . The signal must go through N inverters, each with the delay , for a total time of , to obtain the first 2 π phase shift. Then, the signal must go through each stage a second time to obtain the remaining π phase shift, resulting in a total period of . The frequency is the reciprocal of the period, resulting in the frequency as (1) Fig 2. Existing Ring Oscillator Counter Register Ring Oscillator Level Shifter 10-bit CLOCK