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The increasing significance of thermal issues in modern VLSI motivates the need for a large number of lightweight, robust and power efficient thermal sensors for accurate thermal mapping and management. We propose use of differential ring oscillators (DRO) for thermal sensing, utilizing the temperature dependence of the oscillation frequency. In current starved inverter topology using the 45 nm technology node, they have a resolution of 2degC and a low active power consumption of less than 25 muW which can be reduced further by 60-80% by gating the design. A high threshold design proves to be better in terms of leakage, non-linearity error, overall power consumption as well as sensitivity to power supply variations. The standard deviation in measurement (%) caused by process variations and supply noise is within 3% for low Vt design; it increases to 5% for a high Vt design. In a reduced supply bounce configuration, the measurement error caused due to supply noise can be reduced by 15-60%.