In this paper, some experiments about thermal sensors based on ring-oscillator in low-voltage Virtex series FPGAs are presented. A non linear effect in the frequencytemperature response has been detected, and the sensibility of frequency with respect to voltage variations is greater than the measured in previous works. A quadratic polynomial function fits better the sensor response, and an increment in the number of inverters in the oscillator is effective to reduce the voltage sensibility.