That way every ring oscillator has a different frequency, so an individual calibration process is needed. Therefore, to illustrate this frequency difference some measurements at various temperatures were performed. For all 36 sensors, the frequency versus temperature dependence was determined. Beside the ambient conditions, where package temperature was close to 35°C, an external heat source was used to obtain more measurement points, so the range from 35°C to 90°C (package temperature) was analyzed. The FPGA device was heated the stream of hot air and the package temperature was measured with an infra-red thermometer (IR NC100).