nios-ii-on-die-temperature-sensor example

No problem for me. I had both the v14.1 & 15.0.

Btw, another thing that you can check is the Qsys clock, it should be 50Mhz or higher. While the ADC clock should be 10Mhz (or whatever as you declared in the ADC parameter and configured in your PLL).

Dont confuse both of them.

Otherwise, you may see weird stuffs like temperature reading at minus while it is actually at room temp.