Why the CM403F'ADC don't have the input signal,but also can read some data from the ADCC0_EVT00 register?

Hi all

       When I use the adc from the cm403f ,I find even I don't give the input signal,I also can read some data from the ADCC0_EVT00 register.The data is  0x4040 and sometime is 0xc040.When I give the 1.2VDC to the input ,the data of the ADCC0_EVT00 is also the same.The attachment is my source code , would anyone do me a favor?

Best regards

hlg