Please write to A/D conversion related registers while A/D conversion is stopped (ADST bit = 0).
Note that if the ADST bit is set to 0 to forcibly stop conversion, the A/D conversion result is undefined and an interrupt request is not generated. In addition, in the case of the 3x and Lx Series, an Adi register that is not carrying out A/D conversion may also become undefined. So it is recommended that you do not use the values of any of the Adi registers.
[A/D conversion related registers]
3x and Lx Series: ADMOD, ADINSEL, ADCON0 (ADST bit excluded), ADCON1, and OCVREFCR
1x and 2x Series: ADCON0 (ADST bit excluded), ADCON1, and ADCON2