Hello @Zain Zaidi,
The JAXC6 and JAXC12 provides 3.3V VCC and are not inputs.
The ADC differential input must be with a voltage difference between 0-1V.
The input signal for each JXADC channel of the Pmod must be between 0 - 3.3V.
So if you connect your XA_N channels to ground, then you must apply to the XA_P channels a voltage between 0-1V.
ex : XA1_P= 0.5sin(wt), XA1_N = 0V => XA1_P - XA1_N = 0.5sin(wt) differential input for ADC.
XA1_P= 2.5sin(wt), XA1_N = 2.2sin(wt) => XA1_P - XA1_N = 0.3sin(wt) differential input for +ADC.
Please read ug480 pages 31-32.
If you look into the schematic file of the Basys 3, you will see that the Vp(A12) and Vn(B13) pins are connected to XADCGND.