AD7789 sensor 2.5V - all or nothing issue, 00..00 or 11..11


I have a sensor that provides its value between 0.5V to 2.5V.
I am using an AD7789 with 2.5V reference voltage (ADR391) to REF +, ref- grounded. (AGND = DGND = global GND)
AIN- to ground
AIN + directly to sensor signal

When reading the result of the supply voltage conversion in AD7789 (CH1,CH2=1,1), 24 bits are good, but when playing the AIN conversion (CH1, CH0 = 0.0) I received 0x000000 AIN + = 0V or 0xFFFFFF when  AIN + > 0V. Any intermediate values.

Could you help me, I have to make this product quickly
(If you speak French is even better)
thank you.