Hi Yuquan,
I see now. I'm sorry in that I should have read through the thread again before posting the last time. Please keep in mind that the PGA is before the Offset DAC. When gain is applied the input impedance lowers unless the Buffer is on. When the Buffer is on, the absolute input range lowers by 1.5V from the supply (and the input signal must be between 0.05V and 2.1V). Regardless of the Offset DAC setting, you must maintain the required input levels as specified in the datasheet.
I'm not sure I've seen anyone use the device in quite the way you want to use it. When using the unipolar mode, the analog input is usually a single ended input (referred to ground). In this way you do not lose a bit as the signal would not go into the negative direction.
Can you be more specific on the actual input voltages to the ADS1243 and the range of change these voltages may take? In other words, I would like to know the actual voltages at the + input and the - input with respect to ground. I can then help to determine any issues with the method you are trying to use.
Best regards,
Bob B
