Hi Yuquan,
The ADS1243 resolves to the 24-bit level which is the output response. However due to noise, the effective resolution does decrease from the resolution size.
When writing your code, you can use either method keeping in mind that the LSB size will be determined by the number of bits you are using. Also, the data from the ADS1243 needs to be adjusted accordingly if you uses fewer than 24 bits. One method would be to just truncate the desired number of LSBs. For example, you might have 4 bits of noise. You could take your data and shift the value to the right by 4 (data>>4) and then use the LSB size of 2^20 instead of 2^24 as it relates to full-scale range.
Best regards,
Bob B