AMC1200BDUBR Gain is 4 instead of 8
Hi @Jeroen3, yes I do, and I connected the negative output to the ground directly

The AMC1200 has a differential output. Gain 8 is achieved by using a differential input. Using only one output will seem to have half of total gain.
Shorting the output is not recommended.
If you only have a single ended ADC you need to convert it with a differential amplifier.
Or you can sample both signals with your single-ended ADC and subtract them in software.
You do get some error if you can't sample simultaneously.