AD7609 Input noise


I would like to develop a tool for voltage acquisition using the AD7609. So I bought your EVM board (AD7609EDZ). And reads the data using the SPI interface. Differential input reads several batteries. However, when I read only one channel, it reads very well. If I read several channels, the data moves about + 40mV.
How can I solve this problem? I think the AD7609 is a really optimal solution for the system I want to develop.
Please help me.
Best Regards,