ADS1226: What happens on startup?

Hi,

I'm wondering what happens on startup with ADS1226?

ADS1226 has a different comportement when it startups with Vref with no potential, and when it startups with Vref potential no null.

When ADS1226 startups with Vref connected to a potential, all seem to be ok, datas are valid;

But when ADS1226 startups with Vref not connected to any potential, datas measures are negative voltage, and if Vref is connected to a potential, dats measures are always negative voltage.

Could you help me?

Thanks,