about two step adc missing code

Hello all,
   I am testing a 10bit two step adc chip ,the reference voltage Vref+=1.4v,Vref_=0.4v,Vmid=0.9v,power supply is 1.8v,process is 0.18um, test result shows there are missing code about 960(decimal),when I overdrive the reference voltage Vref+=2.0v,Vref_=0.6v,Vmid=1.3v, power supply change to 2.2v,there are no missing code. I suspicion the Vref_=0.4v is too low,which will lead to the comparator of flash adc work abnormal,because the Nmos Vth is 0.4v for 0.18um process, but the simulation is ok. Could someone tell me the real reason? Thanks in advance.
Best Regards,
Jim

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