Testing my 10bit pipelined ADC,(SMIC 0.18 process)

when diff input both connect dc commen input voltage(0.9V), the output code is not 511.
and the output code in 3 Evaluation Board is different respectively(1# is 485,2# is 540, 3# is 604).

It may because offset of opamp ,but should not so large(about 50mV).

So ,what else may cause the code excursion?

There are a lot of possible causes I can think of, but I think the first thing is to see what the overall transfer function is - effectively what the INL and DNL is, as Berti suggests.