AD7091R-4 offset


Could you give me your advice about value of offset error of AD7091R-4 ?

Datasheet says +-1.5mV offset error at internal reference.

What is value of offset error at  5V external reference ?

As the result of test, offset error is about 1LSB at 2.5V external reference on evaluation board.

But offset error is about ~6LSB at 5V external reference on evalaution board.

(External power supply unit is used as reference).

Please let me know your advice