ADC1175: About ADC1175 and ADC1173 have measurement errors(ADC1175和ADC1173采样错误的问题)

Part Number: ADC1175
Other Parts Discussed in Thread: ADC1173

The VRT is 3.27V,VRB is 0.5--0.7V. When I use ADC1173 and ADC1175,  to sample 2.7V,I get digital about 232 (2.9V). I sample all range ,from 0.6V to 3.2V, The most of value are mistakes( are bigger values: 1.0V sample is 1.2V; 0.7v sample is 1.1V; 2.1V is sample 2.4v). 

My clock frequency are  1.6MHz 2.0MHz 3MHz 11MHz, but it is no effect .

I discover how I adjust the reference voltage range, adc1173 cannot measure correctly.  ADC1175 can operate within the reference voltage range of 0.5V to 3.3V, with only one of 10 circuit boards being able to operate. The others have measurement bias.

I don't know what is it. I see the datasheet said: the max VRT-VRB is 2.8V.  Have anyone faced this problem?