单电源IC，必须注意这个问题。
Maxim有beyond-the-rail，允许输入略负于地电平。
ADI同样的情形如：ADuC824的32～33页---
Analog Input Ranges
The absolute input voltage range on the primary ADC is restricted
to between AGND + 100 mV to AVDD – 100 mV. Care must be
taken in setting up the common-mode voltage and input voltage
range so that these limits are not exceeded, otherwise there will
be a degradation in linearity performance.
The absolute input voltage range on the auxiliary ADC is restricted
to between AGND – 30 mV to AVDD + 30 mV. The slightly
negative absolute input voltage limit does allow the possibility of
monitoring small signal bipolar signals using the single-ended
auxiliary ADC front end. 这种情形不多，但有时非常有效----不需要偏置电压（共模电压）抬高信号电平。
http://www.analog.com/UploadedFiles/Data_Sheets/ADUC824.pdf
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