ADS7142-Q1: How can I select between Full-scale input voltage and absolute input voltage?

Hi JH,

The full-scale input range defines the differential voltage range applied between (AINP - AINN) that will create the zero-scale, and full-scale outputs.  In this case (AINP-AINN) = 0V will be zero-scale (0x000) and (AINP-AINN) = AVDD, will be full-scale (0xFFF).

The absolute input voltage range defines the absolute differential voltages that can be applied to the device, relative to GND.  The absolute differential range shows that voltages applied to the AINN/AINP pins can extend 100mV below GND, and above AVDD.  While these voltages can individually go beyond the supplies by +/-100mV, the full-scale range remains (0V to AVDD) and it's possible to saturate the output if both inputs are extended beyond the range. 

Here's a few examples, assuming the single-channel, single-ended w/ remote GND mode.  AVDD = +3V:

1.) AINN = -0.1V, AINP = 2.9V, (AINP-AINN) = 3V

     ADC output = 0xFFF

2.) AINN = -0.1V, AINP = -0.1V, (AINP-AINN) = 0V

     ADC output = 0x000

3.) AINN = +0.1V, AINP = 3.1V, (AINP-AINN) = 3V

     ADC output = 0xFFF

4.) AINN = +0.1V, AINP =0.1V, (AINP-AINN) = 0V

     ADC output = 0x000