TMS320F28377D: 16-bit differential analog input

Hi Maulik,

The two differential inputs should be complimentary about the common mode voltage. As you have highlighted in the table above, the common mode voltage should be close to (VREFHI - VREFLO) / 2.  For example then if VREFHI is 3.0V then the common mode voltage should be close to 1.5V.

If the common mode voltage is 1.5V and the ADC reference range is 0 to 3.0V then the maximum possible input is Vin+ = 3.0V and Vin- = 0V and the lowest possible input is Vin+ = 0V and Vin- = 3.0V.  The negative input must move to maintain a constant common mode voltage, so if Vin+ = 2.1V then Vin- should be 0.9V.  This is why you can't just tie the negative input to ground.  

The specification you have highlighted above specifies how far off the common mode voltage can be from ideal while allowing the ADC to still function.  If the signal is ideally Vin+ = 1.1V and Vin- = 1.9V, then the DC level of both inputs can drift up or down by up to 50mV.  +50mV would give Vin+ = 1.15V and Vin- = 1.95V and -50mV would give Vin+ = 1.05V and Vin- = 1.85V.  All 3 of these input voltage conditions should give the same ADC output code because the difference between the positive and negative inputs is the same. 

To supply a differential voltage to the ADC, you can either sample a sensor that is already in a differential format, or you can convert a single-ended signal to differential by using either a fully-differential op-amp (e.g. THS4531) or you can use a pair of single-ended op-amps.  The precision amplifier e2e forum can help with designing appropriate signal conditioning circuitry if necessary.