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Fully Differential instrumentation Stage Doubts


I am trying to build fully differential instrumentation circuit with PGA for bandwidth from DC to 15KHz or so only. I have selected PGA280 and OPA1632 before ADC. Here I am not very clear about the following:

1. What is the role of Vocm pin in PGA280 and also on OPA1632, while both require dual power source. As I understand it is used to bias the amplifier, and for dual power source, the biasing is naturally at GND, is that not correct?

2. I believe both PGA280 and OPA1632 can swing its output both side from zero axis? and for no input or zero input, it should stay at zero volt?

3. ADC has one pin Vcom which is AVDD/2 = 2.5V and I saw few schematic connecting Vcom with Vocm of previous stage or connecting 2.5V VREF to Vocm input. What does that mean?

4. ADC takes its power source only single +5V for Analog while its datasheet says the input can be +VREF to -VREF, without a negative supply and for VREF = 3.1V, the actual swing can be +3.1V to -3.1V = 6.2V, while Analog supply is just 5V only. How is that possible? 

Appreciate for clarification of concepts and doubts.