For our application, we are using AD7264 as input stage.
The AD7264 will be fed with four analogue differential channels:
Datasheet defines two specific values for Vcm range
2 V for PGA = 1
(AVcc / 2) for 2 <= PGA <= 128
1. How to identify the "Vcm" run-time?
2. How to design the incoming differential signal attenuator if no Vcm is specified?
3. If Vcm is specified to be zero, how the ADC would handle -ve signal range as per the equation:
Analogue input range = (Vcm) +- (Vref / (2 x Gain)) and single +5 V supply operation?
I would like to clarify on the input voltage of the application is maximum at 100V? Please take note on the abs max rating for the AD7264 from -0.3 to AVcc+0.3V (AVcc = 7V).
The AD7264 is a true differential analog input ADC, this means that the input signal is 180 degrees out of phase. for example, At Vref = 2.5V the typical voltage range for the AD7264 is 2.5V. The common mode voltage would be at 1.25V or 3.75V ( Vcm = Vrange+/-(Vref/(2*gain)).
As specified on the datasheet, the AD7264 has common mode voltage range and going beyond this range cannot guarantee product performance. Having a no Vcm or Vcm =0, for the example above at 2.5V range, this would mean that the input signal is from -1.25V to 1.25V. Having this signal range violates the Abs max rating of the part that may cause damage. There are circuit that converts a unipolar single ended signal into a differential signal. Like as an example, on the Figure 26 of the AD7265 datasheet ( http://www.analog.com/media/en/technical-documentation/data-sheets/AD7265.pdf) it uses the AD8022 to configure to differential signal.