How to determine attenuation divider for Differential input to AD7264?

Question

Hello, 
 
 For our application, we are using AD7264 as input stage. 
 The AD7264 will be fed with four analogue differential channels: 
 
 0 Vdc to 100 Vdc - two channels 
 0 Vdc to 10 Vdc - two 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? 
 
 Best regards, 
 --Utpal

EZAdministrator · Accepted Answer

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jcolao · Answer

Hi Utpal, 
 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.

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How to determine attenuation divider for Differential input to AD7264?