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AD7682 in differential mode driven by a PGA

Question asked by techfreakz on Sep 10, 2012
Latest reply on Sep 11, 2012 by KarenNE

Hello,

I'm designing a converter to sample the input of an RF detector and convert this to a 16 bit value.

 

I'm planning on using the AD8253 as a PGA/buffer with it configured to provide differential outputs with the help of an AD8675 opamp.

 

The common mode voltage of these outputs is set to be half of the ADC full scale reference voltage (Vref/2 = 1.25V).

 

For the ADC function the AD7682 looks good but i'm struggling to understand the datasheet when it comes to differential operation.

 

The datasheet states the converter can operate in differential mode.  As such I image that the inputs can both swing around the common mode voltage as long as they both stay within 0 to Vref (2.5V in may case).  However, on page 4 of the datasheet it implies that the positive input can swing between 0 and Vref, but the negative input is fixed at either 0V or Vref/2.

 

Can you please clarify this point as i'm very confused and perhaps provide a little guidance?

 

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Thanks in advance,

 

TFZ

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