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AD9650 and Internal Reference Connection (Fig 83)

Question asked by jmgilgarcia@mixmail.com on Jul 15, 2013
Latest reply on Jul 23, 2013 by jmgilgarcia@mixmail.com

First thing I would like to apologize if this issue has been solved elsewhere (or if it is not even an issue)... but if so, I coud not find it.

 

I have just designed an AD card based on an AD9650 and I followed the configuration of the figure 83 of the datasheet for the selection of the internal 1.35V reference (pg. 32 rev. A). Basically tie Vref and SENSE.

When I come to check the values measured they tend to be double as expected. I have checked the voltage at Vref pin and its value is 0.673V which would explain why the conversion result is double. But, correct me if I am mistaken, Vref should be 1.35V.

Reading (and not simply watching the figures) seems that SENSE pin should be tied to ground in order to configure the AD for internal reference BUT if the measured voltage at SENSE pin is 0.673V (bigger than 0.2V) (as they both are tied following fig. 83) the AD is not using the internal reference... but something else.

 

Am I mising something or is there something wrong with Figure #83?

 

I have also checked the PCB routes with the test modes and it seems they are ok.

 

On the other side, is there any VHDL code available for capturing big amounts of samples for this AD? May be the ones used for the Evaluation Board? Currently I have managed to read a single sample but i would like a sort of unattended buffer reading through DMA or something similar (kc705 is the target board).

 

Design schematics, board layout or even pictures of the finished card avaliable if required (this aimed to be a proof of concept for something else)

 

Best regards

 

José Miguel Gil-garcía

 

Pd. Sorry If I missed some kind of good practices in the forum with this question.

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