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AD9649 pseudo-random sequence

Question asked by milosoftware on Nov 27, 2015
Latest reply on Dec 3, 2015 by milosoftware

I wanted to use the PN9 sequence built into the AD9649 to evaluate my digital path. In particular, I want to use it to determine that I don't "lose" any samples on the way (e.g. DMA overruns).

 

I'm having trouble determining how to properly interpret this though.

 

My simple thought was that I could just put the ADC into PN9 test mode, and read a bunch of samples. The first sample should repeat every 73 samples (propery of the PN9 sequence), which it usually does, but not always.

 

Since the data is actually a video signal, I can easily see that it never misses any samples when capturing about a megabyte. But when in PN9 test mode, I often see sequences that aren't a repetition of the same 73 values.

 

Just to make sure, is the output of the PN9 sequence really deterministic?

 

Could it be that the timing in the digital path isn't quite correct, and how could I diagnose that?

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