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AD7765 128x Decimation Not Working

Question asked by ALaney on Feb 23, 2018
Latest reply on Feb 27, 2018 by ALaney

I have a board that is daisy chaining 4x AD7765's and have it happily working in DEC_RATE = 0 (256x), BUT I cannot get 128x to work correctly.

 

Q1: What level do I set DEC_RATE to 128x? Do I drive it high or set it floating?

 

Here is a trace of it running in 128x where I have driven DEC_RATE = high

 

 

 

 

 

The bottom 4 signals contain the 4 channels parallel data where the LS status byte is set to 98'h

 

From AD7765 data sheet:

 

 

FILTER-SETTLE = '1', DEC_RATE 1 = '1', Don't care = '1' = 98'h

 

So, the 4x ADC's appear to think they are in 128x decimation.

 

Q2: Why is the NFSO timing incorrect? (i.e. the same as 256x decimation) Unless I misunderstand, NFSO should be running at double the rate. (i.e. no gap between CH1-4 sample bursts)

 

Note: If I set DEC_RATE to float I get 90'h (i.e. Don't Care = '0')

 

Thanks for any help!

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