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AD5380: first time SPI data transfer, giving unexpected results.

Question asked by GOG on Jun 29, 2017
Latest reply on Jul 6, 2017 by GOG

I am using a combination of 3 AD5380, all in stand-alone mode (control via SPI), to get desired voltage levels on any of the channels of this DACs. The SPI has different slave-select lines to select one of the 3 DACs for communication.

 

The SPI communication works as expected, but not for the first SPI transfer.

 

The first SPI transfer actually set the same channel on all the 3 DACs. There are actually 2 issues here

 

(a) in SPI: when I initialize my SPI-SW module for the first time, due to a HW feature!, all the salve select lines are pull low for a very short duration of 5us. This cannot be avoided.

 

(b) in AD5380, now since all the SYNC-N lines were pulled low for 5us, somehow all 3 DACs "become ready" to accept the command on DIN that would follow (note: the SCLK are also shared). Although the command would be targeted at only one of the DACs (whose slave select would be pulled low and the corresponding DACs SYNC-N pulled low), due to this strange issue in AD5380, the remaining 2 DACs also accept this command. Hence in all 3 DACs, that one channel (specified in the 24-bit SPI command) are set to a same voltage. This is not desired.

 

Can you please confirm if my observation is correct. From the data sheet, the value t6 says should be 33 ns (in Standalone), so this actually confirms my observation is correct; but I am not sure. If my observation is incorrect then I have to further look into my HW design if something nasty is happening, because other than this single issue, everything else seems to work fine.

 

Your help will be much appreciated. Thank you.

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