STM32 & ADC AD7699: fading SPI MISO level

Jan Bolting Guest 2011-05-04 15:17 #2170478

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Hey guys,

I'm facing a problem working with an STM32F103RET6 and an ADC AD7699
(datasheet: 
http://www.analog.com/static/imported-files/data_sheets/AD7699.pdf).
I thought maybe some of you might have similar experiences and could 
give me a hint. As you can see in the pictures, the signal level of the 
AD7699's SDO (SerialDataOut) pin doesn't drop straight to LOW during the 
last falling edge -though it does looking at the edges before, so I 
suppose it's not an electric issue - but slowly fades to LOW.
Any idea?


best regards
Jan

Attached files:

SPI_fadingLevel.png (2.2 KB)
SPI_fadingLevel_closeup.png (2.2 KB)

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soundso Guest 2011-05-04 16:28 #2170567

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looks like the sdo pin is going tristate (high impedance) at the end of 
the transmission. if so it is a electrical problem.

use a pulldown or pullup resistor on the line for a defined voltage 
level ...

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