A fews months ago I had some Problem with the SPI communication between an AD7176 and an µC Atmega8. To be able to read faster and simultaneously do others tasks I upgrade my system with an FGPA.
I have the Evalution Board Nexys 4 from Digilent, here you can find the documentation from it. The CS is tied to ground because I just want to read 1 AD.
The Problem is that I need to decouple the MISO pin to ground with an 220pF capacitance so that it works !!!!
By disconnecting the MISO from the FPGA , I monitored the SIgnal from the AD and it send the good signal! And I dont understand why I need this capacitance.
The IO-pin from the FPGA are with an Zener Diode protected! does somebody have an Idea if this zener diode a problem is for the AD7176? Does someone have experience with it?
Thanks a lot!