Is there an existing spice model for the ad9958? Or do i haye to use the ni multisim tool for spice simulation?
Unfortunately, the ADI DDSs doesn't have have available SPICE Models. As mentioned by Jeff Keip in this link: http://ez.analog.com/message/21328#21328 - "SPICE models have the potential to expose important intellectual property in many instances, so for competitive reasons we chose not to offer them." This also applies to the DDS products.
You can download the list of ADI products with SPICE models in this link: MultiSim: Spice Models Support
Regarding DDS simulation, what we have is ADISimDDS to aid the user in selecting and evaluating ADI DDSs.
Hope this helps.
You will typically only see SPICE models for relatively simple analog-type components such as amplifiers, voltage references, etc. Components with a significant digital aspect, like our DDS parts, will not have SPICE models. Even our ADCs and DACs do not have SPICE models, although we have tried looking into how to make SPICE models for them.
Also, SPICE is generally only suitable for frequencies up to a few hundred MHz. Anything above this, specifically RF stuff, you would need S parameters and use a program specifically designed for simulating at RF frequencies.
For digital interfaces, there are IBIS models. I don't know much about them but I believe they basically verify & test the logic levels of say a SPI interface or GPIOs and do not actually model the parts behavior, either.
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