Issue link: https://iconnect007.uberflip.com/i/1256432
JUNE 2020 I DESIGN007 MAGAZINE 43 ance shown by the filter at its output, we short the filter input and connect a one ampere AC current source across the output. With 1A cur- rent, the voltage will be equal to the output impedance. In general, using a 1A swept-fre- quency AC current source is a convenient way to simulate impedance for any linear network. Once we see the performance of the filter suggested in the application note and under- stand the potential risk generated by the peaks in the transfer function and output impedance, we understand what is behind the claims of people who say that ferrite beads should nev- er be used in PDN filters and should not fol- low the advice of application notes. We can ask why those peaky filters are suggested in the first place. Were those circuits not tested on a real board? Most likely, they were, but we need to see the motivation and the constraints behind those efforts: People creating the ap- plication notes for their chips have no way of knowing the many different possible applica- tions their users will come up with. The testing is done in an evaluation board on the bench. This immediately removes most of the external noise and leaves only the noise that is created by the circuit being tested. If the self-generated noise is acceptably low, the circuit will prop- erly function, though we could argue that in a low-noise environment, we would not need such a filter in the first place. We can also see that if we design the filter properly, it should work fine, and it gives such isolation benefits that would be much harder to achieve without a series inductive element. In this short column, I have not looked at the other benefit of using ferrite beads as opposed to regular inductors: Ferrite beads have an AC series loss that increases with frequency sharp- er than what we get from regular power induc- tors, and it further helps blocking the propa- gation of high-frequency noise. If you are in- terested in the full process how to design PDN filters with no peaking, you can view my pre- sentation on "How to Design Good PDN Fil- ters" [4] . DESIGN007 References 1. Alex Waizman, et al., "Novel Isolation Scheme for Miti- gating PDN Coupling," DesignCon 2018. 2. C. Zhang, "Analysis and Design of Input Filter for DC-DC Circuit," Texas Instruments Application Report, SNVA801, November 2017. 3. Murata SimSurfing tool. 4. I. Novak, "How to Design Good PDN Filters," Design- Con 2019. Istvan Novak is the principal signal and power integrity engi- neer at Samtec with over 30 years of experience in high-speed digital, RF, and analog circuit and system design. He is a Life Fellow of the IEEE, author of two books on power integrity, and an instructor of signal and power integrity courses. He also provides a website that focuses on SI and PI techniques. To read past columns or contact Novak, click here. Figure 6: Output impedance with a lossy capacitor added.