I'm trying to select a ferrite bead for the SAMD21:
http://ww1.microchip.com/downloads/en/DeviceDoc/40001882A.pdfOn page 856 it says:
The bead should provide enough impedance (e.g. 50? at 20MHz and 220? at 100MHz)
for separating the digital power from the analog power domain. Make sure to select a ferrite bead
designed for filtering applications with a low DC resistance to avoid a large voltage drop across the
ferrite bead.
Finding a bead that fits this description is easy enough if a select an 0603 part, but I was hoping I could go one smaller and use an 0402 here because I need the room. However at that size I'm having trouble finding an exact match.
So I'm wondering how crucial these parameters are and how far I can push them?
I assume that a higher DCR will result in a voltage drop, but since the max current the AVCC pin can accept is 60mA even if I double the DCR of the bead, the voltage drop will still be only .006V. I would imagine the voltage drop could be 10x that and still not cause an issue. And that's a worst case scenario. I don't imagine the current draw on AVCC would ever approach 60mA in most realistic usage scenarios. This is a general purpose board, so I don't have a specific usage scenario in mind here to design around. If I were to assume 20mA then that 50mOhms could potentially be 30x higher... 1.5ohms, and not cause an issue. And even if AVCC was drawing 60mA I still don't think 1.5ohms would be a problem.
But I don't know for certain.
On the flip side, I don't know how much more is okay on the impedance. 220ohms impedance at 100MHz is what they mention, but is 400 okay? And what about 1K? I assume if I dropped it to 100 it would still be okay but give slightly noisier readings. But it would still be far better than no ferrite at all.
I selected all the 0402 beads with less than 500mOhm DCR and 150-1K impedance:
https://www.digikey.com/short/qqhc72Here for example is the least expensive one which has a lot in stock. It's 500mOhms DCR and 600 ohm impedance @ 100MHz:
https://www.digikey.com/product-detail/en/taiyo-yuden/BK1005HM601-T/587-1839-1-ND/1465309Is that okay? Or would it perform really poorly?
If I choose to sort by power line, then three good contenders with 150-220 mOhm DCR and 220-330 ohm impedance rise to the top:
https://www.digikey.com/short/qqhc3dBut I don't know if this really counts as a "power line" when it's 60mA max.
So, thoughts? What range of values should I be considering here?