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Im in a project, where I will have some noisy digital IC's, and a analog sensor circuit that I want to be a little bit noise free -
Im using a buck regulator to take in some power from a Lipo battery.
For when splitting the supply to analog and digital, how /what would i select for connecting the 2 sources to the buck converter? -
Can you use one of those "EMI filter" beads, which have a built in filter, or how would I select an good ferrit bead for this application? Thanks for your time
You mean that you want the switching noise on the supply made by the DCDC converter (buck regulator) and the digital circuits not to reach the sensitive analog circuits?
Then I would personally ignore EMI filters!
Well, OK, I would only think about EMI filter after at least implementing the suggestions I give below.
Why?
Because these are only reasonably effective at high frequencies, think above 10 MHz or so. Also, the higher frequencies do not tend to travel so much through the wires as through the air!. A wire is about 1 nH (nano Henry) per mm. So a long wire is already an obstacle for any RF signal.
The DCDC converter will switch at a much lower frequency (usually between 50 kHz to 2 MHz) so EMI filters will only suppress part of the noise. For the digital the same is true, it can generate many frequencies also below the range where EMI filters have much effect.
Here are some tips that are more effective:
Choose a ground point and implement a star grounding scheme keeping the digital and analog grounds connected but minimize them influencing each other. If you share the same ground than the return currents cause local, small voltages in the ground. Your sensitive analog IC might not like this. The solution is separate ground wires /connections so that a circuit only "sees" the ground voltages that it has created itself and not the voltages from ground currents from other circuits.
Bypass and decouple your supplies as close as possible to the source of the noise. This keeps the (current) loop for high-frequency signals short and minimizes the chance of noise "escaping".
Use multiple capacitors of different values in parallel for bypass and decoupling. Capacitors are not ideal, to make more ideal ones use multiple, see EEVBlog Dave's video on the subject.
For the analog circuit, consider using a dedicated LDO (non-switching voltage regulator) to make the supply for that circuit only.
What you do is put 0 ohm resistors here and there in the supply lines, then if later it turns out that there is a high-frequency issue you have the chance to add a series resistor or EMI filter (or just an inductor) to solve the issue. Also add (the option) to add a capacitor to ground on both sides of that zero-ohm resistor. No need to add a capacitor just yet but when you do need it, adding one will be easy.
You can filter the voltage going to the analog IC using an RC (if the current is not too high) or a ferrite and capacitor. Ideally, you would have some idea what you are trying to filter, or what you can allow to pass in order to design a filter. Sometimes a ferrite and capacitor can resonate with fairly high Q. So you have to be careful about that. I've never actually seen this happen, but it seems plausible and I have been told that it can happen.
Another approach that can work sometimes is to power the analog IC with an LDO powered from the buck. This would only work if the analog IC can be operated at a lower voltage. The idea is to let the power supply rejection ratio of the LDO be a filter.
Separating supplies can be good, but I recommend you do not separate the grounds.
It might be helpful if you tell us a bit more about your analog sensor circuit. What is it sensing. How sensitive is it, really?
