App note inductor size

[paul23]

I have found an app note (I think someone linked to it on here for somebody else) for an AVR single phase power meter.  The app note itself is very good and does a good job of explaining things.  However, in the BOM they haven't listed the value of inductor L50, which is on the schematic (see attached) and don't mention it in the app note.  How to I work out what the value should be?  I assume it is just there to filter out any noise between the analogue and digital supplies?

I have modified the circuit a bit from the original.  I have used a transformer to isolate and step down the voltage, rather than transformerless design Atmel use.  I have also added a couple of extra ranges on the gain switches and done away with the counter and anti-tamper stuff, I don´t need that.  So I am just reading the voltage/current on the live.

the attached image is Atmel´s original design from app note AVR465

[wraper]

It can be something like 10uH, yes it's for noise filtering.

[paul23]

Thanks, that´s what I thought

[c4757p]

Aack!! What a sloppy schematic! Kind of puts me in the mood for spaghetti...

[CarlG]

My guess is that they suggest to put in a ferrite bead there, perhaps not that critical. You'll probably get some 2x mains switching noise on Vcc that you should try to get rid of, and maybe hf noise from the uC that needs to be filtered out.

I my schematics I use specific bead symbols, and indicate the impedance at 100 MHz just to have a clue.

[c4757p]

Actually, IIRC, many of the AVR datasheets recommend a plain 10uH inductor.

[alm]

What would the advantage of a plain inductor over a ferrite bead be in this application? Is a higher Q really an advantage for wide-band filtering like this?