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Old 14th January 2010, 01:32 PM   #1
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Default Which inductor for IC power supply decoupling

Hi,

I am working on a simple DAC and after reading the article from Guido Tent about supply decoupling I decided that I implement those suggestions.

As I got so far, to finalize the design I had to choose the parts. I need a 47uH inductor. I searched for one. What I found (here in Hungary) is for example a Ferrocore product, with the following parameters:

47uH inductance
Q:50 (test freq:2.52MHz)
Selfresonance freq: 12MHz
DC resistance: 5.8Ohm

As I studied these parameters several questions came to my mind. Guido Tent writes, that the inductor helps to create a local loop by dominating the loop with its bigger value, therefore the resonance frequency will drop and stay away from the possible resonance of the PCB. Better yet, a ferrit bead can be used to introduce some series resistance (about 100Ohm), therefore the Q of the system will be lower.

The questions:
- This inductor has a SRF at 12MHz. That is not so far off from the frequencies used in D/A chips. Isn't that a really bad thing here?
- The resistance parameter is just for DC, there is no data available how this resistance varies with frequency. Guido writes, that 100Ohm at 100Mhz is good. Could be a solution to parallel a 100Ohm with this inductance?

Also, the first thing I can read in the datasheet is: "Special magnetic core structure contributes to high Q and Self-Resonant Frequencies". While I need a solution where Q is low and I am not sure, that in this application the SRF is a good thing.

Any ideas? What kind of inductor should I use for supply decoupling?
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Old 17th January 2010, 08:11 PM   #2
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Just for the record here is what I found after searching further:
- the inductor I found is not recommended in this application. Oscillation can occur as I feared. I got a suggestion to use a Panasonic device (809-4950 - Farnell). That goes to the TORX receiver.
- Supply decoupling for the DAC chip and the receiver can be done with a ferrite bead (and not really an inductor). At least this is what I found at Twisted Pear Audio Opus project. They have about 600Ohm@100Mhz but no inductance is specified. Anyway, it seems to work so probably more than 1uH...(Murata - BLM31PG601SN1L)
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