When are ferrite beads the most approproate component to use?
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From: mook johnson on 26 Dec 2009 22:33 I've never found an application where a I felt a ferrite bead as head ahd shoulders over a resistor or some other means filtering the signal. Most of the develpments Im involved with are power conversion and data acquisition (ADCs and DSPS)from various sensors. For lowish frequency stuff (say sub-10MHz) in what applications would a ferrite bead be the bees knees? Just want to make sure I'm not overlooking a useful electrical component. thanks guys.
From: Phil Hobbs on 26 Dec 2009 22:43 On 12/26/2009 10:33 PM, mook johnson wrote: > I've never found an application where a I felt a ferrite bead as head ahd > shoulders over a resistor or some other means filtering the signal. Most of > the develpments Im involved with are power conversion and data acquisition > (ADCs and DSPS)from various sensors. > > For lowish frequency stuff (say sub-10MHz) in what applications would a > ferrite bead be the bees knees? > > Just want to make sure I'm not overlooking a useful electrical component. > > thanks guys. > > > One good reason is when the noise of the resistor would reduce the SNR significantly. This happens in applications where the desired signal is at lowish frequency and the instability you're trying to prevent is further out. The bead's loss produces Johnson noise just like a separate resistor, but since it appears in parallel with the inductive reactance, which is small at low frequency, it hardly causes any SNR degradation. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Bill Sloman on 26 Dec 2009 22:54 On Dec 27, 4:33 am, "mook johnson" <m...(a)mook.net> wrote: > I've never found an application where a I felt a ferrite bead as head and > shoulders over a resistor or some other means of filtering the signal. > Most of the develpments I'm involved with are power conversion and data > acquisition (ADCs and DSPS)from various sensors. > > For lowish frequency stuff (say sub-10MHz) in what applications would a > ferrite bead be the bees knees? > > Just want to make sure I'm not overlooking a useful electrical component. The nice thing about ferrite beads is that they have very low DC resistance - much lower than a resistor offering the same impedance at RF - and very low parallel capacitance, comparable with an L-trimmed surface mount resistor, and appreciably less than a helically trimmed axial-leaded resistor. They are also lossy at high frequencies, and tend not to have the embarassing self-resonance that you can get with a wound inductor. Keep in mind that they can have enough inductance to resonate with a filter capacitor at some frequency low enough that the losses in the ferrite don't damp the resonance. I got caught that way once, but early enough in the development process that it wasn't too embarassing. -- Bill Sloman, Nijmegen
From: John Larkin on 26 Dec 2009 23:18 On Sat, 26 Dec 2009 21:33:16 -0600, "mook johnson" <mook(a)mook.net> wrote: >I've never found an application where a I felt a ferrite bead as head ahd >shoulders over a resistor or some other means filtering the signal. Most of >the develpments Im involved with are power conversion and data acquisition >(ADCs and DSPS)from various sensors. > >For lowish frequency stuff (say sub-10MHz) in what applications would a >ferrite bead be the bees knees? > >Just want to make sure I'm not overlooking a useful electrical component. > >thanks guys. > > They're great for keeping high frequency stuff out of opamps and such. They have low DC resistance, so don't mess up low frequency circuits, and they have miserable Qs, so kill resonances. ftp://jjlarkin.lmi.net/Ferrite.JPG This bead reduced the RFI sensitivity of a thermocouple front-end by 20 dB or so, specifically by killing some wiring resonances in the 100-300 MHz range. Surface-mount beads are also good, cheap power supply decouplers. Most of them act like lossy inductors in roughly the 5 uH range. John
From: Tim Williams on 27 Dec 2009 01:33
"mook johnson" <mook(a)mook.net> wrote in message news:4AAZm.358$yy2.59(a)newsfe01.iad... > For lowish frequency stuff (say sub-10MHz) in what applications would a > ferrite bead be the bees knees? AFAIK, none. They're for the >10MHz range, and ~100MHz decade especially. That doesn't mean you don't want them; if your circuit is sub-10MHz and you want to *keep it that way*, you might need some LCs for filtering. Story: the college radio station antenna is right on top of my dorm. You know, that station that no one wants to listen to. Well, despite being FM, it's forcing its way into my powered speakers. Either there's enough AM on it as-is, or it's so strong that it's being discriminated despite the low efficiency. Whatever the cause, I added ferrite beads and ceramic caps on all inputs, which shut it up quite excellently. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |