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The Larkin front end

Discussion in 'Electronic Design' started by martin griffith, Nov 6, 2006.

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  1. Hi John
    Just a few questions from the ABSE post

    what are the typical values of L4 and L5, I guess 100uH?
    do you ever use a common mode i/p filter?

  2. John  Larkin

    John Larkin Guest

    They are surface-mount ferrite beads, I think rated 300 ohms (at 100
    MHz, which how they usually rate beads.) Beads are nice because the Q
    is so low, they pretty much never resonate.

    I haven't ever use a common-mode filter for stuff like this. The
    intent was to just squash any RF, common mode or differential.
    Nowadays, cell phones and things are everywhere.

    One of my guys has an older, low-band cell phone, and whenever he
    walks into my office my PC speakers make horrible raspberry noises.

  3. Guest

    If only that were true. I had a ferrite bead in one of my designs that
    had an inductance of 1.6uH, and resonated nicely with a 100nF capacitor
    at 400kHz, with a Q of about 5.

    I ended up having to add a wound 100uH SMD inductor in series, with 22R
    of damping resistance. Dead embarassing. The quick and dirty fix was
    4u7F tantalum capacitor in parallel with the 100nF ceramic - the ESR of
    that tantalum did all the damping required, but the exta capacitance
    chewed up current whenever we intermittently activated the low-power
    system involved.
    In my day it was the electron beam microfabricator that wrote the wrong
    stuff on a $2,000 mask if the security guy went down the corridor with
    his two-way radio. Fixable, eventually.
  4. On 7 Nov 2006 01:53:53 -0800, in

    Hi Bill
    do you mean the 22R was in parallel to the inductor?

  5. Guest

    I didn't specify, but it was in series with both the inductors (who
    were also in series).

    The whole thing was intended to keep the switching spikes from a
    comparator out of the the op amp that was providing one of the inputs
    to the comparator - always worth doing, and particularly so in this
  6. Joerg

    Joerg Guest

    Hello John,
    Might not be a problem with most SMT versions but it is with thru-hole:
    They can exhibit have mechanical resonance. Or in more popular speak a
  7. Rich Grise

    Rich Grise Guest

    Hey, there's something that'd never occurred to me - if the core is loose,
    it will actually become like a little motor, right? :)

  8. John  Larkin

    John Larkin Guest

    Some years ago, the head of R&D for a big NMR company called me and
    asked if we wanted to make sample temperature controllers for them.
    They'd been using units from Oxford Instruments for years, and were
    having problems, one of which was astounding EMI sensitivity (the
    other problem being that they had apparently lost the Z80 source code,
    and couldn't fix bugs!)

    I got one of the Oxfords, and found that I could shut it down from
    across the room with one of those old GR unit oscillators driving a
    banana lead antenna. There were multiple, very narrow bands in the
    150-400 MHz range where it was extraordinarily sensitive. Turns out
    the sensitivity was hugely enhanced by internal wiring and PCB
    resonances. This is the range where beads work well.

    We built a new controller with much better layout and bypassing, and
    it was about 20:1 less sensitive. When we ran the internal
    thermocouple leads (t/c connector to pcb) through a double-hole
    ferrite bead, we got another 10:1, presumable because it blocked entry
    and killed the Q of that path. We've shipped close to 3000 units by

  9. Joel Kolstad

    Joel Kolstad Guest

    I wouldn't be surprised if, even with all this, your build cost is still less
    than The Big Guy's.
    Congratulations! :)
  10. Guest

    Actually, that is a minimal balun, and gives you good differential
    rejection at frequencies where it has a significant impedance.

    If you want more turns, you can use a small ferrite torroid - if you
    confine the wires to a single layer and space them at about their own
    diameter, you are supposed get the maximum availalbe resonant
    frequency. I've not had occasion to try it yet, but the theory seemed
    to make sense.
    Nice work.
  11. mark

    mark Guest

    Any good articles re EMI suppresion you can point us to? Which authors do
    you feel do a good job explaining the problem and solutions?
    (I'll have to remember ferrite beads next time I come across a narrow band
    sensitivity. Thanks for the tip)
    M Walter
  12. Eeyore

    Eeyore Guest

    Ferrite beads are rubbish for EMI on differential inputs since their values are
    never matched and the resulting imbalance can cause more trouble than previously

    I know because I've run real tests.

  13. John  Larkin

    John Larkin Guest

    Does that mean I have to give all the money back?

  14. Guest

    That why you can buy two-hole beads, not to mention six hole beads,
    rings and tubes - I doubt if the ferrite is perfectly uniform within
    these parts, but the matching is a lot closer than it would be from
    part to part.
  15. John  Larkin

    John Larkin Guest

    I don't understand this "matching" thing. If you lowpass filter both
    sides effectively, you're way better off, even if the lowpass
    filtering isn't exactly the same on both sides. The transistors in the
    front-end of an opamp are essentially square-law RF rectifiers. If you
    have 100 mv of RF, they will rectify it big-time. If you have 1 mv,
    they barely will at all. 100^2 = 10000, and 10000:1 is usually
    considered to be a pretty big improvement.

    The really insidious RF is in the 100+ MHz range, where amp internal
    feedback and bias mechanisms can't track, and things go nonlinear...
    every junction drives stray capacitance to form a peak detector.
    Ferrite beads attenuate the RF and kill resonant Q's.

    Wound inductors can be useful, too, but you've got to make sure they
    don't resonate anywhere dangerous, make sure their parasitic
    capacitance doesn't let hf stuff blow through, and make sure they're
    not hum detectors.

  16. Eeyore

    Eeyore Guest

    You're not as smart as I once thought.

    Consider the inherent rejection of common mode signals in a differential front end. It
    doesn't work so well if the signals are different sizes ( rather obviously ).

    I'll use 1% caps here too.

  17. SioL

    SioL Guest

    I think I understand John.
    If the first stage is a balanced opamp input, its a biq question how "balanced" its function
    is as a rf detector, the interference signals are likely way out of the operational range
    of that opamp. Thermocouple sounds like a very slow device. Pretty much killing
    anything even remotely RF should work pretty well.

  18. Eeyore

    Eeyore Guest

    A ferrite bead won't do that.

  19. John  Larkin

    John Larkin Guest

    If you're referring to capacitive imbalances turning common-mode stuff
    into differential, that's a different issue. In the case of load
    cells, with ballpark 200 ohm drives and 0.1 uF caps, and a deep 50/60
    Hz notch in the delta-sigma adc response, it's not an issue at all; do
    the math. The ferrites are dead shorts at these frequencies. With a 25
    mV full-scale load-cell signal, I can weigh things to 5-digit noise
    and linearity with *unshielded* leads.

    Line rejection is a trivial issue here, something obvious that simple
    math can handle. RF rectification is far more insidious, less
    predictable, and more dangerous (a little AC noise is nothing compared
    to hard railing the ADC) and what you see on the schematic is just
    part of the story.

    Another thing I don't understand is why you insist on calling me
    stupid when you disagree with me. I've never called you stupid.

  20. John  Larkin

    John Larkin Guest

    The only complication being that it does.

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