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synchronizing SMPS for ADC sampling noise reduction

Discussion in 'Electronic Design' started by Jamie Morken, Apr 4, 2008.

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  1. Jamie Morken

    Jamie Morken Guest

    Hi,

    I am working on a switcher with ADC sampling at the same frequency as
    the main switcher PWM frequency, they are already synchronized to reduce
    noise when sampling the ADC. There are also several small flybacks
    (10watts at 12Volts output) on the board, I am thinking of syncing their
    PWM as well, but I am wondering if syncing a flyback is the same
    as syncing a forward converter for timing? Is it worthwhile to do this
    or is the noise reduction going to effectively be minimal since there is
    noise all the time in SMPS due to the fast current changes in the
    inductors, or is switching noise really something that is worth taking
    into account even for small switchers?

    cheers,
    Jamie
     
  2. Before bothering with synchronization, make sure you have fully
    applied all the other mitigation measures.

    - Minimise area of switched current loop

    - Minimise area of sensitive circuit loops

    - Use shielded, toroidal magnetics

    - maximise the physical separation (of the SMPS magnetics from the
    sensitive circuit)

    - Make sure the SMPS output is suffiently well filtered
     
  3. legg

    legg Guest

    If multiple converter circuits are constant frequency, its a normal
    precaution to sync them, whether A-D conversion is being attempted or
    not, simply to avoid beat frequency effects in regulation.

    Depending on the number if bits, syncing A-D conversion can reduce
    jitter, but it won't reduce noise-induced error. If it is timed to a
    certain part of the noise period, repeatable error can be reduced.

    Even your humble dual slope A-D converter multimeter uses a conversion
    rep rate that is loosely related to the local mains for reduced LSB
    dither.

    RL
     
  4. MooseFET

    MooseFET Guest

    It is a good idea to sync them.

    On may flyback converters, you are operating them with "current mode"
    controller. It is bad if one converter switches on or off just at the
    time that another is deciding to switch off. It is extra bad if two
    are switching off at exactly the same time. You can end up with an
    small oscillation from the interaction. When you sync them, you want
    to be sure you don't put the converters into that situation.

    In the past, I have solved this issue by using the switch off of one
    converter as the sync pulse for syncing the turn on of the other.
    This puts an unwanted cross talk between the servos of the two
    converters but it does avoid the oscillation issue because it removes
    the feedback path in one direction between the converters.

    If you don't sync the converters on purpose, it can be a good idea to
    make sure that they run at very different frequencies. This pushes
    the beat frequency up high enough that the output filter can reduce
    it.
     
  5. Tom Bruhns

    Tom Bruhns Guest

    Others have mentioned the desirability of not having switchers
    operating on different but nearly the same frequency. I can attest to
    that being a very good idea. I just got bit by it. ;-) Two supplies
    running from the same input bus, physically separated by a few inches
    but "talking" to each other on the input bus were the problem.
    Apparently the control loop in them is noisy (marginally stable?) at
    around 4-5kHz; there's a broad peak in the spectral noise out at their
    outputs around that freq. It's low enough frequency to be impractical
    to L-C filter. But when we got a board with those two switchers
    running at frequencies different by about 5kHz, the 3.3V-output one
    (which supplied the ADCs through some filtering that was quite good at
    the switching freq) had a couple millivolts of 5kHz on its output, and
    that caused sideband spurs at 5kHz on the digitized samples.

    Something not mentioned by other posters, at least as far as I could
    see, is that you can make things much better if you not only
    synchronize them but run them at phase offsets to interleave the
    pulses drawn from the input side. This is a really big advantage.
    Some switcher controller manufacturers such as Linear Technology offer
    parts that do this for you.

    If the current drawn from each supply is essentially constant so that
    the switcher runs with each cycle like every other cycle, then if the
    ADC sampling is synced to the switching frequency, effects from the
    switchers alias to DC, and to the degree they are constant, they can
    be calibrated out (subtracted out--one nice thing about aliasing to DC
    is that you have no phase to worry about!). Another trick is to put
    the (aliased) spur signal in a frequency band you aren't interested
    in.

    But I can also offer you some hope: the board I've been working on
    has several switchers on it, to supply 1.0V, 1.2V, 1.8V and 3.3V, for
    some fairly heavy-duty digital processing. There are two analog -->
    ADC channels that run at about 100Ms/s. I can see spurs on one
    channel at the switching frequencies and harmonics, with max amplitude
    around -120dBm for one of the fundamentals, rapidly trailing down to
    much lower levels. Almost all that is picked up by amplifiers and
    filters in front of the ADC. On the other channel, I see three power
    supply fundamental spurs at -135dBm or less, and practically
    everything else is lost in the noise floor at -155dBm (this with a
    10Hz resolution bandwidth). I believe almost all of what I see is
    being coupled in magnetically at this point. We did kill the 3.3V
    switcher noise by going to a linear regulator down from 5V for the
    critical analog parts; the very slight overall lowering of efficiency
    was well worth the performance gain.

    Cheers,
    Tom
     
  6. Jamie Morken

    Jamie Morken Guest


    I am measuring the current through an inductor using a shunt resistor
    feeding opamp and ADC, the inductor current is up to 55amps with 15amp
    max ripple at 100kHz. The ADC sampling is synchronized to always occur
    right before switching. Is it best to run long traces from the shunt
    to the opamp/ADC, or is it best to put the opamp and ADC as close to the
    shunt kelvin terminals as possible (and thus next to the large planar
    inductor).

    Also I am measuring the output voltage of the inductor synchronized with
    the switching at 100kHz and filtered by a capacitor, using a resistor
    divider to measure the voltage and an opamp/ADC. Is it best to have the
    resistor divider close to the inductor output voltage in this case, or
    also move it away and have a longer trace for the inductor output
    voltage?

    cheers,
    Jamie
     

  7. Ah, I had not realised the ADC was to measure the SMPS itself.

    My experience has been mainly with smaller buck switchers. But quite
    low noise circuits, and the same principles should apply.

    The magnetic field from the inductor will impose a voltage on any
    loops created by the PCB tracks, according to the area of magnetic
    field they see (and the geometry). And the magnetic field drops off
    very quickly as you move away from the inductor.

    There will also be capacitive coupling from the big voltage change on
    the switching node. But the shunt is low impedance (yes?) so we only
    need to consider the magnetic (low impedance) coupling.

    You need to minimise the length of the low-level signal tracking,
    which means putting the opamp right next to the shunt.

    If you are filtering out the ripple, have a passive filter at the ADC
    which can then be far away.

    Hey, I bet you could measure that inductor current using the magnetic
    field picked up by a "probe" track. You wouldn't need the shunt at
    all!

    I made a little H-probe out of a 10mm loop of wire soldered across the
    end of a piece of coax. Connect to a sensitive scope input set to 50
    ohms. If you have a prototype board you might want to make one and see
    what you find - it gives you a nice feel for the magnitude of the
    effects and how the field behaves.
    In general you want to minimise the length of the low-level signal
    wires. So I would move the divider away. Although if it is high
    voltage there might be safety reasons why you would do the division
    locally instead.
     
  8. [...]
    Well, the AC component of it anyway....
     
  9. Jamie Morken

    Jamie Morken Guest

    Ya I thought that sounded kind of like a current transformer.
    I have to measure 60Hz current as well (too low frequency for
    a probe track I think, thanks for the cool idea though, I might
    put one under the ferrite inductor just to see what the scope
    shows :) (probably noise as usual on my cheap USB one)

    cheers,
    Jamie
     
  10. I used my big old Tek 7000 series beast with 7A22 differential plugin
    - goes to ~50uV/division and has a nice switchable filter so you don't
    see the switching noise (just the ripple).

    Perfect for this, also good for looking at the residual ripple on the
    output voltage.
     
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