Noise on power supply net

Hi All, Ok, more on the dreaded color reader problems... ;-)

Now that I have the scope, been troubleshooting this thing a bit. One thing I am seeing is that there is a lot of noise on my power net. While the chip's specs say it should have a ripple of around 40mVs, I am also seeing a noise signal around every 100ns of 200mV p-p. I haven't been able to figure out what is causing it. It is there even when the mcu is in break or asleep, so it doesn't seem to be from therre. The LED power supply is shut down, so it shouldn't be from there. The only things power up are the opamps and the digital pot.

I have a 10uF next to the PSU chip, another 10uF on the same rail by the other PSU chip. I have a full copper pour for both ground and Vcc, and 1uF bypasses next to each chip (and two by the MCU.)

So, any ideas? That noise pulse is only around 100us wide, but fairly regular. If it was the opamp oscilating, I would have expected more of either a square or sine wave, instead of just regular pulses...

Thanks, Charlie

Reply to
Charlie E.
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Hmmm... units error? "100us" pulse occurring every "100ns" Perhaps you meant 100us occurring every 100ms?

Reply to
D Yuniskis

Actually, they are every .1ms, and only 100ns wide...

Reply to
Charlie E.

Since you had clarified that it's 100usec repetion rate and 100nsec pulse width a suspicion arises: The charge pump in idle mode, and you've got one in there that supplies the board (U7).

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Those things are hysteretic and theoretically should have more than one pulse but who knows. Oh, did I mention that I don't like charge pumps? ... :-)

One trick to figure this out is to make two probes. One a coax with about 1/4" center conductor exposed and BNC on the other side. Wrap some tape around the tip so you can't touch and bzzzt anything. The other is the same but 3-4 turns soldered to the tip, maybe 1/4" diameter. You can take a 6ft-8ft BNC cable, cut it in the middle and make those two probes. Now plug them into your scope and hunt. Once you found the culprit keep those probes, Murphy says you'll need them again. At least I do, so I bought some rugged commercial ones.

Sometimes the results are rather surprising. Once at a client I found the source of similar stuff to be a Tektronix TDS220 scope. In another it was an unmarked huge building across the street, with armed guards and lots of antennas on the roof.

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Regards, Joerg

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Reply to
Joerg

Is there any thing running at 10KHz? Clock source? ISR?

When testing for noise, make sure you don't have a long ground lead that can pick up noise. I take of the grabber clip and take off the alligator clip ground lead. Then wrap some bus wire around where the ground clip was making contact. If you want to see actual noise out of a power supply. Use this to probe directly across the output cap of the supply. Then probe across the power leads of each chip. If the pulses are more pronounced by one chip, that is likely the chip.

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Reply to
mook johnson

Good ideas. My scope has these little springy thingies that go on the end of the probe for that. Using those, the noise is down to only

100mA at all the chips, and I don't see the big, regular spikes anymore...

Wonder if I was picking up my laptop sitting next to it...

Charlie

Reply to
Charlie E.

Jeorge, What do you prefer to do a little boost like this? Do you just roll your own?

Charlie

Reply to
Charlie E.

When it has to be very cheap and it's for a mass product I often roll my own but that costs a lot of engineering time. Otherwise I try to find something reasonable that can be simulated and that generally boils it down to a chip from LTC. Not sure about your requirements and I haven't looked in detail, maybe this one:

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Two things are important in a noise-sensitive application:

a. It should not have hysteretic control, causes too much ripple and thus noise.

b. Should ideally not have burst mode. While that reduces idle power it does increase noise, often big time.

So in the example you should prefer the B version because that does not have burst mode. Beats me why they called it "B" :-)

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Reply to
Joerg

100mV? That would still be a lot. In your application a whole lot.
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Regards, Joerg

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Reply to
Joerg

e
y

yep rather annoying to spend hours trying to find a mysterious source of noise only to find out that turning off the scope makes it disappear, tried that too ..

-Lasse

Reply to
langwadt

Most annoying was an Agilent spectrum analyzer of noble heritage and corresponding price, for EMC analysis, that produced ... you guessed it ... EMI. Lots of it, from its display. I had to ask a client engineer where the next hardware store was. Bought some chimney spark screen and draped it over the whole thing.

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Regards, Joerg

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Reply to
Joerg

Hmmm...first you had 40mV of ripple with 200mV p-p of spikes.

Now the spikes are gone but the ripple has gone up to 100mVp-p?

Or has the spikes reduced in amplituse to 100mVp-p?

At either rate you're are now looking at something more like the real noise.

Reply to
mook johnson

Just curious, did you get this noise issue fixed?

[...]
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Reply to
Joerg

Hi Joerg, Not yet. I at present have put a .1uF cap on the net, and just now tried replacing one of the 1uF bypasses by the MCU with a 47uF tant. It reduces the noise to what appears to be a 20-30KHz sawtooth about

60mVs high. I think I will probably redesign to a chip like the one you suggested using an inductor for the next version.

My next effort is to try averaging a lot of samples (maybe 20-30) to see if that stabilizes the measurerd value.

Charlie

Reply to
Charlie E.

One

fairly

Sounds like the MCU to me. Try adding one or two 1000 pF (yes picofarad) caps just as close to the MCU power/ground pins as possible.

Reply to
JosephKK

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