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From: Joerg on 28 May 2010 20:24
langwadt(a)fonz.dk wrote:
> On 29 Maj, 01:03, Joerg <inva...(a)invalid.invalid> wrote:
>> Charlie E. wrote:
>>> 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...
>> 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).
>>
>> http://ww1.microchip.com/downloads/en/DeviceDoc/21752a.pdf
>>
>> 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 <gasp> 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.
>>
>
> 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 ..
>

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: mook johnson on 28 May 2010 22:07

"Charlie E." <edmondson(a)ieee.org> wrote in message
news:7ik006d1oej6tb30sj0l0bnv46n4mpu363(a)4ax.com...
> On Fri, 28 May 2010 18:06:03 -0500, "mook johnson" <mook(a)mook.net>
> wrote:
>
>>"Charlie E." <edmondson(a)ieee.org> wrote in message
>>news:ire0065ndgpkjvp0ioe94acmcmm2m10pm4(a)4ax.com...
>>> 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
>>
>>
>>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.
>>
>>http://www.national.com/rap/images/noisefig1.gif
>>
>>http://www.national.com/rap/Story/0,1562,18,00.html
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
> 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


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.








From: Joerg on 1 Jun 2010 21:03
Charlie E. wrote:
> 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.)
>

Just curious, did you get this noise issue fixed?

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Charlie E. on 3 Jun 2010 14:14
On Tue, 01 Jun 2010 18:03:52 -0700, Joerg <invalid(a)invalid.invalid>
wrote:

>Charlie E. wrote:
>> 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.)
>>
>
>Just curious, did you get this noise issue fixed?
>
>[...]

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
From: JosephKK on 6 Jun 2010 16:37
On Fri, 28 May 2010 15:43:31 -0700, Charlie E. <edmondson(a)ieee.org>
wrote:

>On Fri, 28 May 2010 15:24:53 -0700, D Yuniskis
><not.going.to.be(a)seen.com> wrote:
>
>>Hi Charlie,
>>
>>Charlie E. wrote:
>>> 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
>>
>>--------------------------------------------------^^^^^
>>
>>Hmmm... units error? "100us" pulse occurring every "100ns"
>>Perhaps you meant 100us occurring every 100ms?
>>
>>> regular. If it was the opamp oscilating, I would have expected more
>>> of either a square or sine wave, instead of just regular pulses...
>Actually, they are every .1ms, and only 100ns wide...

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.
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