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From: Charlie E. on 28 May 2010 18:03
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
From: D Yuniskis on 28 May 2010 18:24
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...
From: Charlie E. on 28 May 2010 18:43
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...
From: Joerg on 28 May 2010 19:03
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.)
>
> 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.

--
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 19:06
"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











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