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From: Charlie E. on 28 May 2010 19:37
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
From: Charlie E. on 28 May 2010 19:38
On Fri, 28 May 2010 16:03:47 -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.)
>>
>> 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.

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

Charlie
From: Joerg on 28 May 2010 20:09
Charlie E. wrote:
> On Fri, 28 May 2010 16:03:47 -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.)
>>>
>>> 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.
>
> Jeorge,
> What do you prefer to do a little boost like this? Do you just roll
> your own?
>

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:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1031,C1060,P2130,D3957

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" :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Joerg on 28 May 2010 20:11
Charlie E. wrote:
> 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...
>

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

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: langwadt on 28 May 2010 20:19
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 ..

-Lasse
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