[DIYbio] Re: [plots-spectrometry] Re: [dorkbotpdx-blabber] Open-Source Spectrometer

Thanks Dave for the kinds words, suggestions and tips!

I hadn't thought much this time around regarding trace widths... the
analog section has its own LDO tied to the USB + after it passes by
some caps and an inductor at the microUSB port. The FTDI has a
regulator built-in, and the processor, level shifter (which I might
not need, I can't remember but it was on a Propeller demo board that
also had an FTDI), EEPROM and ADC digital lines are all running off of
another LDO (same type as Analog section uses). The ADC and processor
and FTDI all have their caps super close (as much as I thought I
could) and have the smallest-value cap closest to the IC, and am
making sure the power line passes under the cap mount on its way to
the pin (I don't have the caps on a side-leg of the power trace, like
a T with the caps on one side and the right side going to the pin, the
bottom coming from LDO).

I also didn't do any signal length matching, I tried to do this for
the USB data + and -... but from the FTDI to the processor was
autorouted (since I'm not actually needing USB high-speed, I think,
right now at least). I'd like to get the ADC control lines and CCD
control lines all length-matched... my thoughts are it will reduce
sampling error/noise and maybe remove clocking/control-line jitter. I
know the clocks/controls are relatively slow compared to radio or
other matched-pair interfaces... but I'm trying to watch all angles
for a good/robust overall design.

Thanks again!

P.S. here's a link to the project I posted on hackaday, it has some
schematic/PCB pics (they're in the github repo too) for easy viewing:
http://hackaday.io/project/1342-open-Spectrometer


On Wed, Jun 4, 2014 at 3:12 PM, <dstoft@gmail.com> wrote:
> Nathan, Congrats on the design and build -- that's a lot of work.
> On the noise/grounding/4-layer -- The only need to go 4 layer would be if
> you (later on) wanted to physically shrink the layout and put parts on both
> sides of the board to make a really compact module. I don't have the layout
> software for your design to look in details, but 2-layers is cheaper and
> quite sufficient in terms of noise. The more important issue is impedance.
> (I've done a lot of RF design.)
> You want very, very low impedance for bypassing (DC) so short distances for
> SM caps and avoid long ground lines and 'loops' -- which it looks like you
> are doing. Mohm impedance signal lines (ADC input?) can be isolated by
> ground area but avoid adding a lot of extra capacitance to ground by
> trace-length over/next to ground -- it looks like you've probably done ok.
> The default 8-mil trace widths of many layout programs is an inherent
> pitfall for RF design so use wide traces for power, grounds, etc.and say
> 15-mil or so for important high-impedance signal lines -- though it sounds
> like you've kept them short which is good.
> Finally, use a super-clean DC supply or add secondary on-board regulation
> for an added 30+ dB supply noise immunity. Wall-warts can be rather noisy
> and infect your design but local bypassed regulation can do wonders.
> As you likely suspect, noise will probably not be your biggest issue first
> time around. Good luck!
> Cheers,
> Dave
>
>
>
> On Wednesday, June 4, 2014 2:40:56 PM UTC-7, Nathan McCorkle wrote:
>>
>> I used KiCad, did the analog stuff and some more by hand, then pulled
>> it into the http://www.freerouting.net/ autorouter for the rest.
>>
>> I didn't want to go 2-layer as all the low-noise analog design stuff I
>> read seemed like enough reason to go 4-layer. I think I might break
>> the board into two, which would let me choose different number of
>> layers for each, if 4 are really needed.
>>
>> I'm also not sure if I need the datasheet-recommended audio
>> transistor, since I didn't seem to noticed any loading effect when I
>> connected my o-scope with and without the transistor. The way I've
>> arranged the analog section, if I remove the transistor there's about
>> 2 or 3mm between the Analog output and the ADC input... while I
>> believe the datasheet may be covering cases where the CCD is in a
>> hand-held barcode reader and several feet from the rest of the
>> circuitry. I could be wrong though, so its in there for now.
>>
>> Like I said earlier, I'm not even using the second inner layer, just
>> top and bottom, with an unbroken ground plane (this is what I really
>> was going for, I didn't want to cut into the plane, as a lot of analog
>> design stuff recommends that over the tying two plane sections at the
>> ADC).
>>
>> --
>> -Nathan
>
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--
-Nathan

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