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Meindert Sprang
Guest
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 Posted:
Fri Dec 02, 2005 5:15 pm Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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"joep" <bungalow_steve@yahoo.com> wrote in message
news:1133534768.519247.103980@f14g2000cwb.googlegroups.com...
| Quote: | In general I don't think mixed signal device manufacturers recommend
separation of AGND and DGND, so I'm not sure where everyone is getting
this advice from
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Well, what about the datasheets of the components?
Here is a quote from Analog Devices (AD9952 DDS):
--------
Layout Considerations
For the best performance, the following layout guidelines
should be observed. Always provide the analog power supply
(AVDD) and the digital power supply (DVDD) on separate
supplies, even if just from two different voltage regulators
driven by a common supply. Likewise, the ground connections
(AGND, DGND) should be kept separate as far back to the
source as possible (i.e., separate the ground planes on a localized
board, even if the grounds connect to a common point in
the system).
--------
Meindert |
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Bo
Guest
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| Posted:
Fri Dec 02, 2005 11:19 pm Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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"joep" <bungalow_steve@yahoo.com> wrote in message
news:1133534768.519247.103980@f14g2000cwb.googlegroups.com...
| Quote: | In general I don't think mixed signal device manufacturers recommend
separation of AGND and DGND, so I'm not sure where everyone is getting
this advice from
This is the best collection of articles I have seen concerning the
problem of mix signal devices and grounding, from the horses mouth
(analog devices)
http://www.analog.com/UploadedFiles/Associated_Docs/217348021sect7b.pdf
For mixed signal devices you should connect the digital and analog
ground to the analog ground with the shortest leads as possible.
|
Where to make this connection? Especially if you have multiple devices that
require both AGND and DGND?
And a 'direct' connection? ie I've had some old timers tell me to 'stitch'
the two planes together around the perimeter of the board (every inch or so)
using small resistors and/or ferrite beads....
Thoughts?
| Quote: | Its a
compromise but the best compromise possible with a mixed signal device
(and the reason why mixed signal devices are inherently inferior to
separate pure analog and digital devices, all other things being
equal). Refer to the referenced document for good info and explanations
of what's really happening and the logic behind it. (see page 3).
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Joerg
Guest
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| Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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Hello Meindert,
| Quote: | Well guys, thanks for all the input. Especially that document Joep referred
to, was very interesting. As for my design, I now have one single grounplane
and all DGND and AGND pins are all connected to them through their own via,
less than 0,5mm away from each pad. And indeed Joerg, it saved me a lot of
aspirin :-)
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Great. But you guys don't have to worry about Aspirin since you've
already got Hoegaarden and Beerenburger :-D
Regards, Joerg
http://www.analogconsultants.com |
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Meindert Sprang
Guest
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| Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:S_2kf.21640$BZ5.1234@newssvr13.news.prodigy.com...
| Quote: | Hello Joep,
In any case I wouldn't second guess the datasheet writer, ...
I always do. In fact, when the layout guidelines section of the data
sheet starts I usually stop reading ;-)
|
Well guys, thanks for all the input. Especially that document Joep referred
to, was very interesting. As for my design, I now have one single grounplane
and all DGND and AGND pins are all connected to them through their own via,
less than 0,5mm away from each pad. And indeed Joerg, it saved me a lot of
aspirin :-)
Meindert |
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Joerg
Guest
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| Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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Hello Bo,
| Quote: |
Where to make this connection? Especially if you have multiple devices that
require both AGND and DGND?
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That's exactly the point. The more complex a system becomes the less
clear a split ground structure will be. I guess that's why you can find
a jumbo package of Aspirin in many engineers' desk drawers.
| Quote: | And a 'direct' connection? ie I've had some old timers tell me to 'stitch'
the two planes together around the perimeter of the board (every inch or so)
using small resistors and/or ferrite beads....
Thoughts?
|
Oh boy. But I won't complain because stuff like that keeps our phones
busy and puts food on the table. Because somebody is going to have to
fix it afterwards.
Regards, Joerg
http://www.analogconsultants.com |
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Joerg
Guest
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| Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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Hello Joep,
| Quote: | In any case I wouldn't second guess the datasheet writer, ...
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I always do. In fact, when the layout guidelines section of the data
sheet starts I usually stop reading ;-)
Regards, Joerg
http://www.analogconsultants.com |
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joep
Guest
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| Posted:
Sat Dec 03, 2005 1:15 am Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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| Quote: | Where to make this connection? Especially if you have multiple devices that
require both AGND and DGND?
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Connect AGND and DGND together at each device to the analog ground
plane. Note that I am referring to AGND and DGND as the pins coming out
of the mix signal device (not the ground planes). The digital ground
plane is tied, at one point, to the analog gound plane at the power
source. Buffers between the A/D and the DSP are needed to prevent
digital noise feeding into the mix signal device (and then to the
analog ground). The buffers (and of course the DSP) would use the
digital ground plane as returns. Put all the mixed signal devices and
analog devices on one side of the board, the buffers in middle and the
DSP on the other side. I would probably would further seperate the
analog side between the quiet analog and semi quiet mixed signal,
watching the flow of current back to the return paths as a guide to
where to put everything. The digital power pin of the mix signal
devices would be decoupled to the DGND pin of the mixed signal device,
you can use the analog supply to power the digital side of the mixed
signal device, with a ferrite bead in the path.
This doesn't apply to very high power mixed signal devices (like high
speed DSP's with integrated A/D's), there really isn't a good solution
with those.
Well thats how I approach it anyhow :)
Real life constraints (board size, board shapes, size limitations etc)
all can force me to break alot of rules I just descirbed |
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Meindert Sprang
Guest
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| Posted:
Sat Dec 03, 2005 9:15 am Post subject:
Re: Ever 'souped up' the ADC on Renesas uC? |
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"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:bt5kf.21662$BZ5.10685@newssvr13.news.prodigy.com...
| Quote: | Hello Meindert,
Great. But you guys don't have to worry about Aspirin since you've
already got Hoegaarden and Beerenburger :-D
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Yuck, I don't like Beerenburgen (anymore). Been very drunk once from that
stuff.....
Hertog Jan is my favourite beer and as for the stronger stuff, you may wake
me up any time for a good Islay whisky.. :-))
Cheers,
Meindert |
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