Analog and Digital Ground
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Posts: 46,084
Hi
In ADCs we keep the DIGITAL and ANALOG grounds
seperate (to minimize the switching Noise for the
analog part)eg ADC0838 . But i am confused as to how
the grounds are physically seperate. I mean the
reference has to common .right?
If we seperate them physically the ckt stops working
hence i have to join the analog and digital grnds.
WHich defeats the puropse of providing two different
grnds in the first place.
Pls shed some light as to how to seperate the grnds?
thanks
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In ADCs we keep the DIGITAL and ANALOG grounds
seperate (to minimize the switching Noise for the
analog part)eg ADC0838 . But i am confused as to how
the grounds are physically seperate. I mean the
reference has to common .right?
If we seperate them physically the ckt stops working
hence i have to join the analog and digital grnds.
WHich defeats the puropse of providing two different
grnds in the first place.
Pls shed some light as to how to seperate the grnds?
thanks
__________________________________
Do you Yahoo!?
Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
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Comments
of your power supply. They are seperate in that no digital grounds are
attached to the analog leg and visa versa.
The important part if how many noisy things you have between the ADC and the
ground. The further from the ground you get, the noisier the ground leg
itself will be. If you hook up the ADC at the very end of a digital ground,
its results will be quite noisy. The closer you get to the power supply and
the fewer digital sources between it and the ground, the less noise you'll
see. In many cases just having the ADC ground first on a ground leg is
enough to mitigate noise. The simplest/safest thing is to run another leg
for analog only to ensure no digital grounds get between your analog device
and the ground.
To truly understand the topic you'll have to brush up on electricity. AC
characteristics are what you're looking for, the affect of expanding and
collapsing electromagnetic fields etc.
If noise is a problem then the first step is an analog ground. The second
thing is to remember that you have electricity flowing through long wires.
Whenever a chip becomes active, it suddenly draws more power. As the current
fluctuates, so to does the electromagnetic field around the wire and you
effectively have a broadcasting antanna. Wires around this antanna are
affected by this broadcast and current is induced in them, causing more
noise in the circuit.
You can try to keep your traces far apart and minimize propagation but thats
not entirely practical in many situations. What you can do though is shorten
the amount of trace that is actually experiencing large current fluctations.
Thats where decoupling capacitors come into play. If you stick a small cap
(say .1uF) right at the power pin of each chip, the capacitor can supply the
higher current when required instead of the power supply. The trace between
the power supply and the cap has a fairly constant current so field
fluctuations are quite small and broadcasts very weak. Between the cap and
the chip the current fluctuations are far larger but the trace length is
quite short - again, weak broadcasts. The closer to the power pin of the
chip you can get, the better it works.
When I added decoupling caps to my pH sensor circuit the ADC readings went
from a fluctuation of roughly +/- 0.05 pH to +/- 0.002. Major improvement!
Meridian 59 is back! Sacred Haven - server 200
www.skotos.net
Original Message
From: "harshit suri" <suri_list@y...>
To: "stamp" <basicstamps@yahoogroups.com>
Sent: Wednesday, June 11, 2003 2:08 PM
Subject: [noparse][[/noparse]basicstamps] Analog and Digital Ground
> Hi
>
> In ADCs we keep the DIGITAL and ANALOG grounds
> seperate (to minimize the switching Noise for the
> analog part)eg ADC0838 . But i am confused as to how
> the grounds are physically seperate. I mean the
> reference has to common .right?
> If we seperate them physically the ckt stops working
> hence i have to join the analog and digital grnds.
> WHich defeats the puropse of providing two different
> grnds in the first place.
> Pls shed some light as to how to seperate the grnds?
>
> thanks
>
> __________________________________
> Do you Yahoo!?
> Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
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Body of the message will be ignored.
>
>
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>
>
> (say .1uF) right at the power pin of each chip, the capacitor can supply
the
Oops, make that 1uF.
The reason for all this bypass capacitor mumbo-jumbo is that big filter caps
have a lot of series inductance (they're wrapped like a jelly roll inside),
which basically unhooks them from the circuit at high frequencies. That's
why little disc capacitors (which have great high-frequency response) are
often piggy-backed on big 10,000 uF caps, and added near each IC or PCB
supply line. Long power supply leads and traces add serial resistance, as
well as inductance on occasion, plus add the possibility of
capacitive/inductive coupling, etc. And yes, all electron flow returns via
the grounds, so once digital noise gets on an analog ground, it's very
difficult to get rid of it. That's why keeping your digital and analog
supplies separate is the only way to fly.
Mike Sokol
mikes@m...
www.ModernRecording.com
"Yes, we're mum and dad - and good and bad -
and everyone's happy to be here.
Genesis-
Original Message
From: "Jim Forkin" <jjf@p...>
To: <basicstamps@yahoogroups.com>
Sent: Thursday, June 12, 2003 9:04 AM
Subject: RE: [noparse][[/noparse]basicstamps] Analog and Digital Ground
> What you need to do is keep current flow in the ground circuits for the
> analog and digital from affecting each other. (mostly digital affecting
> analog). In other words, run a return for analog in directly to an op amp
> and then to a common ground point and digital input ground directly to a
> buffer chip ground and then to a common ground point. Don't daisy chain
> grounds together. Keep the signals balanced even though both may be at the
> same potential. This can be difficult to do in common practice. Keep in
> mind that all noise interferance does not propagate on ground returns.
> Noise can be radiated directly, coupled inductively or capacitively or any
> combination of these ways. As a practical matter, just use shielded
> twinlead for low level analog inputs (ground the shield only at the higher
> level end), keep signal and power wiring away from each other, and isolate
> analog and digital power using filtering or regulation and by all means,
> place a capacitor at each device to bypass + to ground as close to the
> device as possible. Analog circuit design is a different world than
digital
> and things are not always predictable without the experience to recognize
> potential problems.
>
> I hope I didn't confuse you more!
>
> Good luck,
>
> Jim
> http://www.geocities.com/jimforkin2003/
>
>
>
Original Message
> From: harshit suri [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=BoDxDBL8823sMoHGMK83WOhf8FydP8B2ebekCns1JxPhNRJ-6WLDAUboKEglfHpKYRwXCsKo5b1OVg]suri_list@y...[/url
> Sent: Wednesday, June 11, 2003 5:09 PM
> To: stamp
> Subject: [noparse][[/noparse]basicstamps] Analog and Digital Ground
>
>
> Hi
>
> In ADCs we keep the DIGITAL and ANALOG grounds
> seperate (to minimize the switching Noise for the
> analog part)eg ADC0838 . But i am confused as to how
> the grounds are physically seperate. I mean the
> reference has to common .right?
> If we seperate them physically the ckt stops working
> hence i have to join the analog and digital grnds.
> WHich defeats the puropse of providing two different
> grnds in the first place.
> Pls shed some light as to how to seperate the grnds?
>
> thanks
>
> __________________________________
> Do you Yahoo!?
> Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
> http://calendar.yahoo.com
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
> Body of the message will be ignored.
>
>
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>
>
>
> To UNSUBSCRIBE, just send mail to:
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Body of the message will be ignored.
>
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>
analog and digital from affecting each other. (mostly digital affecting
analog). In other words, run a return for analog in directly to an op amp
and then to a common ground point and digital input ground directly to a
buffer chip ground and then to a common ground point. Don't daisy chain
grounds together. Keep the signals balanced even though both may be at the
same potential. This can be difficult to do in common practice. Keep in
mind that all noise interferance does not propagate on ground returns.
Noise can be radiated directly, coupled inductively or capacitively or any
combination of these ways. As a practical matter, just use shielded
twinlead for low level analog inputs (ground the shield only at the higher
level end), keep signal and power wiring away from each other, and isolate
analog and digital power using filtering or regulation and by all means,
place a capacitor at each device to bypass + to ground as close to the
device as possible. Analog circuit design is a different world than digital
and things are not always predictable without the experience to recognize
potential problems.
I hope I didn't confuse you more!
Good luck,
Jim
http://www.geocities.com/jimforkin2003/
Original Message
From: harshit suri [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=8qnJ0-zA64js-COuYMqmvyRC-7AvtfjVVzYoIEWpUg3CmgMcfzLyW2dVSFl1pvPYZWgEanLoaXCwUXJT]suri_list@y...[/url
Sent: Wednesday, June 11, 2003 5:09 PM
To: stamp
Subject: [noparse][[/noparse]basicstamps] Analog and Digital Ground
Hi
In ADCs we keep the DIGITAL and ANALOG grounds
seperate (to minimize the switching Noise for the
analog part)eg ADC0838 . But i am confused as to how
the grounds are physically seperate. I mean the
reference has to common .right?
If we seperate them physically the ckt stops working
hence i have to join the analog and digital grnds.
WHich defeats the puropse of providing two different
grnds in the first place.
Pls shed some light as to how to seperate the grnds?
thanks
__________________________________
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place.
The recommended layout of high performance mixed - signals ICs is outlined
in the evaluation board that the manufacturers offer.
I use AD7730 24 bit A/D and the ground planes should be connected at the
AGND and DGND pins of the AD7730 like this:
AGND || DGND
area || area
_____ ||____
|_____ ____| AD7730
* ||
||
||
Regards
Mohamed Refky
>From: harshit suri <suri_list@y...>
>Reply-To: basicstamps@yahoogroups.com
>To: stamp <basicstamps@yahoogroups.com>
>Subject: [noparse][[/noparse]basicstamps] Analog and Digital Ground
>Date: Wed, 11 Jun 2003 14:08:32 -0700 (PDT)
>
>Hi
>
>In ADCs we keep the DIGITAL and ANALOG grounds
>seperate (to minimize the switching Noise for the
>analog part)eg ADC0838 . But i am confused as to how
>the grounds are physically seperate. I mean the
>reference has to common .right?
>If we seperate them physically the ckt stops working
>hence i have to join the analog and digital grnds.
>WHich defeats the puropse of providing two different
>grnds in the first place.
>Pls shed some light as to how to seperate the grnds?
>
>thanks
>
>__________________________________
>Do you Yahoo!?
>Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
>http://calendar.yahoo.com
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
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place.
The recommended layout of high performance mixed - signals ICs is outlined
in the evaluation board that the manufacturers offer.
I use AD7730 24 bit A/D and the ground planes should be connected at the
AGND and DGND pins of the AD7730 like this:
DGND || AGND
area || area
_____ ||____
this is the correct not the last
|_____ ____| AD7730
* ||
||
||
Regards
Mohamed Refky
>From: harshit suri <suri_list@y...>
>Reply-To: basicstamps@yahoogroups.com
>To: stamp <basicstamps@yahoogroups.com>
>Subject: [noparse][[/noparse]basicstamps] Analog and Digital Ground
>Date: Wed, 11 Jun 2003 14:08:32 -0700 (PDT)
>
>Hi
>
>In ADCs we keep the DIGITAL and ANALOG grounds
>seperate (to minimize the switching Noise for the
>analog part)eg ADC0838 . But i am confused as to how
>the grounds are physically seperate. I mean the
>reference has to common .right?
>If we seperate them physically the ckt stops working
>hence i have to join the analog and digital grnds.
>WHich defeats the puropse of providing two different
>grnds in the first place.
>Pls shed some light as to how to seperate the grnds?
>
>thanks
>
>__________________________________
>Do you Yahoo!?
>Yahoo! Calendar - Free online calendar with sync to Outlook(TM).
>http://calendar.yahoo.com
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
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