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Author Topic: DSP IC supply  (Read 3559 times)

Offline 'yus

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DSP IC supply
« on: April 23, 2008, 01:57:35 PM »
sa DSP/DSC (or any similar controllers),
ok lang ba na i-connect ang VSSA sa VSS, saka ang VDDA sa VDD ? ???
like this pin diagram:



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DSP IC supply
« on: April 23, 2008, 01:57:35 PM »

Offline zer0w1ng

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Re: DSP IC supply
« Reply #1 on: April 23, 2008, 02:31:24 PM »
Yes you could short Vssa to Vss and Vdd to Vdda.  But should only on one point at the supply capacitor preferably to reduce noise due to digital switching.  You could also place an inductor in series + a cap to the Vdda or Vssa.  All reference ADC ground must also be connected to the Vssa or a source voltage for ADC to Vdda.
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Re: DSP IC supply
« Reply #1 on: April 23, 2008, 02:31:24 PM »

Offline motion55

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Re: DSP IC supply
« Reply #2 on: April 23, 2008, 02:41:17 PM »
You ultimately must connect Vdda to Vdd and Vssa to Vss. You can't have too much DC voltage difference between Vssa and Vss. Otherwise you might destroy the DSP chip.

For PCB layout purposes, you can use different nodes for the analog and digital supply rails and tie them together through a zero ohm resistor (a jumper).



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Re: DSP IC supply
« Reply #2 on: April 23, 2008, 02:41:17 PM »

Offline 'yus

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Re: DSP IC supply
« Reply #3 on: April 23, 2008, 03:22:00 PM »
@zer0w1ng & motion55
thanks sa inyo!

hmmm... pwede naman palang i-connect directly, eh bakit pinaghiwalay pa nila ? ???
sayang naman yung 2 (or 3) extra pins, pwede pang gamiting sa ibang functions...
wala na siguro silang maisip na ilagay na ibang pins/functions, kaya inulit na lang nila.. ahihihi  :D
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Re: DSP IC supply
« Reply #3 on: April 23, 2008, 03:22:00 PM »

Offline rdpzycho

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Re: DSP IC supply
« Reply #4 on: April 23, 2008, 04:39:55 PM »
noise coupling issues kaya merong separate supplies yung analog at digital... ;)
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Re: DSP IC supply
« Reply #4 on: April 23, 2008, 04:39:55 PM »

Offline motion55

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Re: DSP IC supply
« Reply #5 on: April 23, 2008, 05:14:16 PM »
Quote
hmmm... pwede naman palang i-connect directly, eh bakit pinaghiwalay pa nila ?

It is one of concepts na dapat matutunan. At DC, the two points are connected. But at high frequencies magkahiwalay dapat sila or else noise from the digital ground will mix with the analog signal. The A/D conversions magkaka-error.

Secondly, the ground line is not a single point. Sa schematic diagram, it seems to be. But sa actual PCB, it is a trace of copper. Depending on where the current is flowing, there are small differences in voltage along the trace. You must know where to connect the analog ground to the digital ground where it picks up the least noise.





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Offline rdpzycho

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Re: DSP IC supply
« Reply #6 on: April 23, 2008, 05:34:44 PM »
as what an EMC mentor told me "you must always see the hidden schematic". ;D

malaki rin effects nung small current ng switching nung digital circuits. hindi man kasing laki ng sa high power switching, malaki pa rin ang effect nila, especially sa A/D conversion.

some application notes even recommend an additional inductor para mas maliit effects nung digital switching.
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Offline 'yus

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Re: DSP IC supply
« Reply #7 on: April 23, 2008, 05:38:14 PM »
isolated din ba sila sa loob ng IC? ???
pa'no yung paths ng mga signals ? meron parang optocoupler ? ???
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Offline motion55

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Re: DSP IC supply
« Reply #8 on: April 23, 2008, 06:22:34 PM »
Quote from: julznc on April 23, 2008, 05:38:14 PM
isolated din ba sila sa loob ng IC? ???
pa'no yung paths ng mga signals ? meron parang optocoupler ? ???

I think the analog circuit would be "isolated" within the die. It would be expensive to isolate it fully with optoisolators. You might as well buy a separate A/D converter.

Remember the IC is a semiconductor. The die is n-channel doped and so for it not to conduct, it has to be connected to the most negative supply which in most cases is Vss. If Vssa is applied a voltage more negative than Vss (greater than a diode forward voltage), then current will flow from Vss to Vssa within the die, breaking the isolation.
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Offline 0b00000111

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Re: DSP IC supply
« Reply #9 on: April 24, 2008, 07:00:22 PM »
tungkol sa grounding connection ng analog at digital sa PCB, sinusunod ko yung nabasa ko sa pdf ni rdpzycho doon sa PCB design...
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Offline 'yus

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Re: DSP IC supply
« Reply #10 on: April 24, 2008, 11:09:30 PM »
Quote from: sevenstring on April 24, 2008, 07:00:22 PM
tungkol sa grounding connection ng analog at digital sa PCB, sinusunod ko yung nabasa ko sa pdf ni rdpzycho doon sa PCB design...

ito pala yun: http://www.electronicslab.ph/forum/index.php?topic=1433.0
ngayon ko lang nabasa..  ahihi :D   di pa ako nakaka-encounter nito dati..
pagnagpapa-layout kasi ako ng PCB, usually ang binabatayan ko lang ay yung mga "loop" (e.g. parasitic inductances)
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Offline beginner wan

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Re: DSP IC supply
« Reply #11 on: April 24, 2008, 11:31:47 PM »
OT: wow.. sige.. magdaldalan lang kayo, marami akong natututunan.. hehehe... ;D ;D ;D

mga sir,lagi ko po naririnig na hiwalay yung digital at analog grounds, parehas silang ground, bakit po kailangan pa nating paghiwalayin to? ??? ??? ??? anong epekto ni Digital ground kay analog ground and vice-versa? examples naman po..

@sir rdpzycho: "you must always see the hidden schematic" pwede po paki-elaborate? parang mahiwaga itong parilala na ito. salamat po. ;D ;D ;D
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Offline rdpzycho

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Re: DSP IC supply
« Reply #12 on: April 25, 2008, 12:57:33 AM »
hidden schematic = parasitics not shown on your actual schematic.

example ang PCB traces when considered as ideal conductor, 0 ohm wire lang siya. sa actual circuit merong parasitic inductance (pwedeng ma-compute) at parasitic capacitance parang sa mga transmission lines din. sa DC, medyo pwedeng balewalain ang effects nila. sa switching circuits (digital) na mataas ang frequency malaki ang effects nila.

yung Analog Ground at Digital Ground, noise issues yan.

example, merong common point ground na iisa ang supply / decoupling capacitor. kung may inductance yung common point mataas ang impedance nito at high frequency, kahit sa small switching current magkakaroon ng ground bounce or yung supposed to be ground mo, a few voltage above your actual reference point i.e. 0V ng main supply. kung nakataas yung ground apektado kaagad ang conversion ng ADC.

pwedeng gawing common ground yung dalawa pero dapat magkaiba pa rin ng decoupling capacitor para localized ang kanya kanyang current. kailangan din kaya mong tantyahin kung ano ang return path ng current para hindi dumaan sa ground nung kabila.

maraming papers nito online si Henry Ott at Mark Montrose. IIRC, yung kay Mark Montrose summarized rule of thumbs yung iba para hindi na iisipin. ;D
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Offline beginner wan

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Re: DSP IC supply
« Reply #13 on: April 25, 2008, 08:20:17 PM »
salamat sir rdpzycho, kagabi ko lang nakita yung post mo bout this PCB design.hehehe.. ayos!! salamat!!! ;D ;D ;D
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Offline rdpzycho

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Re: DSP IC supply
« Reply #14 on: April 25, 2008, 09:27:50 PM »
complex mathematics talaga ang Electromagnetic Emissions. good thing merong mga taong nagsi-simplify ng work natin at sumusulat ng mga papers at application notes. ;D
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Offline beginner wan

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Re: DSP IC supply
« Reply #15 on: April 25, 2008, 09:58:10 PM »
oo nga e..hehehe.. salamat sa kanila at salamat din sa inyo... marami pa talga kong hindi alam... ;D ;D ;D
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Offline 'yus

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Re: DSP IC supply
« Reply #16 on: May 19, 2008, 06:28:57 PM »
bakit kaya yung sa TI DSP (e.g.TMS320F2801) magkaibang level yung dalawang Vdd? ???
Quote
CPU and Logic Digital Power Pins (1.8 V)
Digital I/O Power Pin (3.3 V)
???
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Offline rdpzycho

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Re: DSP IC supply
« Reply #17 on: May 19, 2008, 08:29:22 PM »
baka may conversion lang ng logic level para madaling mag-interface with other IC. malamang maliit na micron process na yung CPU kaya mababa voltage.
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Offline 'yus

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Re: DSP IC supply
« Reply #18 on: May 19, 2008, 08:37:39 PM »
bat kaya di na lang nila nilagyan ng internal linear reg. para meron ng 1.8V sa loob? ???
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Offline rdpzycho

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Re: DSP IC supply
« Reply #19 on: May 19, 2008, 09:02:02 PM »
kung for battery powered applications ayaw nila gumamit ng linear regulator. motto na ata kasi ng TI yang patagalan ng battery life. ;D
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Re: DSP IC supply
« Reply #19 on: May 19, 2008, 09:02:02 PM »
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