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Kai Klaas
03/28/06 05:50
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Germany


 
Msg Score: +1
 +1 Good Answer/Helpful
#113289 - Traversing the gap using a pi-filter
Responding to: Prahlad J. Purohit's previous message

Normally you could locate the common regulator at the analog section and feed the 5V over the gap to digital section via a pi-filter.
But as LDOs don't like complex impedances at output to ground, I would run the 7.2V over the gap from analog to digital section via a pi-filter and locate an additional LDO near the gap. At output of this LDO use the decoupling caps recommended for this LDO.

Use a 6-hole choke in series to 2R2 resistor and two 47F/35V aluminium electrolytics paralleled by 100n/X7R ceramics to ground for the pi-filter.

If you still have digital noise on analog ground, then add additional pi-filters at supply pins of each digital chip. But take care, use a LDO, which can handle complex loads at output!


One additional word to this scheme:


It looks like both supply voltage systems are totally separated, because of using different secondary windings. But they are not! Interwinding capacitance of both secondary windings of mains transformer of several 100pF looks like a short circuit for high frequency digital noise, which makes both ground planes being connected again by two paths. So, digital ground return currents can again flow across the analog section!
One remedy is to use a common mode ferrite choke for each connection from secondary winding to according voltage regulator. But as common mode filtering is always tricky, because of the occurence of resonances, I would recommend to use the same scheme as above, as I do it always.

By the way, what cables are leaving the board?

Kai

List of 13 messages in thread
TopicAuthorDate
Location of Supply section.      Prahlad J. Purohit      03/27/06 21:15      
   Traversing the gap using a pi-filter        Kai Klaas      03/28/06 05:50      
      Making a new dsPIC board.      Prahlad J. Purohit      03/28/06 06:15      
         7 AAAs better than one block      Kai Klaas      03/28/06 06:45      
            7.2V Pack      Prahlad J. Purohit      03/28/06 06:51      
               for handheld/battery      Erik Malund      03/28/06 06:57      
                  I am planning to use 700mAh rechargeable      Prahlad J. Purohit      03/28/06 07:23      
               7.2V block = 120mAh!      Kai Klaas      03/28/06 08:53      
                  700mAh 7.2V block available here.      Prahlad J. Purohit      03/28/06 20:55      
      transformers        Steve M. Taylor      03/29/06 07:28      
         Screens      Kai Klaas      03/29/06 09:02      
         How do you do it?      Jon Ledbetter      03/29/06 09:12      
            Manufacturer      Kai Klaas      03/29/06 09:16      

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