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???
06/25/08 13:48
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Msg Score: +1
 +1 Good Answer/Helpful		 #156199 - Again, do not add any impedance in the GND line
Responding to: ???'s previous message
Richard said:
If even a very small part of that total resistance is in the GND path between the two bypass cap's, then much less current will flow through the R in the GND rail than through the local cap.

You mean something like that?



Sorry, Richard, again, the current is entirely limited by the resistor R in the Vcc line. Inserting "even a very small part of that resistance in the GND line", as you wrote, will not have any relevant influence. What counts is the total impedance in the left, blue marked loop, which is best provided by the shown R only.

Please do not add any impedance in the GND line. This would be totally counter-productive and violate all design rules dealing with decoupling.

Kai

List of 31 messages in thread
TopicAuthorDate
Resistance in the supply rails            01/01/70 00:00      
   inductor instead of resistor            01/01/70 00:00      
      yes, and no and I think the theory is flawed            01/01/70 00:00      
         Spice?            01/01/70 00:00      
            That would probably help, but ...            01/01/70 00:00      
      That's what I said            01/01/70 00:00      
         Not in the gnd path!            01/01/70 00:00      
            Well ... this normally doesn't work so well, but            01/01/70 00:00      
   If this scheme is used it would also be well to...            01/01/70 00:00      
      This does increase the supply impedance            01/01/70 00:00      
         please clarify with a simple schematic            01/01/70 00:00      
            the bypass cap\'s always go between Vcc and Gnd            01/01/70 00:00      
               Which Vcc, Which gnd            01/01/70 00:00      
                  There's a cap on either side of the resistor(s)            01/01/70 00:00      
                     is it            01/01/70 00:00      
                        Not quite ...            01/01/70 00:00      
                           don't use a resistor in the GND connection            01/01/70 00:00      
                              What >good reason< could this be??            01/01/70 00:00      
                                 What good reason could this be??            01/01/70 00:00      
                                    Using a resistor in the GND connection...            01/01/70 00:00      
                              How would that come to pass?            01/01/70 00:00      
                                 Why should I insert a resistor in the GND line??            01/01/70 00:00      
                                    Seldom andonly if it is of benefit            01/01/70 00:00      
                                       No disagree, but ...            01/01/70 00:00      
                                          The regulator can't pull the Vcc down            01/01/70 00:00      
                                             This has nothing to do with imbalance!            01/01/70 00:00      
                                                Kai, I don\'t disagree ...            01/01/70 00:00      
                                                   Again, do not add any impedance in the GND line            01/01/70 00:00      
   tantalum capacitor across the MAX232 power pins            01/01/70 00:00      
      It's a matter of careful choice            01/01/70 00:00      
      Yes they does            01/01/70 00:00      

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