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???
06/19/08 19:03
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Msg Score: +1
 +1 Good Answer/Helpful		 #156046 - don't use a resistor in the GND connection
Responding to: ???'s previous message
unless you have a very good reason.

What does a resistor in the GND connection mean?

Let both outputs T1OUT and T2OUT be on -10V and R1OUT be at 0V and R2OUT be at 5V and R1OUT being connected to RX of an 8051.

Now switch both T1OUT and T2OUT to +10V (against a resistive or capacitive load).

The current needed to drive both lines will cause a voltage drop over your resistor in the GND connection.
This very voltage drop will cause a lower potential for R2OUT (where it does not hurt) and a lower potential for R1OUT (where it does hurt).
Essentially you are feeding (part of) the two output driver currents into the protection diodes of a single 8051 input.

Plus the rationale "lower the noise" is only partly met:
you'll find part of the switching noise on R1OUT and R2OUT (which typically are connected to the heart of your circuit).
Likewise you'll find part of (external) noise applied on T1OUT and T2OUT to propagate on R1OUT and R2OUT.

(ESD not yet mentioned)

In short: don't use a resistor in the GND connection unless you have a very good reason.

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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