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Richard Erlacher
03/14/12 20:00
Read: 511 times
Denver, Co
USA


 
#186675 - Some of those rules are showing their age
Responding to: Erik Malund's previous message
First of all, I don't think 100 nF is "right" for IC bypass unless in parallel with about 200pF. The sharp edges on modern CMOS signals cause considerable ground bounce and, unless the cap's are extremely high-quality, they won't respond quickly enough to suppress the switching noise internal to most parts, especially if they're registered parts.

Additionally, I believe one should avoid too much power-gnd capacitance, as it will cause far too slow a decay of Vcc. This means there will be lots of time for circuits to misbehave during the period in which Vcc is still high enough to allow them to operate, but not high enough to ensure they operate correctly. Not all components, including MCU's behave the in the same way once Vcc is out of specified limits. All one can say is that they behave unpredictably, and since their limits are often quite different, that problem has to be mitigated in some way. I prefer to put a short circuit across Vcc, and, especially, the input to the last regulator.

I've been using very small-valued cap's to bypass HC, HCT, AC, ACT, and similar parts for decades with surprisingly good results. Your mileage may vary, but I doubt adding the tiny cap's will hurt, and you might actually find that 200 pF works well with those fast-clocked counters, shift registers, etc. After all, that 4.7 uF at the power entry will keep things smoothed with today's low-power circuits.

RE


List of 66 messages in thread
TopicAuthorDate
Sine wave detection on microcontroller      Yagnesh Mehta      03/07/12 04:59      
   Detect or measure?      Per Westermark      03/07/12 05:25      
      not detection, measurement of sine wave signal      Yagnesh Mehta      03/07/12 06:14      
         What are you measuring?      Andy Peters      03/07/12 10:05      
            RMS and Average - 1st Priority      Yagnesh Mehta      03/07/12 20:59      
               415 v      Mahmood Elnasser      03/08/12 22:07      
                  I have to do using MCU      Yagnesh Mehta      03/08/12 22:48      
                     Start simple      Mahmood Elnasser      03/08/12 23:17      
                        if time is major isssue than.....      Yagnesh Mehta      03/08/12 23:31      
                           People responding to specific questions      Per Westermark      03/09/12 02:14      
                              Same Technique....      Yagnesh Mehta      03/09/12 04:32      
                                 RMS      Maarten Brock      03/09/12 05:49      
                                    Sorry for my laziness        Yagnesh Mehta      03/09/12 20:50      
                                 So what have you done?      Per Westermark      03/09/12 06:19      
                                    Require more work...      Yagnesh Mehta      03/10/12 00:50      
                                    Working on that      Yagnesh Mehta      03/10/12 06:12      
                                       options, options      Per Westermark      03/10/12 06:49      
   Help me      Yagnesh Mehta      03/08/12 21:31      
      Okay      Joseph Hebert      03/09/12 06:13      
         Are many threads available      Per Westermark      03/09/12 06:29      
      when you save a char ...        Erik Malund      03/09/12 08:09      
      Why don't you specify your problem COMPLETELY?      Richard Erlacher      03/09/12 11:36      
         Already specify it      Yagnesh Mehta      03/10/12 02:07      
            there are some details?      Richard Erlacher      03/10/12 07:51      
               comparisions      Yagnesh Mehta      03/12/12 01:16      
                  How are you calculating your needed sample rate?      Per Westermark      03/12/12 01:54      
                     start with simple one....      Yagnesh Mehta      03/12/12 08:02      
                        You do know sin(90+angle)? Or 2-log computations?      Per Westermark      03/12/12 08:48      
                           ADC resolution............      Yagnesh Mehta      03/12/12 12:27      
                              impossible or wrong      Erik Malund      03/12/12 13:30      
                              double post, ignore      Erik Malund      03/12/12 13:30      
                  Precision Rectify:      Erik Malund      03/12/12 06:44      
                     SCR Chopped Wave      Yagnesh Mehta      03/12/12 07:48      
                        What accuracy do you need?      Kalpak Dabir      03/12/12 08:43      
                           CT accuracy      Yagnesh Mehta      03/12/12 12:58      
                              the fundamental      Erik Malund      03/12/12 13:41      
                              50Hz      Per Westermark      03/12/12 14:27      
                              Precision rectifier is suitable for any waveform      Kalpak Dabir      03/13/12 01:47      
                                 are u sure??      Yagnesh Mehta      03/13/12 08:03      
                                    Depends      Per Westermark      03/13/12 09:08      
                                    must be severely bandwidth limited      Erik Malund      03/13/12 09:11      
                                       Maybe was circuit with top-voltage capacitor      Per Westermark      03/13/12 09:17      
                                       Sorry for confusion      Yagnesh Mehta      03/14/12 00:18      
                                          What accuracy do you need?      Kalpak Dabir      03/14/12 01:23      
                                             360/1024      Yagnesh Mehta      03/14/12 01:41      
                                                Not what he (or me) have asked      Per Westermark      03/14/12 04:39      
                                                   you are right....      Yagnesh Mehta      03/14/12 11:51      
                                                Do you believe you can protect your circuit from noise?      Richard Erlacher      03/14/12 08:19      
                                                   Very Noisy environment      Yagnesh Mehta      03/14/12 11:57      
                                                      re Very Noisy environment      Erik Malund      03/14/12 12:23      
                                                         Some of those rules are showing their age      Richard Erlacher      03/14/12 20:00      
                                                            here we go again      Erik Malund      03/15/12 08:15      
                                                               think about this for a minute      Richard Erlacher      03/15/12 17:10      
                                                                  when discussing a FAQ      Erik Malund      03/16/12 06:49      
                                                                     Well, when were those cap sizes determined?      Richard Erlacher      03/16/12 09:53      
                                                                        Technology      Per Westermark      03/16/12 10:13      
                                                                           Steady state currents do not need decaps      Kalpak Dabir      03/16/12 22:48      
                                                                              Often low capacitance so limited charge. But remember load.      Per Westermark      03/17/12 06:41      
                                                                        fixations      Erik Malund      03/16/12 10:40      
                                                                           Nothing. has been proven yet.      Richard Erlacher      03/18/12 13:15      
                                                                              once more for the peanut gallery      Erik Malund      03/19/12 07:23      
                                                                                 Where you sit determines what you see ...      Richard Erlacher      03/20/12 01:56      
                                                                                    Off-topic        Per Westermark      03/20/12 02:01      
                                                                                       difficult when ...      Erik Malund      03/20/12 06:59      
                                                                                       Yes ... I don't know how we got here either      Richard Erlacher      03/20/12 09:32      
                                                                                          i do      Erik Malund      03/20/12 09:45      

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