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Richard Erlacher
03/16/12 09:53
Read: 441 times
Denver, Co
USA


 
#186734 - Well, when were those cap sizes determined?
Responding to: Erik Malund's previous message
I'm persuaded that the cap sizes popular when people were using TTL or LSTTL are considerably too large and likely to cause problems of their own when used in combination with mostly CMOS logic.

Adding more capacitance between logic power and gnd will increase the rise and fall time of Vcc with respect to Gnd, but won't help much with noise induced from the environment. Small cap's will help somewhat more but won't do the job entirely either.

I'd guess it wouldn't hurt for someone to take a close look at those cap values in the FAQ so they don't mislead newbies. I'd guess the values presented are, generally speaking, and for the technology popular today, an order of magnitude too large. Further, since people are using 0603's and 0402's, size causing increased real estate isn't that big a problem. after all, there are still smaller sizes.

There's more to dealing with supply bypass than just swamping supply input ripple. The ground bounce created by sharp edges on clocks and on signals in general contribute to overall system noise, but what's needed in this particular case is the need for mitigating externally induced noise, since it's a thoroughly "industrial" environment, with large currents moving around outside the control circuit under discussion.

Managing externally imposed noise might be as simple as using a darlington-input Vcc regulator rather than an LDO. Careful Gnd routing would also be important. The circuit in question might even need to be in a Faraday cage.

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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