ground reference changing, how??

[mess123]

I wish to know when do the ground reference in a circuit changes?? How is it determined?? Suppose a battery and another power source (say a power from wall or a PC) is connected, how do the common ground reference work??

---------- Post added at 14:38 ---------- Previous post was at 14:11 ----------

I have heard about "Ground bounce". Could it be a permanent phenomenon??

 

[mister_rf]

http://www.ultracad.com/articles/g_bounce.pdf
http://www.home.agilent.com/upload/cmc_upload/All/Ground_Bounce.pdf?&cc=RO&lc=eng
http://www.fairchildsemi.com/an/AN/AN-640.pdf

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**broken link removed**
http://www.xilinx.com/support/documentation/application_notes/xapp689.pdf

http://www.te.com/documentation/whitepapers/pdf/3jot_2.pdf