*From*: Steven J Ohlsen <sjohlsen@xxxxxxxxxxxx>*To*: si-list@xxxxxxxxxxxxx*Date*: Thu, 13 Jun 2013 13:32:15 -0700

Hello, I am somewhat new to the world of Power Integrity. I hope this isn't a dumb question, but I can not find any answer in the books I have or online either. I understand that the equation for Target Impedance is (allowed Voltage Variation)/(max change of current). However this seems like a "50,000 ft" rule-of-thumb. If your design meets this criteria then all is well. But if your design has difficulty reaching this impedance target then what. My observation is that when you get down to the board level for example, there can be 30-40 balls on the voltage regulator (and maybe multiple voltage regulators), and 50-100 balls on the device under test, and numerous decoupling capacitors scattered around, between what points does the target impedance apply? If you look at the board geometries then the current from Voltage Regulator to Device may travel through a 1-inch (or more) wide path on the power plane. Current is also provided by the decoupling capacitors whose current (probably) has a different path to the device under test . It seems that the current portion of the target impedance equation varies from point to point on the board depending on a host of very complicated relationships. Simply dividing the current portion of the target impedance equation by the number of balls on the voltage regulator or by the number of balls on the device under test also seems incorrect. Is there a more design specific method to compute the target impedance? Thank You all for your responses. Steven Ohlsen ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: http://www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:

- » [SI-LIST] A Question about Target Impedance Threshold computation - Steven J Ohlsen
- » [SI-LIST] Re: A Question about Target Impedance Threshold computation - steve weir
- » [SI-LIST] Re: A Question about Target Impedance Threshold computation - Smith, Larry
- » [SI-LIST] Re: A Question about Target Impedance Threshold computation - steve weir
- » [SI-LIST] Re: A Question about Target Impedance Threshold computation - Dhamija, Naresh
- » [SI-LIST] Re: A Question about Target Impedance Threshold computation - steve weir
- » [SI-LIST] Re: A Question about Target Impedance Threshold computation - Dhamija, Naresh