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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> Downbond and Isolation https://designers-guide.org/forum/YaBB.pl?num=1183054745 Message started by fz2101 on Jun 28th, 2007, 11:19am |
Title: Downbond and Isolation Post by fz2101 on Jun 28th, 2007, 11:19am Hello Everyone, In an application note from Maxim (APPLICATION NOTE 3630 Power Supply and Ground Design for a WiFi Transceiver), it emphasizes the need to isolate the charge pump (CP) from the VCO through proper PCB layout. My question refers to Fig. 6 of the app note which I redrew below on the left-hand side. The app note says that GND_CP and GND_VCO have multiple vias to provide a low impedance path to the ground plane, which is the second layer of the board. However, downbonding both GND_CP and GND_VCO to the exposed paddle, which is contacted to the ground plane through multiple vias also provides a low impedance path (right figure). I suppose the argument is that, for the figure on the right, noise current from CP can develop a noise voltage across L_via and therefore can affect the VCO, so is it true that if L_via is reduced (by using multiple vias on the paddle), then the configuration on the right can be used? Thanks, frank |
Title: Re: Downbond and Isolation Post by mg777 on Jun 28th, 2007, 9:40pm I congratulate you on stating your question so clearly. As signal integrity guru Howard Johnson emphasizes, inductance is a property of spaces between wires, and not the wires per se. Here is a link to one of his articles which you should find useful: http://signalintegrity.com/Pubs/edn/SpreadYourReturns.htm Following Figs. 1 & 2 in that article, it appears that the answer lies in the entire geometry i.e; we must include the position of the VDD pins for the CP and VCO as well. M.G.Rajan |
Title: Re: Downbond and Isolation Post by fz2101 on Jun 29th, 2007, 9:04am I guess my question boils down to whether you want small ground inductance or large ground inductance. 1. Small ground inductance gives you less degeneration, which is important for single-ended circuits AND circuits where input impedance matching is critical. 2. Large ground inductance gives you better isolation, since it tends to prevent noise current from going between circuit blocks . As Howard Johnson's article points out, this statement is not entirely true, since noise can still couple magnetically between ground bondwires. Therefore, if a circuit block can tolerate the reduced gain and the increased input impedance from the extra ground inductance, then it is better to bondwire the ground to the package pin first and guide the current back though a decoupling capacitor to VDD (figure on the left). If we downbond all the ground wires to the paddle (figure on the right), then the situation is too similar to shorting all the grounds together on chip and therefore isolation suffers. Is this generalization correct? Thanks, frank |
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