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Design >> RF Design >> Downbond and Isolation
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Message started by fz2101 on Jun 28^th, 2007, 11:19am

Title: Downbond and Isolation
Post by fz2101 on Jun 28^th, 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 28^th, 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 29^th, 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