[SI-LIST] Re: Decoupling Capacitor for LNA and PA
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 30 May 2006 09:26:56 -0400
> ... So I thought that we
> better not apply the decoupling capacitors in the analog chips such as LNA
> or PA, but I am not sure about that.
I'm going to guess that decoupling capacitors were known to be necessary for
analog circuits at least as long as digital ones. After all, some of the
earliest ICs were analog, op-amps and such, which tend to work very poorly
(if at all) without decoupling capacitors at each IC.
If your LNA chip really had no power/ground noise, then adding capacitors
would have no effect. Since you noticed a change, there must be something
going on there, whether or not you can measure it. Small amounts of
feedback from one IC to another can lead to things you didn't anticipate,
including changes in gain, or oscillation.
Adding capacitors can have a detrimental effect if the ones you added (in
conjunction with layout) happen to form a resonance or somesuch, which might
increase noise or feedback rather than reducing it. That means you need to
work harder on choosing capacitors and/or the layout.
Regards,
Andy
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