[SI-LIST] Re: basic question regarding inductor before Vcc
- From: steve weir <weirsi@xxxxxxxxxx>
- To: roymesi@xxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Jun 2005 11:37:33 -0700
Roy, sure a series impedance decouples the IC from the surrounding
interconnect. Done properly, that can be a good thing for issues such as
EMI. When a thirsty IC inhales lots of current that wave propagates trough
the interconnect until it either dissipates due to resistive, and radiated
losses, or encounters an impedance discontinuity such as a decoupling
inductor. That works two ways and can isolate digital return currents from
traversing a sensitive analog path in an IC such as are used for PLLs.
The bad news is that many people design bypass / decoupling networks by
rote rules and happenstance. An inductor that isolates an IC that is not
matched with enough capacitance to supply the IC is a bad
thing. Similarly, a combined inductor and capacitor network can peak badly
making power quality much worse than without the inductor in the first place.
Steve.
At 07:59 PM 6/17/2005 +0200, Roy M. wrote:
>Hi,
>can someone explain to me why the board designers put inductors just
>before the Vcc connection? is it some kind of filter against noises?
>I understand that when you put a resistor, it is intended for lowering the
>voltage or for pull-downs... but what's the logical explanation for
>putting an inductor?
>Thanks,
>Roy
>
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