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bk11
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Bipolar supplies
bk11   12/18/2013 1:20:00 PM
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You mention that it is not a good idea to share vias between caps, but what about decoupling devices that use bipolar supplies?  Thre are (at least) three options:

a. One cap from V+ to V-

b. Two caps, each to ground, but sharing a common ground via

c. Two caps with seperate ground vias

I often use option B, with the thought that only the imbalanced current flows through the via, but each supply is decoupled to ground should the imbalance be large.

zeeglen
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Isolation Resistors
zeeglen   12/18/2013 2:13:51 PM
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Any thoughts on the effectiveness of a few ohms resistance between each power pin and its Vdd plane?  This does force the PCB designer into placing a cap close to each pin, but does this confining of the shoot-through current loop to the local capacitor work to keep the planes quieter and reduce radiated EMI?

Adam-Taylor
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Re: Bipolar supplies
Adam-Taylor   12/18/2013 2:57:25 PM
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I would be go with option C as sharing via's increases the inductance and can cause issue if the instantenous demands are large.

This will also increase the inductance due to the tracking required to share the via.

You want to minimise the loop area so treat each supply to ground as its own loop and attempt to minimise it 

I hoope this helps and makes sense

Adam-Taylor
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Re: Isolation Resistors
Adam-Taylor   12/18/2013 3:01:50 PM
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Sadly a few ohms resistance might cause issues of voltage drop at high currents and complicate the anaylsis of the power distribution network.

If you need to have quieter planes, say for a ADC or DAC then ferrite beads can be used.

 

 

salbayeng
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Via placement and FB's
salbayeng   12/18/2013 5:55:27 PM
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Hi all, 

I try to put pairs of via's either side of the pad where possible, this lowers the effective loop area and makes the "inductor width" wider, both of which reduce inductance. 

And I use ferrite beads wherever possible, a good place for these is on rapidly switching outputs (particularly capacitive loaded ones) this not only prevents ringing , but rounds off the edegs of the circulating currents through VCC and gnd. 

er.vikasjha
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Re: Bipolar supplies
er.vikasjha   12/20/2013 12:25:38 AM
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My experience is better to use Decoupling Cap NW wrt FB as during EMC , Ferrites behavior relatively to Parasitic Impedance is Unpredictable

 

Sanjib.A
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Choosing smaller Decoupling capacitor packages
Sanjib.A   12/21/2013 5:45:57 AM
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Is there an advantage of using smaller decoupling capacitor packages e.g. 0402 over the bigger packages such as 0805 from signal/power integrity, EMI stand point? Obviously the important electrical parameters such as voltage, capacitance shall meet requirement first. But I was thinking if smaller sizes would reducing the loop & inductance even further, and hence should be the first priority from EMI point of view, apart from the fact that these take lesser space.

MeasurementBlues
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Re: Choosing smaller Decoupling capacitor packages
MeasurementBlues   1/1/2014 12:51:32 PM
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@Sanjib, that's the general thinking, redce loops wherever possible and you reduce EMI emissions and susceptibility problems. There's also the inductance of the capacitor itself, depending on the frequencies in your circuit.

MeasurementBlues
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Rise time of digital signals
MeasurementBlues   1/1/2014 12:58:38 PM
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Adam,

I usually think of decoupling caps in relation to digital devices. Can you relate wavelength to the rise time of a digital signal? That will help in understanding both the value of a decoupling cap and the distance from the IC that needs power supplied by the cap.



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