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reduce inductance effects in layout

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MohEllayali

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dear all,

i have a layout very sensitive to inductances
i want to reduce thier effect, can you please give me some advice to minimize that effect

for instance to make the resistance low, i draw wide tracks in upper thick metal layers, what about inductances can you give me some clues, guidelines

many thanks in advance
 

ljcox

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Is your problem with the inductance of the Vcc & Gnd lines?

If so, then a 2 sided PCB with gnd and Vcc planes will reduce the L since the current can flow via multiple paths.

If it is the signal lines, you can use strip lines (I can't recall the exact name) but it is a transmission line like a telephone line or a coaxial cable eg. an antenna to TV feeder. If properly designed and terminated, the charteristic impedance is resistive since the L & C effectively cancel each other.
 

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MohEllayali

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Okay thank you for ythe suggestion
my problem is in the signal path , but i am surprised that inductance may cause probleme for vdd ans vss, it's a dc voltage so the inductance at that frequency is zero, am i wrong ?
about the signal path it's quite crowded neighbourhood, i dont know if i can afford a transmission-like layout
 

erikl

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i dont know if i can afford a transmission-like layout
Can you afford wide signal wires? ("wide" means w=width»height). Because - in this case: L ~ ln(1/w)
 

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ljcox

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Okay thank you for ythe suggestion
my problem is in the signal path , but i am surprised that inductance may cause probleme for vdd ans vss, it's a dc voltage so the inductance at that frequency is zero, am i wrong ?
about the signal path it's quite crowded neighbourhood, i dont know if i can afford a transmission-like layout
The point you're missing is that the signal currents also flow in the Vcc & Gnd lines. These can cause unwanted effects such as oscillation in analogue circuits and odd effects in digital ones.

It is therefore essential to provide adequate bypassing (ie. bypass capacitors) and in critical circuits, you also need to consider the inductance.

The object is to keep the impedence of the supply line as low as possible.
 

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MohEllayali

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thank you guys and sorry about the delayed answers

@ erikl

yes, indeed, but the problem is that the effect is in log (1/W) only , so the effect is very weak

finally i followed Ijcox suggestion i used ground shields so they provide a low impedance return path at high frequencies and the self is virtually canceled out by the mutual of the ground return path
 
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