[sdiy] Nice info on decoupling caps

[David G Dixon]

Very nice (as always)!  Thanks, Richie!

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Richie Burnett
> Sent: Thursday, January 19, 2012 3:34 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Nice info on decoupling caps
> 
> When planning supply rail decoupling on a PCB it is often 
> inductance in the supply wiring that is more troublesome than 
> resistance.  After all, you can minimise resistance by using 
> progressively wider copper traces or thicker copper weight, 
> but both of these changes do little to reduce self inductance 
> in the current path.
> 
> Power supply stray inductance is particularly apparent where 
> rapidly changing current draws are encountered.  (By rapidly 
> changing I mean going from one value to the other in a short 
> time [high di/dt], rather than something that is necessarily 
> oscillating at some high frequency but might vary smoothly.)  
> Voltage developed across an inductor is equal to L times 
> di/dt.  So if the current changes abruptly enough it will 
> generate a large voltage spike across stray inductance 
> anywhere in the current's path!  If there happens to be 
> significant stray inductance in the supply wiring then all 
> devices connected at that point will see large voltage spikes 
> due to L di/dt whenever the load current changes abruptly.
> 
> This is a classic problem for digital circuits as they 
> frequently draw large current pulses as their transistors 
> switch from one logic state to the other passing through the 
> linear region.  The solution is to place decoupling 
> capacitors locally to all devices that exhibit high di/dt.  
> The local capacitance stiffens the supply and helps to absorb 
> the packets of energy stored up in supply wiring inductance.  
> Design engineers frequently place 100nF 0805 multi-layer SMD 
> caps across every other IC in digital systems simply because 
> they are dirt cheap in the hundreds of thousands, and it is 
> much cheaper than a product recall and rework excercise if 
> they are found to be necessary at a later date.  The reason 
> why they have to be placed *right* up next to the IC?  To 
> minimise stray inductance.
> 
> You can also go a long way to minimising stray inducance in 
> supply wiring by making sure the V+ and V- wires take as 
> closely as possible the same routes. 
> (It is "enclosed area" that makes for lots of inductance, so 
> if you minimise the area between the V+ and V- wires you 
> minimise the inductance.)  For example, you can twist 
> together the red and black wires coming from the power supply 
> to keep them as close together as possible, then you could 
> use power planes on two layers of your PCB to take power "to" 
> and "from" each part that requires it.  This close placement 
> of the two supply nets miminises inductance and also builds 
> in some distributed decoupling capacitance.
> 
> On a 4 layer board the V+ and V- rails can take the outer two 
> layers, with all high speed signals on the internal two 
> layers.  This makes a PCB with very low supply inductance, 
> and minimal RF radiation or susceptibility because the power 
> planes on either side screen the internal signals from the 
> outside world.  For a mixed analogue and digital PCB these 
> layers can carry Analogue V+ and V- in one area of the board 
> and Digital V+ and V- in another, and shouldn't overlap.  
> Such a layout goes a long way to ensuring that any voltage 
> spikes on the supply rails due to high di/dt in the digital 
> circuit don't make it into the sensitive analogue parts.
> 
> Hopefully some useful info on the decoupling and RF 
> susceptibility thread,
> 
> -Richie Burnett, 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy