3D Visualization Of Bad Decoupling Capacitor Placement

Placement matters: what happen when a decoupling capacitor being placed far far away from the IC that it is supposed to decouple? Compare diagram below to those in previous posts:

• http://electroniccircuitdesignsharing.blogspot.com/2012/07/3d-current-flow-for-inverting-op-amp.html
• http://electroniccircuitdesignsharing.blogspot.com/2012/07/3d-visualization-of-better-decoupling.html

This time, I have removed all traces that that is not related to positive supply decouling capacitor discharge path. Obviously in this placement, the decoupling capacitor discharge path is much longer that those in posts stated above.  In this case there are much larger series inductance that discourage good decoupling performance (as it is more troublesome for capacitor to discharge).

Top view of top signals – positive supply decoupling capacitor discharge path

Top view of bottom signals – positive supply decoupling capacitor discharge path

3D Visualization Of Better Decoupling Capacitor Placement

Placement matters: One way to decide how good a decoupling capacitor placement is by looking at the discharge current loop. Continue from previous post (http://electroniccircuitdesignsharing.blogspot.com/2012/07/3d-current-flow-for-inverting-op-amp.html), re-placing the decoupling capacitor near IC pin6 to bottom and just under supply pin will allow much better decoupling performance due to smaller discharge current loop. Compare diagrams below with the ones shown in previous post for details.

Top view of top signals

Top view of bottom signals

3D current flow for inverting op-amp with negative input pulse

I have always wanted to draw a 3-dimensional illustrative diagram on how the current flow – for the SMT circuit – with the Google Sketch up (mentioned in http://electroniccircuitdesignsharing.blogspot.com/2012/03/tools-for-circuit-design.html), finally I can create something shown in this post. Hopefully it is clear enough for you to understand how current flows after reading this.

Input and output voltages:

How the current flow – Inverting Amp with negative input pulse:

Top view of bottom signals:

Where and how does the current flow when the load resistor is being replaced by realistic next stage?

What’s the current loop for a step positive voltage at vin?

The flow:

  

Note that there’s no ground current involve other than the interconnect of 2 decoupling capacitors. – current does not always flows in ground interconnect

Why do we want to understand the current loop?

Because it allows us to build the physical circuit with the best result, we should always minimize the distance fast current travels – the smaller the loop the better it is. See the difference of the good or bad placement – for the bad placement sometimes it does works, depending on the specification of the circuit.

Bad one:

Good one:

Where and how does the current flow?

All circuit has different current loop, let’s start with something simple such as a voltage buffer circuit shown above. Before we even try, we need to redo the schematic into some sort of diagram as shown below :

Positive pulse at output:

Negative pulse at output: