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Quiet Power: Do Not Measure PDN Noise Across Capacitors!

Some application notes will tell you that to measure the output ripple of a DC-DC converter, the best way to make the connection is to attach the probe across the top of one of the output capacitors. While there is a legitimate argument for measuring the noise this way, be aware: if you measure the noise across a capacitor, it will most likely alter the noise signature.

Take, for instance, the evaluation board shown in Figure 1. It holds a small, encapsulated step-down, non-isolated DC-DC converter--the black rectangular package in the middle of the board. I use this board in my courses to show live measurements of converter loop stability and output impedance. This is very convenient for desktop demos, because the input voltage range is a few volts, so the entire setup can be powered simply with three AA batteries. The converter can be loaded with up to 4A of current. In addition to the converter module, which contains a fully functioning DC-DC converter, the evaluation module has banana receptacles for connecting the input supply and the load, a few ceramic capacitors across the input and output terminals, jumpers for setting the output voltage, and two BNC receptacles to connect oscilloscopes.Read the full column here.Editor's Note: This column originally appeared in the February 2013 issue of The PCB Design Magazine.