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Quiet Power: Don't Forget to Terminate Cables

We know that in high-speed signal integrity measurements, one of the first rules is to properly terminate traces and cables. However, many of our PDN measurements may be limited to lower frequencies, such as measuring the switching ripple of a DC-DC converter. Do we really need to terminate our measurement cables if the signal we want to measure is the switching ripple of a converter running at 1 MHz? We may think that termination is not needed or does not matter in this case, but we may be in for some surprises!

Let us look at the output ripple of the LTM4604 converter, which was our device under test in an earlier column. The converter is powered from a 4.5V battery pack, and its output voltage is set to 1.2V. An approximate 1A DC load is created by a small incandescent bulb. The output ripple is measured by a Tektronix TDS540B oscilloscope that I recently bought on eBay. The 1 GS/s maximum real-time sampling rate and 500 MHz analog bandwidth, together with a maximum 1 mV/div vertical sensitivity makes this category of oscilloscopes very useful for many PDN measurements even today. Read the full column here.Editor's Note: This column originally appeared in the August 2013 issue of The PCB Design Magazine.