7/30/2019 7254655 Measure Droop

http://slidepdf.com/reader/full/7254655-measure-droop 1/1

You can simply apply a volt-meter across the power andground terminals of any bypasscapacitor in the area you wish tomeasure. To measure the power toa chip, apply the voltmeter to the

actual vias that touch your chip’spower and ground balls or pins.The difference between the meas-ured voltage at the output termi-nals of your power supply and thatat the input terminals of yourCPU is called the droop in yourpower system. Values as large asseveral tens or even hundreds of millivolts are common.

The dc source impedance of thepower system lies far below 1⍀, so you do not need a fancy high-

input-impedance voltmeter. Any meter with lots of digits of preci-sion will do.

Before making extremely finecomparative measurements,experiment with your voltmeter tosee how temperature and proxim-ity to the emissions from your sys-tem affect it. Keep the instrumentin a well-protected, consistentenvironment. Long leads runningto the voltmeter work fine forordinary dc measurements. Prac-

tically no current runs down thelead wires so their resistance isnegligible. Your board performsits worst when hot .

With your leads touching powerand ground terminals, every measurement you make includes afixed dc offset equal to the power-supply voltage. When trackingdown millivolts of power-supply droop, that dc offset obfuscates your measurements. For example,using a three-digit meter to read

voltages on a 2.5V rail, the small-

est observable value of droopequals the minimum resolution of the meter, 0.01V. That value is notgood enough.

Why not more directly measuredroop? With one terminal of your

meter touching the output termi-nal of the power supply and theother at the power entrance to your CPU, your meter showsdroop in the power rail withoutthe annoying dc offset. Now turn

up the gain on your three-digitmeter to show much smallerobservable changes. Don’t forgetto also measure droop in theground connection from theground terminal of the powersupply to the ground on the CPU,and add that to the power-rail-droop number to compute totaldroop.

Here’s how you can measure abare pc board. This techniquerequires that you know the worst-

case power-supply current thateach chip in your system draws.

First, short from power toground at the power-supply ter-minals. Next, connect a regulatedpower source to Chip A. Connectto every power via and every ground via in that chip’s footprint.Place an ammeter in series withthe power source, and adjust itscurrent limit to the exact currentthat the target chip draws. Sourcecurrent now circulates backward

from the chip through the power

system, which is the exact opposite

of what happens in a real system.You can now observe realisticdroop voltages produced at any location due to the current thatChip A draws. This four-terminaltechnique removes all dc bias from your measurement, so you obtainfull meter resolution. Repeat themeasurements for current injectedat other chip locations. Superim-posing all the results produces aperfect map of total droop in thefinished system.˿

20 ed n | February 3 , 2005 www.edn.com

Measuring droop

Rajen Murugan of Dell writes: What is the best 

technique to make dc-voltage measurement on a

 power rail? Will a four-point measurement technique

be the most accurate? 

 YOU DO NOT NEED A FANCY HIGH-INPUT-

IMPEDANCE VOLTMETER. ANY METER WITH LOTS

OF DIGITS OF PRECISION WILL DO.

signalintegrity  By Howard Johnson, PhD

Howard Johnson, PhD, author of High-Speed Digital Design and High-Speed Signal Propagation , frequently conducts technical work-shops for digital engineers at Oxford University and other sites world-wide. www.sigcon.com, howie03@

sigcon.com.

Talk to usPost comments via TalkBack at theonline version of this article at www.edn.com.