IntroductionOscilloscope users often needto make “floating” measure-ments where neither point ofthe measurement is at ground(earth) potential. “Signalcommon” may be elevated tohundreds of volts from earth.In addition, many of thesemeasurements require therejection of high common-mode signals1 in order toevaluate low-level signals.Unwanted ground currentscan also add bothersome humand ground loops. Too often,users resort to the use ofpotentially dangerous mea-surement techniques to over-come these problems.

Limitations of TraditionalOscilloscopesTraditional oscilloscopedesigns cannot effectivelyhandle these problems alone.Let’s examine why:

Most oscilloscopes have their“signal common” terminalconnected to the protectivegrounding system, commonlyreferred to as “earth” groundor just “ground.” This is doneso that all signals applied toor supplied from the oscillo-scope have a common con-nection point. This is usuallythe oscilloscope chassis andis held at (or very near to)zero volts by virtue of thethird-wire ground in thepower cord for AC-poweredequipment. It also meansthat, with few exceptions, allmeasurements must be madewith respect to “earth”ground. This constrains thetypical oscilloscope (at leastin a single measurement)from being used to measurepotential differences betweentwo points where neitherpoint is at earth ground. Tektronix provides severalsolutions for “floating” mea-

surements which are not onlysafe but also much moreaccurate than the sometimesused potentially dangerousprocedures (see Table 1). These solutions meet the safetyengineering principles statedin the sidebar on page 2. Theyfall into four general cate-gories: • Battery-powered oscillo-

scopes• Monolithic isolation ampli-

fiers• Differential measurement

systems• Isolated input oscillo-

scopes

Copyright © 1998 Tektronix, Inc. All rights reserved.

Floating OscilloscopeMeasurements …And Operator Protection

Technical Brief

__________

1 A “common-mode signal” is defined asa signal which is present at both pointsin a circuit. Typically referenced toground, it’s identical in amplitude, fre-quency, and phase. Making a floatingmeasurement between two pointsrequires rejecting the “common-modesignal” so the difference signal can bedisplayed.

WARNINGWhile the subject of this tech-nical note is about FloatingMeasurements, some defini-tions of terms and general pre-cautions must be understoodbefore proceeding. Histori-cally, floating measurementshave been made by knowinglydefeating the built-in safetyground features of oscillo-scopes or measurement instru-ments in various manners.THIS IS AN UNSAFE ANDDANGEROUS PRACTICE ANDSHOULD NEVER BE DONE!Instead, this technical notedescribes instruments, acces-sories, and practices which canmake these measurementssafely as long as standardsafety practices and precau-tions are observed.

WARNINGWhenever making measure-ments on instruments or cir-cuits which are capable ofdelivering dangerously high-voltage, high-current power,measurement techniciansshould always treat exposedcircuits, bus-bars, etc., as beingpotentially “live,” even whencircuits have been shut off ordisconnected. This is particu-larly true when connecting ordisconnecting probes or testleads.

Floating An Oscilloscope: ADefinition“Floating” a ground referenced oscil-loscope is the technique of defeatingthe oscilloscope’s protective ground-ing system – disconnecting “signalcommon” from earth, either by defeat-ing the grounding system or using anisolation transformer. This allowsaccessible parts of the instrumentsuch as chassis, cabinet, and connec-tors to assume the potential of theprobe ground lead connection point.This is dangerous, not only from thestandpoint of elevated voltages pre-sent on the oscilloscope (a shock haz-ard to the operator), but also due tocumulative stresses on the oscillo-scope’s power transformer insulation.This stress may not cause immediatefailure, but may lead to future danger-ous failures (a shock and fire hazard),even after returning the oscilloscopeto properly grounded operation!Not only is floating a ground-refer-enced oscilloscope dangerous, butthe measurements are often inaccu-rate. This results from the totalcapacitance of the oscilloscope chas-sis being directly connected to thecircuit under test at the point wherethe common lead is connected.TEKTRONIX RECOMMENDS ONLYTHOSE MEASUREMENT TECH-NIQUES THAT COMPLY WITHSAFETY ENGINEERING PRINCIPLESAND ENSURE ACCURATE MEA-SUREMENTS.

Safety Engineering PrinciplesTektronix has adopted many safetyprinciples in the design of theirproducts. Of particular concern tomaking electrical and electronicmeasurements are these principles:• When the instrument is used

properly, accessible parts shall notbecome hazardously live, even inthe event of the single worst-casefault.

• Electronic devices (those devicesemploying conduction in a vac-uum, gas, or semiconductor) shallnot be relied upon for providingoperator protection from electricshock.

• Products should not develop insidi-ous hazards during proper opera-tion (an insidious hazard is a haz-ard which can develop in a way asto be well established beforebecoming apparent).

• The operator shall not need todefeat a protective system to makethe measurement.

Safety – A Shared ResponsibilityThe operator and employer share inthe responsibility of meeting theseprinciples – through proper opera-tion and measurement techniques.

page 2

Management And Safety In The Workplace

Never attempt to defeat theprotective grounding sys-tem of your oscilloscope byusing an isolation trans-former (left) or disconnect-

ing the ground connector onthe power plug (right). Fail-ure to follow safety warn-ings can result in seriousinjury or loss of life.

WARNING

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Battery-Powered OscilloscopesThe TDS3000 Series (see Fig-ure 1), when operated fromAC line power using its stan-dard power cord, exhibits thesame limitations as tradi-tional oscilloscopes dis-cussed previously. However, AC power is notalways available where youwant to make oscilloscopemeasurements. The TDS3000Series optional battery pack(TDS3BAT) allows you tooperate the oscilloscopewithout the need for ACpower. Observing the follow-ing precautions will providesafe operation of theTDS3000 Series oscilloscopeswhen battery powered.

Figure 2 shows the warningthat is displayed on theTDS3000 Series screen whenin battery operation mode. This warning applies to cir-cuits that have voltages ofgreater than 30 VRMS (>42 Vpk).If the circuit under test hasvoltages that exceed 30 VRMS(>42 Vpk), the TDS3000 Serieschassis needs to be connectedto earth ground using thegrounding wire provided withthe instrument (Battery Pack)to prevent electrical shock tothe operator.If you do not attach thegrounding wire, you are notprotected against electricalshock if you connect the oscil-loscope to a hazardousvoltage.

WARNINGSuch electrical shock couldresult in serious personalinjury or loss of life.

While in battery operation andfollowing environmental speci-fication limits for the TDS3000Series, it is safe to “float” the“signal common” for makingmeasurements provided you donot connect a signal greaterthan 30 VRMS (>42 Vpk) fromearth ground to either theprobe tip or common lead. Formeasurements where highervoltages {>30 VRMS, >42 Vpk)are present, the instrument’schassis must be connected toearth ground using the sup-plied grounding wire to pre-vent electrical shock to theoperator.

WARNINGIf there is any doubtwhether more than 30 VRMSis present or not, the sup-plied grounding wire shouldalways be used and floatingmeasurements SHOULDNOT BE ATTEMPTED! Beaware that hazardous volt-ages may exist in unex-pected places due to faultycircuitry in the device-under-test.

Figure 2. Warning displayed by TDS3000 Series when operating inbattery mode.

Table 1. Methods of Making “Floating” Measurements Using Tektronix InstrumentationMeets Safety Dynamic Range/ Ease of Operation Common-Mode Differential Mode Common-Mode

Engineering Principle Bandwidth Voltage (Float) Voltage (Signal) Reject

Isolation Amplifiers ✔ High Medium <850 VRMS <850 VRMS High(A6907 and A6909)

Active High Voltage ✔ Medium High <2,200 VRMS <4,400 VRMS MediumDifferential Probes(P5200 Series)

Differential Amplifiers ✔ Medium (+) Medium (–) <353 VRMS <353 VRMS High(P6135A)

Battery Powered ✔ Low High <30 VRMS <30 VRMS LowOscilloscopes CAUTION! ONLY ONLY(TDS3000 Series)

Isolated Input ✔ High High <600 VRMS <1,000 VRMS LowOscilloscopes(THS700 Series)

Isolation Transformers WARNING! This Is An Unsafe And Dangerous Practice And Should Never Be Done!

Defeating Grounds WARNING! This Is An Unsafe And Dangerous Practice And Should Never Be Done!

Figure 1. TDS30xx Digital Phosphor Oscilloscope (above). Optional TDS3BAT Battery Pack(right).

It’s important to rememberthat the “signal commons”for all channels are at thesame potential and are NOTindependent. Ensure that allprobe common leads are con-nected to the same voltage orcommon point.Do not connect a groundeddevice, such as a printer orcomputer, to the oscilloscopeunless the oscilloscope’sgrounding wire is connectedto earth ground!In order to perform differen-tial measurements with inde-pendent “signal grounds,”true differential probes suchas the P5205 or P5210 mustbe used.

Monolithic Isolation AmplifiersThe Tektronix A6900 Seriesmonolithic isolation ampli-

fiers (see Figure 3) connectbetween the oscilloscope andthe circuit-under-test. Thesignal is coupled across anelectro-optical isolation bar-rier, providing the necessaryisolation. Monolithic voltageisolators provide multiplechannels with different“common” potentials in aconvenient portable package.These units are designed tobe used next to the measure-ment instrument where phys-ical separation is not arequirement.Because multiple channelsare contained within a singleunit, the cost per channel islower and the actual circuit-to-instrument connectionsare simplified.The A6900 Series monolithicvoltage isolators are essential

tools for power conversiondesign. The A6907 andA6909 offer safety, perfor-mance, multiple isolatedchannels (A6907 – four chan-nels, A6909 – two channels),direct connection to the mea-surement instrument, andsimplified controls in a singleinstrument package.Total Galvanic isolation isaccomplished through theuse of electro-optical andoptical-electro converters.The isolator chassis is refer-enced to ground to ensuresafety while making floatingmeasurements.The A6900 Series isolatorsfeature a bandwidth of60 MHz and GPIB control(standard on the A6907;available as Option 10 for theA6909), in a compact pack-age. The CMRR is 105 dB(178,000:1) at 60 Hz, 60 dB(1,000:1) at 1 MHz and 50 dB(316:1) at 10 MHz. The 850 Vprobes plug directly into theisolators and are specificallydesigned for safe connectionto floating circuits andenhanced CMRR.These microprocessor con-trolled instruments featurepush-button self-calibrationof offset and gain forincreased accuracy. Couplingand attenuation for eachchannel can be individuallyset.The A6907 and A6909 satisfythird party safety standards.

Figure 3. A6905 two-channel isolation amplifier (right); A6907 four-channel isolation amplifier (left). Shown withprobes.

page 4

Differential Measurement SystemPseudo-Differential Tech-niques. The most popularsolution to the need for a“floating” measurement is the“A minus B” pseudo-differen-tial technique. Most general-purpose dual-trace oscillo-scopes have an ADD Modewhere the two channels canbe electrically subtracted(invert CH 2), giving a displayof the difference signal.Higher voltage probes such asthe P5100 (2,500 V, 100X –see Figure 4) are used, butthey limit minimum sensitivi-ties. This can be a problemwhen attempting to examinelow-level control signals inthe presence of high common-mode voltages. Also, thecommon-mode dynamic rangeis severely limited (+1 divi-sion beyond screen height)and common-mode rejectionratio (CMRR) is low – approxi-mately 20 to 1.True Differential Techniques.True differential amplifiersfeature complementary inputs(+ and –) and offer highCMRR – as high as 10,000 to 1or higher for many instru-ments. Since the amplifier’sability to reject the common-mode component depends onthe degree to which the twochannels remain balanced, itscommon-mode rejection ratiowill decrease with frequency(imbalance due to effects ofstray C, etc.) and with themagnitude of the common-mode signal (imbalance dueto effects of amplifier over-drive).Use of a differential probepair such as the P6135A (seeFigure 5) is essential to main-tain maximum CMRR.

Figure 4. P5100 High Voltage Passive Probe.

page 5

Figure 5. P6135A Differential Passive Voltage Probe pair.

page 6

High-Voltage Active Differen-tial Probes. The P5200 SeriesHigh Voltage Active Differen-tial Probes are economical,heavy-duty solutions for mak-ing safe, accurate floatingmeasurements with any oscil-loscope. With true differen-tial amplifiers in the probeheads, the compact P5200Series are rated for differen-tial voltage measurements upto 4,400 VRMS (5,600 V (DC +pk AC)) depending on theprobe chosen.Circuit connections are madeby leads terminated by stan-dard 4 mm shrouded bananaplugs. The included crocodileclips and insulated plungerstyle clamps enable safe, easyconnections to a wide rangeof test points from bus bars toIC legs, even when “hot.”The P5200 (see Figure 6) con-nects directly to the BNCinput of the measurementinstrument and is powered bya 9 V wall adapter. The P5200has a bandwidth of 25 MHzwith CMRR of 50 dB at1 MHz.The P5205 (Figure 7) andP5210 (Figure 8) use theTektronix, Inc. TekProbe®

interface system found on theTDS3000, TDS 400, TDS 500,TDS 600, and TDS 700 oscillo-scope systems. The TekProbeinterface provides probepower, readout, and manyother features not readilyavailable on common probes.The P5205 has a bandwidthof 100 MHz and a voltagelevel of 1,300 VRMS. At4,400 VRMS, the P5210 canmeasure frequencies up to50 MHz. Both probes providea CMRR of 50 dB at 1 MHz.The P5200 Series High Volt-age Active Differential Probessatisfy today’s third partysafety standards

Figure 6. P5200 High Voltage Active Differential Probe.

Figure 7. P5205 High Voltage Active Differential Probe.

Figure 8. P5210 High Voltage Active Differential Probe.

page 7

Isolated-input OscilloscopesIsolated-input oscilloscopessuch as the TektronixTHS700 Series (THS710A,60 MHz; THS720A, 100 MHz;THS730A, 200 MHz;THS720P, 100 MHz – see Fig-ure 9) are hand-held, batteryoperated oscilloscopes thatfeature dual input channels,individually isolated fromthe oscilloscope’s chassis aswell as from each other. Thisallows dual-trace waveformcomparisons to be made,with each of the two chan-nels referenced to its owncommon. The safe operatingvoltage of these oscilloscopesdepends on the probes used.The standard 10X P6117probe is rated for IEC cate-gory II applications up to300 VRMS tip-to-common,with the common lead float-

ing up to 30 VRMS above earthground. This range is suitablefor most typical measure-ments on low-voltage elec-tronics circuits.For higher voltage applica-tions, the THS710A,THS720A, THS720P, andTHS730A, with the 10XP5102 Probe (see Figure 10),are rated for IEC category IIapplications up to 1,000 VRMStip-to-common, with thecommon lead floating up to600 VRMS above earth ground.This combination of scopeand probe takes full advan-tage of the THS700 Series Iso-latedChannel™ architectureand allows safe probing offloating signals, providingprotection to both the userand the equipment undertest.

ConclusionTektronix has reviewed thesemethods in an effort toincrease user awarenessregarding the potential dan-gers inherent in the improperoperation of oscilloscopes.Our commitment to Test andMeasurement Product Safetyhas resulted in the isolatedinput oscilloscope, isolationamplifiers, and differentialproducts described here. Ifyou feel your applicationswould be more safely andaccurately served by theseproducts, please contact yournearest Tektronix representa-tive.

Figure 9. THS720P TekScope Isolated Channel oscilloscope.

Figure 10. P5102 High Voltage Probe for the THS700 Series.

11/98 TD/XBS 51W–10640–1

Copyright © 1998, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in thispublication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registeredtrademarks of Tektronix, Inc. All other trade names referenced are the service marks, trademarks, or registered trademarks of their respective companies.

For further information, contact Tektronix:Worldwide Web: For the most up-to-date product information, visit our web site at www.tektronix.com.

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