Digital Multimeter Measurement Errors Series

System Cabling Errors and DCVoltage Measurement Errorsin Digital Multimeters

Application Note AN 1389-1

When making measurements with a digital multimeter(DMM), common errors will crop up. The followingdiscussion will help you eliminate potential measure-ment errors and achieve the greatest accuracy witha DMM. This paper covers system cabling errors anddc voltage measurement errors. For an overview ofac voltage measurement errors, see Application NoteAN 1389-3. For a discussion of resistance; dc current;ac current; and frequency and period measurementerrors, see Application Note AN 1389-2. (NOTE: TheAgilent 34401A, a 6-1/2-digit, high-performance DMMwith both benchtop and system features, will be usedas an example throughout this article).

Introduction

Radio Frequency Interference Most voltage-measuringinstruments can generate falsereadings in the presence of large,high-frequency signal sourcessuch as nearby radio and televisiontransmitters, computer monitors,and cellular telephones. High-frequency energy can also becoupled to the multimeter onthe system cabling. To reduceinterference, try to minimize theexposure of the system cabling tohigh-frequency RF sources. If theapplication is extremely sensitiveto RFI radiating from the multi-meter, use a common mode chokein the system cabling, as shown inFigure 1, to attenuate multimeteremissions.

Thermal EMF Errors Thermoelectric voltages, the mostcommon source of error in low-level voltage measurements, aregenerated when circuit connectionsare made with dissimilar metalsat different temperatures. Eachmetal-to-metal junction forms athermocouple, which generatesa voltage proportional to thejunction temperature. It is agood idea to take the necessaryprecautions to minimizethermocouple voltages andtemperature variations in low-level voltage measurements.The best connections are formedusing copper-to-copper crimpedconnections. Figure 2 showscommon thermoelectric voltagesfor connections between dissimilarmetals.

Noise Caused by Magnetic Fields When you make measurementsnear magnetic fields, take pre-cautionary steps to avoid inducingvoltages in the measurementconnections. Voltage can beinduced by either movement ofthe input connection wiring ina fixed magnetic field, or bya varying magnetic field. Anunshielded, poorly dressedinput wire moving in the earthsmagnetic field can generateseveral millivolts. The varyingmagnetic field around the acpower line can also inducevoltages up to several hundredmillivolts. Be especially carefulwhen working near conductorscarrying large currents.

Where possible, route cablingaway from magnetic fields, whichare commonly present aroundelectric motors, generators,televisions and computer monitors.In addition, when you areoperating near magnetic fields,be certain that the input wiringhas proper strain relief and is tieddown securely. Use twisted-pairconnections to the multimeter toreduce the noise pickup loop area,or dress the wires as closelytogether as possible.

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System Cabling Errors

Figure 1.

Figure 2.

Copper-to- Approx. V/ C

Copper < 0.3Gold 0.5Silver 0.5Brass 3Beryllium Copper 5Aluminum 5Kovar or Alloy 42 40Silicon 500Copper-Oxide 1000Cadmium-Tin Solder 0.2Tin-Lead Solder 5

Noise Caused by Ground Loops When you measure voltages incircuits where the multimeter andthe device-under-test (DUT) areboth referenced to a commonearth ground, a ground loop isformed. As shown in Figure 3,any voltage difference betweenthe two ground reference points(Vground) causes a current to flowthrough the measurement leads.This causes errors, such as noiseand offset voltage (usually power-line related), which are added tothe measured voltage.

The best way to eliminate groundloops is to maintain the multimetersisolation from earth; do notconnect the input terminals toground. If the multimeter mustbe earth-referenced, be sure toconnect it and the DUT to thesame common ground point.This will reduce or eliminate anyvoltage difference between thedevices. Also, whenever possible,make sure the multimeter andDUT are connected to the sameelectrical outlet.

3

Figure 3.

RL = Lead ResistanceR i = Multimeter Isolation ResistanceVground = Voltage Drop on Ground Bus

Common Mode Rejection Ideally, a multimeter is completelyisolated from earth-referencedcircuits. However, there is finiteresistance between the multimetersinput LO terminal and earthground, as shown in Figure 4.This can cause errors when youmeasure low voltages that arefloating relative to earth ground.

Noise Caused by Injected Current.Residual capacitances in themultimeters power transformercause small currents to flow fromthe LO terminal to earth ground.The frequency of the injectedcurrent is the power line frequencyor possibly harmonics of thepower line frequency. The injectedcurrent is dependent upon thepower line configuration andfrequency. A simplified circuitis shown in Figure 5.

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DC Voltage Measurement Errors

Figure 4.

Vf = Float VoltageRs = DUT Source Resistance ImbalanceRi = Multimeter Isolation Resistance (LO-Earth)Ci = Multimeter Input Capacitance ( 200 pF LO-Earth)

Error (v) =Vf x RsRs + Ri

Figure 5.

Injected Current(50/60 Hz ac line leakage current)

With Connection A (see Figure 6),the injected current flows fromthe earth connection provided bythe circuit to the LO terminal ofthe DMM, adding no noise to themeasurement. However, withConnection B, the injected currentflows through the resistor R,thereby adding noise to themeasurement. With Connection B,larger values of R will worsenthe problem.

Loading Errors Due to InputResistance Measurementloading errors occur whenthe resistance of the DUT is anappreciable percentage of themultimeters own input resistance.Figure 7 shows this error source.

To reduce the effects of loadingerrors, and to minimize noisepickup, set the Agilent 34401Asinput resistance to greater than10 G for the 100 mVdc, 1 Vdc,and 10 Vdc ranges. The inputresistance is maintained at 10 Mfor the 100 Vdc and 1000 Vdcranges.

5

Figure 7.

Vs = Ideal DUT VoltageRs = DUT Source ResistanceRi = Multimeter Input Resistance (10 M or >10 G)

Error (%) = 100 x RsRs + Ri

Figure 6.

Note: Themeasurementnoise caused byinjected currentcan be significantlyreduced by settingthe integrationtime of the DMMto 1 power linecycle (PLC) orgreater.

Loading Errors Due to Input BiasCurrent The multimeters inputcapacitance will "charge up" dueto input bias currents when theterminals are open-circuited (ifthe input resistance is 10 G).The multimeters measuringcircuitry exhibits approximately30 pA of input bias current forambient temperatures from 0 Cto 30 C. Bias current will double(x2) for every 8 C change inambient temperature above 30 C.This current generates smallvoltage offsets dependent uponthe source resistance of the DUT.This effect becomes evident for asource resistance of greater than100 k, or when the multimetersoperating temperature is signifi-cantly greater than 30 C.

6

Figure 8.

ib = Multimeter Bias CurrentRs = DUT Source ResistanceCi = Multimeter Input Capacitance

Error (v) ib x Rs=

A modern DMM such as the34401A has many features to helpprotect you from many commonsources of error. However, whenmaking low-level or precisemeasurements, you must bemore aware of sources thatcontribution to measurementerrors. Environmental conditions such as changes in temperature,RF signals and electromagneticfields will have a significantimpact on the quality of DMMmeasurements. Once themeasurement environmenthas been optimized, propercabling can significantly reducemeasurement errors. The remainingsources of error can be calculatedand added to the measurementuncertainty.

For more information aboutthe Agilent 34401A, go towww.agilent.com/find/34401a

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Conclusion

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Product specifications and descriptions in this document subject to change without notice. Agilent Technologies, Inc. 2002Printed in the USA January 31, 20025988-5511EN

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