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Low Level Measurements Handbook Precision DC Current, Voltage, and Resistance Measurements Low Level Measurements Handbook 6th Edition LLM Keithley Instruments, Inc. Corporate Headquarters 28775 Aurora Road Cleveland, Ohio 44139 440-248-0400 Fax: 440-248-6168 1-888-KEITHLEY (534-8453) Copyright 2004 Keithley Instruments, Inc.No. 1559 Printed in U.S.A.80440KSI Specifications are subject to change without notice. All Keithley trademarks and trade names are the property of Keithley Instruments, Inc. All other trademarks and trade names are the property of their respective companies. 6thEdition RadioFans.CN 收音机爱 好者资料库 Low Level Measurements Handbook Precision DC Current, Voltage, and Resistance Measurements S I X T H E D I T I O N AG R E A T E R M E A S U R E O F C O N F I D E N C E RadioFans.CN 收音机爱 好者资料库 RadioFans.CN 收音机爱 好者资料库 Low Level Measurements Handbookiii SECTION 1Low Level DC Measuring Instruments 1.1Introduction .1-3 1.2Theoretical Measurement Limits.1-3 1.3Instrument Definitions.1-5 1.3.1The Electrometer.1-5 1.3.2The DMM.1-7 1.3.3The Nanovoltmeter .1-7 1.3.4The Picoammeter.1-8 1.3.5The Source-Measure Unit .1-8 1.3.6The SourceMeterInstrument .1-9 1.3.7The Low Current Preamp .1-9 1.3.8The Micro-ohmmeter.1-9 1.4Understanding Instrument Specifications .1-10 1.4.1Definition of Accuracy Terms .1-10 1.4.2Accuracy.1-10 1.4.3Deratings .1-13 1.4.4Noise and Noise Rejection .1-14 1.4.5Speed.1-15 1.5Circuit Design Basics .1-16 1.5.1Voltmeter Circuits .1-16 1.5.2Ammeter Circuits.1-17 1.5.3Coulombmeter Circuits.1-22 1.5.4High Resistance Ohmmeter Circuits.1-22 1.5.5Low Resistance Ohmmeter Circuits .1-25 1.5.6Complete Instruments.1-29 SECTION 2Measurements from High Resistance Sources 2.1Introduction .2-2 2.2Voltage Measurements from High Resistance Sources.2-2 2.2.1Loading Errors and Guarding .2-2 2.2.2Insulation Resistance.2-11 TABLE OFCONTENTS iv 2.3Low Current Measurements.2-14 2.3.1Leakage Currents and Guarding .2-14 2.3.2Noise and Source Impedance .2-19 2.3.3Zero Drift.2-21 2.3.4Generated Currents.2-22 2.3.5Voltage Burden .2-28 2.3.6Overload Protection .2-30 2.3.7AC Interference and Damping .2-31 2.3.8Using a Coulombmeter to Measure Low Current.2-33 2.4High Resistance Measurements .2-36 2.4.1Constant-Voltage Method .2-36 2.4.2Constant-Current Method.2-37 2.4.3Characteristics of High Ohmic Valued Resistors.2-43 2.5Charge Measurements.2-44 2.5.1Error Sources.2-44 2.5.2Zero Check .2-45 2.5.3Extending the Charge Measurement Range of the Electrometer .2-46 2.6General Electrometer Considerations .2-47 2.6.1Making Connections .2-47 2.6.2Electrostatic Interference and Shielding.2-49 2.6.3Environmental Factors.2-52 2.6.4Speed Considerations .2-53 2.6.5Johnson Noise .2-58 2.6.6Device Connections.2-62 2.6.7Analog Outputs .2-66 2.6.8Floating Input Signals .2-67 2.6.9Electrometer Verification.2-68 SECTION 3Measurements from Low Resistance Sources 3.1Introduction .3-2 3.2Low Voltage Measurements.3-2 3.2.1Offset Voltages.3-2 3.2.2Noise .3-10 3.2.3Common-Mode Current and Reversal Errors.3-15 Low Level Measurements Handbookv 3.3Low Resistance Measurements.3-16 3.3.1Lead Resistance and Four-Wire Method .3-16 3.3.2Thermoelectric EMFs and Offset Compensation Methods .3-19 3.3.3Non-Ohmic Contacts.3-23 3.3.4Device Heating .3-24 3.3.5Dry Circuit Testing.3-25 3.3.6Testing Inductive Devices .3-26 SECTION 4Applications 4.1Introduction .4-2 4.2Applications for Measuring Voltage from High Resistance Sources.4-2 4.2.1Capacitor Dielectric Absorption .4-2 4.2.2Electrochemical Measurements.4-5 4.3Low Current Measurement Applications .4-9 4.3.1Capacitor Leakage Measurements.4-9 4.3.2Low Current Semiconductor Measurements .4-11 4.3.3Light Measurements with Photomultiplier Tubes.4-14 4.3.4Ion Beam Measurements.4-16 4.3.5Avalanche Photodiode Reverse Bias Current Measurements .4-18 4.4High Resistance Measurement Applications.4-20 4.4.1Surface Insulation Resistance Testing of Printed Circuit Boards.4-20 4.4.2Resistivity Measurements of Insulating Materials .4-22 4.4.3Resistivity Measurements of Semiconductors.4-26 4.4.4Voltage Coefficient Testing of High Ohmic Value Resistors .4-35 4.5Charge Measurement Applications .4-36 4.5.1Capacitance Measurements.4-37 4.5.2Using a Faraday Cup to Measure Static Charge on Objects.4-38 4.6Low Voltage Measurement Applications .4-39 4.6.1Standard Cell Comparisons.4-39 vi 4.6.2High Resolution Temperature Measurements and Microcalorimetry .4-42 4.7Low Resistance Measurement Applications .4-44 4.7.1Contact Resistance.4-44 4.7.2Superconductor Resistance Measurements .4-47 4.7.3Resistivity Measurements of Conductive Materials.4-50 SECTION 5Low Level Instrument Selection Guide 5.1Introduction .5-2 5.2Instrument and Accessory Selector Guides .5-2 APPENDIX ALow Level Measurement Troubleshooting Guide APPENDIX BCable and Connector Assembly APPENDIX CGlossary APPENDIX DSafety Considerations INDEX S E C T I O N 1 Low Level DC Measuring Instruments FIGURE 1-1: Standard Symbols Used in this Text 1-2SECTION 1 UnitSymbolQuantity Quantities V A C s W F Hz K EMF current resistance charge time power capacitance frequency temperature volts amperes ohms coulombs seconds watts farads cycles/s degrees PrefixSymbolExponent Prefixes y z a f p n m (none) k M G T P E Z Y yocto- zepto- atto- femto- pico- nano- micro- milli- (none) kilo- mega- giga- tera- peta- exa- zetta- yotta- 1024 1021 1018 1015 1012 109 106 103 100 103 106 109 1012 1015 1018 1021 1024 1.1Introduction DC voltage, DC current, and resistance are measured most often with digi- tal multimeters (DMMs). Generally, these instruments are adequate for measurements at signal levels greater than 1V or 1A, or less than 1G. (See Figure 1-1 for standard symbols used in this text.) However, they dont approach the theoretical limits of sensitivity. For low level signals, more sen- sitive instruments such as electrometers, picoammeters, and nanovolt- meters must be used. Section 1 offers an overview of the theoretical limits of DC measure- ments and the instruments used to make them. It includes instrument descriptions and basic instrument circuit designs. For easier reference, this information is organized into a number of subsections: 1.2 Theoretical Measurement Limits: A discussion of both the theoretical measurement limitations and instrument limitations for low level meas- urements. 1.3 Instrument Definitions: Descriptions of electrometers, DMMs, nano- voltmeters, picoammeters, source-measure units, SourceMeterinstru- ments, low current preamps, and micro-ohmmeters. 1.4 Understanding Instrument Specifications: A review of the terminology used in instrument specifications, such as accuracy (resolution, sensi- tivity, transfer stability), deratings (temperature coefficient, time drift), noise (NMRR and CMRR), and speed. 1.5 Circuit Design Basics: Describes basic circuit design for voltmeter cir- cuits (electrometer, nanovoltmeter) and ammeter circuits (shunt amme- ter, feedback picoammeter, high speed picoammeter, logarithmic picoammeter). 1.2Theoretical Measurement Limits The theoretical limit of sensitivity in any measurement is determined by the noise generated by the resistances present in the circuit. As discussed in Sections 2.6.5 and 3.2.2, voltage noise is proportional to the square root of the resistance, bandwidth, and absolute temperature. Figure 1-2 shows the- oretical voltage measurement limits at room temperature (300K) with a response time of 0.1 second to ten seconds. Note that high source resist- ance limits the theoretical sensitivity of the voltage measurement. While its certainly possible to measure a 1V signal that has a 1 source resistance, its not possible to measure that same 1V signal level from a 1T source. Even with a much lower 1M source resistance, a 1V measurement is near theoretical limits, so it would be very difficult to make using an ordinary DMM. In addition to having insufficient voltage or current sensitivity (most DMMs are no more sensitive than 1V or 1nA per digit), DMMs have high Low Level DC Measuring Instruments1-3 input offset current1when measuring voltage and lower input resistance compared to more sensitive instruments intended for low level DC meas- urements. These characteristics cause errors in the measurement; refer to Sections 2 and 3 for further discussion of them. Given these DMM characteristics, its not possible to use a DMM to measure signals at levels close to theoretical measurement limits, as shown in Figure 1-3. However, if the source resistance is 1M or less, or if the desired resolution is no better than 0.1V (with low source resistance), the signal level isnt “near theoretical limits,” and a DMM is adequate. If better voltage sensitivity is desired, and the source resistance is low (as it must be because of theoretical limitations), a nanovoltmeter provides a means of measuring at levels much closer to the theoretical limits of measurement. With very high source resistance values (for example, 1T), a DMM isnt a suitable voltmeter. DMM input resistance ranges from 10M to 10Gsev- eral orders of magnitude less than a 1T source resistance, resulting in severe input loading errors. Also, input currents are typically many picoamps, creating large voltage offsets. However, because of its much high- er input resistance, an electrometer can make voltage measurements at lev- els that approach theoretical limits. A similar situation exists for low level current measurements; DMMs generally have a high input voltage drop 1-4SECTION 1 FIGURE 1-2: Theoretical Limits of Voltage Measurements 103 100 103 106 109 1012 1kV 1V 1mV 1V 1nV 1pV 1001031061091012 11k1M1G1T Within theoretical limits Near theoretical limits Prohibited by noise Noise Voltage Source Resistance 1Input current flows in the input lead of an active device or instrument. With voltage measurements, the input current is ideally zero; thus, any input current represents an error. With current measurements, the signal current becomes the input current of the measuring instrument. However, some background cur- rent is always present when no signal current is applied to the instrument input. This unwanted current is the input offset current (often called just the offset current) of the instrument. The source and test connections can also generate unwanted offset currents and offset voltages. A leakage current is another unwanted error current resulting from voltage across an undesired resist- ance path (called leakage resistance). This current, combined with the offset current, is the total error current. (input burden), which affects low level current measurements, and DMM resolution is generally no better than 1nA. Thus, an electrometer or picoam- meter with its much lower input burden and better sensitivity will operate at levels much closer to the theoretical (and practical) limits of low current m