Low Level Measurements Handbook

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TABLE OF C ONTENTSSECTION 1 Low Level DC Measuring Instruments1.1 Introduction ..................................................................................1-31.2 Theoretical Measurement Limits..............................................1-31.3 Instrument Definitions................................................................1-51.3.1 The Electrometer..........................................................1-51.3.2 The DMM ......................................................................1-71.3.3 The Nanovoltmeter ......................................................1-71.3.4 The Picoammeter..........................................................1-81.3.5 The Source-Measure Unit ............................................1-81.3.6 The SourceMeter ® Instrument ....................................1-91.3.7 The Low Current Preamp ............................................1-91.3.8 The Micro-ohmmeter....................................................1-91.4 Understanding Instrument Specifications ..........................1-101.4.1 Definition of Accuracy Terms ....................................1-101.4.2 Accuracy......................................................................1-101.4.3 Deratings ....................................................................1-131.4.4 Noise and Noise Rejection ........................................1-141.4.5 Speed ..........................................................................1-151.5 Circuit Design Basics ................................................................1-161.5.1 Voltmeter Circuits ......................................................1-161.5.2 Ammeter Circuits........................................................1-171.5.3 Coulombmeter Circuits..............................................1-221.5.4 High Resistance Ohmmeter Circuits ..........................1-221.5.5 Low Resistance Ohmmeter Circuits ..........................1-251.5.6 Complete Instruments................................................1-29SECTION 2 Measurements from High Resistance Sources2.1 Introduction ..................................................................................2-22.2 Voltage Measurements fromHigh Resistance Sources ............................................................2-22.2.1 Loading Errors and Guarding ......................................2-22.2.2 Insulation Resistance..................................................2-11Low Level Measurements Handbookiii
2.3 Low Current Measurements....................................................2-142.3.1 Leakage Currents and Guarding ................................2-142.3.2 Noise and Source Impedance ....................................2-192.3.3 Zero Drift ....................................................................2-212.3.4 Generated Currents....................................................2-222.3.5 Voltage Burden ..........................................................2-282.3.6 Overload Protection ..................................................2-302.3.7 AC Interference and Damping ..................................2-312.3.8 Using a Coulombmeter to Measure Low Current......2-332.4 High Resistance Measurements ............................................2-362.4.1 Constant-Voltage Method ..........................................2-362.4.2 Constant-Current Method ..........................................2-372.4.3 Characteristics of High Ohmic Valued Resistors ........2-432.5 Charge Measurements..............................................................2-442.5.1 Error Sources..............................................................2-442.5.2 Zero Check ................................................................2-452.5.3 Extending the Charge Measurement Rangeof the Electrometer ....................................................2-462.6 General Electrometer Considerations ..................................2-472.6.1 Making Connections ..................................................2-472.6.2 Electrostatic Interference and Shielding....................2-492.6.3 Environmental Factors................................................2-522.6.4 Speed Considerations ................................................2-532.6.5 Johnson Noise ............................................................2-582.6.6 Device Connections....................................................2-622.6.7 Analog Outputs ..........................................................2-662.6.8 Floating Input Signals ................................................2-672.6.9 Electrometer Verification............................................2-68SECTION 3 Measurements from Low Resistance Sources3.1 Introduction ..................................................................................3-23.2 Low Voltage Measurements ......................................................3-23.2.1 Offset Voltages ..............................................................3-23.2.2 Noise ..........................................................................3-103.2.3 Common-Mode Current and Reversal Errors ............3-15iv
Page 1 and 2: www.keithley.comLLM6 th EditionLow
Page 6 and 7: 3.3 Low Resistance Measurements....
Page 8 and 9: SECTION 1Low Level DCMeasuringInstr
Page 10 and 11: 1.1 IntroductionDC voltage, DC curr
Page 12 and 13: FIGURE 1-3: Typical Digital Multime
Page 14 and 15: Coulombmeter FunctionCurrent integr
Page 16 and 17: 1.3.6 The SourceMeter ® Instrument
Page 18 and 19: TABLE 1-1: Specification Conversion
Page 20 and 21: Transfer StabilityA special case of
Page 22 and 23: FIGURE 1-6: Common Mode NoiseMeasur
Page 24 and 25: FIGURE 1-8: Voltage Amplifier+-V 2A
Page 26 and 27: Picoammeter amplifier gain can be c
Page 28 and 29: Using a small-signal transistor in
Page 30 and 31: FIGURE 1-17: High Resistance Measur
Page 32 and 33: FIGURE 1-20: High Resistance Measur
Page 34 and 35: Micro-ohmmeterThe micro-ohmmeter al
Page 36 and 37: FIGURE 1-26: Typical Digital Electr
Page 38 and 39: In order to cancel internal offsets
Page 40: Sense circuitry is used to monitor
Page 43 and 44: 2.1 IntroductionAs described in Sec
Page 45 and 46: cause significant loading errors. I
Page 47 and 48: Cable leakage resistance is a commo
Page 49 and 50: FIGURE 2-6: Guarding Leakage Resist
Page 51 and 52: Example: Assume R S = 10GΩ and C S
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MaterialTABLE 2-2: Properties of Va
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QuartzQuartz has properties similar
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FIGURE 2-11: Guarding as Applied to
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FIGURE 2-13: Guarding the Leakage R
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FIGURE 2-15: Simplified Model of a
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time and/or temperature. Zero offse
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open-circuited, allow the reading t
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to equalize charges and minimize ch
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If insulators become contaminated,
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The input resistance of a feedback
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ence. However, in some cases, shiel
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FIGURE 2-27: Feedback Coulombmeter
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Advantages of Using a Coulombmeter
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FIGURE 2-30: Constant-Voltage Metho
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In addition to the voltage drop lim
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FIGURE 2-34a: Effects of Cable Resi
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Such devices require extreme care i
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charge will be lost through the zer
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FIGURE 2-40: Proper ConnectionCurre
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Figure 2-43 shows an example of AC
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fering voltage or current. A guard
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esponse is the rise time of the ins
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FIGURE 2-47: Shunt Capacitance Effe
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Resistance Measurements (Constant-C
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FIGURE 2-52: Noise Voltage vs. Band
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Source ResistanceAfter the bandwidt
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a. ConfigurationShieldCenterconduct
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• Shielding and Guarding: The fix
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floating circuits, a second grounde
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3.1 IntroductionLow voltage and low
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Thermoelectric EMFsThermoelectric v
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Reversing Sources to Cancel Thermoe
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quency spectrum of these interferen
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ment is almost entirely determined
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FIGURE 3-9: Low Voltages Generated
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FIGURE 3-11a: Multiple Grounds (Gro
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Common-Mode Reversal ErrorsReversin
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FIGURE 3-14: Two-Wire Resistance Me
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FIGURE 3-16: Canceling Thermoelectr
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Both V A and V B are affected by th
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If using a micro-ohmmeter or DMM to
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FIGURE 3-20: Dry Circuit Testing Us
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SECTION 4Applications
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For timing and integrating applicat
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FIGURE 4-2:Using an Electrometer to
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If current flows, the electrodes wi
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4.3 Low Current Measurement Applica
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DC. The Model 6517A can also be use
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voltage (V DS ) and measures the re
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The Keithley Model 248 High Voltage
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FIGURE 4-16: Ion Collector with Gro
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Figure 4-19 shows a Model 6430 conn
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FIGURE 4-21: SIR Test System to Mea
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FIGURE 4-22: Volume ResistivityElec
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these problems, the Alternating Pol
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FIGURE 4-25: Four-Point Collinear P
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Using two electrometers eliminates
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A plot of this function is shown in
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FIGURE 4-31: van der Pauw Measureme
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Through interactive programming, th
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objects. As discussed in Section 2.
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FIGURE 4-36: Faraday CupOutside Ele
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FIGURE 4-38: Connections for Standa
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FIGURE 4-40: Microcalorimeter with
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Measurement MethodFigure 4-42 illus
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FIGURE 4-43: Using a Nanovoltmeter
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exceed the critical current of the
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equation for V/I. Most materials ha
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FIGURE 4-49: van der Pauw Connectio
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5.1 IntroductionChoosing a specific
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TABLE 5-1a: Low Current/High Resist
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Table 5-1b: Source-Measure Instrume
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Table 5-2: High Speed Power Supplie
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Table 5-3: Connectors, Adapters, an
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Table 5-4: CablesTERMINATIONS LENGT
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Table 5-5: Test Leads and ProbesMOD
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Table 5-6: Switching Cards for the
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APPENDIX ALow LevelMeasurementTroub
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Measurement Type andTypical Applica
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Proper cable and connector assembly
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APPENDIX CGlossary
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channels. For matrix cards, a chann
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R. Buckminster Fuller. Sometimes ca
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NORMAL-MODE REJECTION RATIO (NMRR).
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SOURCEMETER. A SourceMeter instrume
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APPENDIX DSafetyConsiderations
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The types of product users are:Resp
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are the same. Other components that
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1/f noise, 3-7 to 3-83dB point, 2-5
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Normal mode rejection ratio (NMRR),
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Specifications are subject to chang
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Specifications are subject to chang
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Low Level Measurements Handbook

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