Low Level Measurements Handbook

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FIGURE 1-3: Typical Digital Multimeter (DMM), Nanovoltmeter (nVM), NanovoltPreamplifier (nV PreAmp), and Electrometer Limits of Measurement atVarious Source Resistances1V10 0NoiseVoltage1mV10 –3ElectrometerDMMnVMnV PreAmp1µV1nV10 –610 –91pV10 –31mΩ10 0 10 3 10 6 10 9 10 121Ω 1kΩ 1MΩ 1GΩ 1TΩSource Resistance10 151PΩ10 –12(input burden), which affects low level current measurements, and DMMresolution is generally no better than 1nA. Thus, an electrometer or picoammeterwith its much lower input burden and better sensitivity will operateat levels much closer to the theoretical (and practical) limits of low currentmeasurements.1.3 Instrument DefinitionsA number of different types of instruments are available to make DC measurements,including electrometers, DMMs, nanovoltmeters, picoammeters,SMUs (source-measure units), SourceMeter instruments, low current preamps,and micro-ohmmeters. The following paragraphs discuss and comparethe important characteristics of these instruments.1.3.1 The ElectrometerAn electrometer is a highly refined DC multimeter. As such, it can be usedfor many measurements performed by a conventional DC multimeter.Additionally, an electrometer’s special input characteristics and high sensitivityallow it to make voltage, current, resistance, and charge measurementsfar beyond the capabilities of a conventional DMM.An electrometer must be used when any of the following conditionsexist:1. The task requires an extended measurement range unavailable withconventional instruments, such as for detecting or measuring:• Currents less than 10nA (10 –8 A).• Resistances greater than 1GΩ (10 9 Ω).<strong>Low</strong> <strong>Level</strong> DC Measuring Instruments 1-5
2. Circuit loading must be minimized, such as when:• Measuring voltage from a source resistance of 100MΩ or higher.• Measuring current when input voltage drop (burden) of less than afew hundred millivolts is required (when measuring currents fromsources of a few volts or less).3. Charge measurement is required.4. Measuring signals at or near Johnson noise limitations (as indicated inFigure 1-2).In addition to their versatility, electrometers are easy to operate, reliable,and rugged.Voltmeter FunctionThe input resistance of an electrometer voltmeter is extremely high, typicallygreater than 100TΩ (10 14 Ω). Furthermore, the input offset current isless than 3fA (3×10 –15 A). These characteristics describe a device that canmeasure voltage with a very small amount of circuit loading.Because of the high input resistance and low offset current, the electrometervoltmeter has minimal effect on the circuit being measured. As aresult, the electrometer can be used to measure voltage in situations wherean ordinary multimeter would be unusable. For example, the electrometercan measure the voltage on a 500pF capacitor without significantly dischargingthe device; it can also measure the potential of piezoelectric crystalsand high impedance pH electrodes.Ammeter FunctionAs an ammeter, the electrometer is capable of measuring extremely low currents,limited only by theoretical limits or by the instrument’s input offsetcurrent. It also has a much lower voltage burden than conventional DMMs.With its extremely low input offset current and minimal input voltageburden, it can detect currents as low as 1fA (10 –15 A). Because of this highsensitivity, it’s suitable for measuring the current output of photomultipliersand ion chambers, as well as very low currents in semiconductors, massspectrometers, and other devices.Ohmmeter FunctionAn electrometer may measure resistance by using either a constant-currentor a constant-voltage method. If using the constant-current method, theelectrometer’s high input resistance and low offset current enables measurementsup to 200GΩ. When using the constant-voltage method, the electrometerapplies a constant voltage to the unknown resistance, measuresthe current, and then calculates the resistance. This is the preferred methodbecause it allows the unknown resistor to be tested at a known voltage. Anelectrometer can measure resistances up to 10PΩ (10 16 Ω) using thismethod.1-6 SECTION 1
Page 1 and 2: www.keithley.comLLM6 th EditionLow
Page 4 and 5: TABLE OF C ONTENTSSECTION 1 Low Lev
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 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
Page 53 and 54: MaterialTABLE 2-2: Properties of Va
Page 55 and 56: QuartzQuartz has properties similar
Page 57 and 58: FIGURE 2-11: Guarding as Applied to
Page 59 and 60: FIGURE 2-13: Guarding the Leakage R
Page 61 and 62: 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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