Low Level Measurements Handbook

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Coulombmeter FunctionCurrent integration and measurement of charge are electrometer coulombmetercapabilities not found in multimeters. The electrometer coulombmetercan detect charge as low as 10fC (10 –14 C). It’s equivalent to an activeintegrator and, therefore, has low voltage burden, typically less than 100µV.The coulombmeter function can measure lower currents than theammeter function can, because no noise is contributed by internal resistors.Currents as low as 1fA (10 –15 A) may be detected using this function. SeeSection 2.3.8 for further details.1.3.2 The DMMDigital multimeters vary widely in performance, from low cost handheld 3 1 ⁄2-digit units to high precision system DMMs. While there are many modelsavailable from a wide variety of manufacturers, none approaches the theoreticallimits of measurement discussed previously. These limitations don’timply that DMMs are inadequate instruments; they simply point out the factthat the vast majority of measurements are made at levels far from theoreticallimits, and DMMs are designed to meet these more conventional measurementneeds.Although low level measurements are by definition those that are closeto theoretical limits, and are thus outside the range of DMMs, advances intechnology are narrowing the gap between DMMs and dedicated low levelinstruments. For example, the most sensitive DMMs can detect DC voltagesas low as 10nV, resolve DC currents down to 10pA, and measure resistancesas high as 1GΩ. While these characteristics still fall far short of the correspondingcapabilities of more sensitive instruments like the electrometerdescribed previously, all the measurement theory and accuracy considerationsin this book apply to DMM measurements as well as to nanovoltmeter,picoammeter, electrometer, or SMU measurements. The difference is only amatter of degree; when making measurements close to theoretical limits, allmeasurement considerations are vitally important. When measuring at levelsfar from theoretical limits, only a few basic considerations (accuracy,loading, etc.) are generally of concern.1.3.3 The NanovoltmeterA nanovoltmeter is a very sensitive voltage meter. As shown in Figure 1-3,this type of instrument is optimized to provide voltage measurements nearthe theoretical limits from low source resistances, in contrast to the electrometer,which is optimized for use with high source resistances.Compared to an electrometer, the voltage noise and drift are much lower,and the current noise and drift are much higher. Input resistance is usuallysimilar to that of a DMM and is much lower than that of an electrometer.As is the case with electrometers, nanovoltmeters are just as reliable andeasy to operate as DMMs. Their distinguishing characteristic is their voltagesensitivity, which can be as good as 1pV. Most nanovoltmeters aren’t multi-<strong>Low</strong> <strong>Level</strong> DC Measuring Instruments 1-7
function instruments and are correspondingly less complex thanelectrometers.1.3.4 The PicoammeterA picoammeter is an ammeter built along the lines of the ammeter functionof an electrometer. When compared with an electrometer, a picoammeterhas a similar low voltage burden, similar or faster speed, less sensitivity, anda lower price. It may also have special characteristics, such as high speedlogarithmic response or a built-in voltage source.1.3.5 The Source-Measure UnitAs its name implies, a source-measure unit (SMU) has both measuring andsourcing capabilities. Adding current and voltage sourcing capabilities to ameasuring instrument provides an extra degree of versatility for many lowlevel measurement applications. For example, very high resistance valuescan be determined by applying a voltage across a device and measuring theresulting current. The added sourcing functions also make a SMU more convenientand versatile than using separate instruments for such applicationsas generating I-V curves of semiconductors and other types of devices.The typical SMU provides the following four functions:• Measure voltage• Measure current• Source voltage• Source currentThese functions can be used separately or they can be used together inthe following combinations:• Simultaneously source voltage and measure current, or• Simultaneously source current and measure voltage.SMUs have a number of electrometer-like characteristics that makethem suitable for low level measurements. The input resistance is very high(typically 100TΩ or more), minimizing circuit loading when making voltagemeasurements from high impedance sources. The current measurementsensitivity is also similar to that of the electrometer picoammeter—typicallyas low as 10fA.Another important advantage of many source-measure units is theirsweep capability. Either voltage or current can be swept across the desiredrange at specified increments, and the resulting current or voltage can bemeasured at each step. Built-in source-delay-measure cycles allow optimizingmeasurement speed while ensuring sufficient circuit settling time tomaintain measurement integrity.1-8 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 12 and 13: FIGURE 1-3: Typical Digital Multime
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
Page 63 and 64: 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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