Low Level Measurements Handbook

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FIGURE 1-8: Voltage Amplifier+–V 2AR AV OV 1R BV O = V 2 (1 + R A /R B )Nanovoltmeter PreamplifierThe same basic circuit configuration shown in Figure 1-8 can be used as aninput preamplifier for a nanovoltmeter. Much higher voltage gain isrequired, so the values of R A and R B are set accordingly; a typical voltagegain for a nanovoltmeter preamplifier is 10 3 .Electrometer and nanovoltmeter characteristics differ, so the operationalamplifier requirements for these two types of instruments are alsosomewhat different. While the most important characteristics of the electrometervoltmeter operational amplifier are low input offset current andhigh input impedance, the most important requirement for the nanovoltmeterinput preamplifier is low input noise voltage.1.5.2 Ammeter CircuitsThere are two basic techniques for making current measurements: these arethe shunt ammeter and the feedback ammeter techniques. DMMs and olderelectrometers use the shunt method, while picoammeters and the AMPSfunction of electrometers use the feedback ammeter configuration only.Shunt AmmeterShunting the input of a voltmeter with a resistor forms a shunt ammeter, asshown in Figure 1-9. The input current (I IN ) flows through the shunt resistor(R S ). The output voltage is defined as:V O = I IN R S (1 + R A /R B )For several reasons, it’s generally advantageous to use the smallest possiblevalue for R S .First, low value resistors have better accuracy, time and temperature stability,and voltage coefficient than high value resistors. Second, lower resistor<strong>Low</strong> <strong>Level</strong> DC Measuring Instruments 1-17
values reduce the input time constant and result in faster instrumentresponse time. To minimize circuit loading, the input resistance (R S ) of anammeter should be small, thus reducing the voltage burden (V 2 ). However,note that reducing the shunt resistance will degrade the signal-to-noise ratio.FIGURE 1-9: Shunt AmmeterR SI INV O = I IN R S (1 + R A /R B )V 2+–AR AV OV 1R BFeedback AmmeterIn this configuration, shown in Figure 1-10, the input current (I IN ) flowsthrough the feedback resistor (R F ). The low offset current of the amplifier(A) changes the current (I IN ) by a negligible amount. The amplifier outputvoltage is calculated as:V O = –I IN R FThus, the output voltage is a measure of input current, and overall sensitivityis determined by the feedback resistor (R F ). The low voltage burden(V 1 ) and corresponding fast rise time are achieved by the high gain op amp,which forces V 1 to be nearly zero.FIGURE 1-10: Feedback AmmeterR FI INInput+V 1–AV OOutputV O = –I IN R F1-18 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 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 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
Page 65 and 66: open-circuited, allow the reading t
Page 67 and 68: to equalize charges and minimize ch
Page 69 and 70: If insulators become contaminated,
Page 71 and 72: The input resistance of a feedback
Page 73 and 74: 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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