Low Level Measurements Handbook

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known for their low input bias current, which is usually a few femtoamps.Picoammeters and SMUs also have very low input bias currents, althoughusually not as low as an electrometer’s.Although input bias current is a common source of this type of error,currents generated by external circuits can also result in errors due to voltagedrops across the source resistance. Typical sources of such offset currentsare insulators and cables.Shunt Resistance Loading and GuardingExternal shunt resistances, such as leaky cables and dirty insulators, mayalso cause loading errors.Any external shunt resistance across the voltage source will attenuatethe measured voltage, as shown in Figure 2-3. As in the case of input resistancevoltage loading, the shunt resistance (R SHUNT ) and the source resistance(R S ) form a voltage divider that reduces the measured voltage (V M ) asfollows:R SHUNTV M = V S ––––––––––––––( RSHUNT + R S)For example, assume R S = 10GΩ and R SHUNT = 100GΩ. If V S has a valueof 10V, the measured voltage (V M ) is:10V 11M = 10( 10)–––––––––––– 1011+ 10V M = 9.09VIn this instance, the error due to shunt loading is approximately 9%.FIGURE 2-3: Effects of Shunt Resistance on Voltage Measurement AccuracyHIR SV SShuntResistanceR SHUNTV MLOVoltage SourceVoltmeter Measuring V SIndicating V MR SHUNTV M = V S R S + R SHUNT<strong>Measurements</strong> from High Resistance Sources 2-5
Cable leakage resistance is a common source of shunt resistance loading,as shown in Figure 2-4. In this case, the measured voltage (V M ) is attenuatedby the voltage divider formed by R S and the cable resistance (R L ):( RS + R L)R LV M = V S –––––––––To reduce errors due to shunt resistance, use cables, connectors, andtest fixturing with the highest possible insulation resistance. In addition, theuse of guarding will eliminate any residual errors.FIGURE 2-4: Effect of Cable Leakage Resistance on Voltage Measurement AccuracyConnecting CableR SR LCableShieldHIV SCableLeakageResistanceV MLOVoltage SourceVoltmeter Measuring V SIndicating V MV M = V SR LR S + R LThe error due to cable leakage can be greatly reduced by the use ofguarding, as shown in Figure 2-5. In the guarded configuration, the cableshield is now connected to the output of the guard buffer instead of themeter LO terminal. R G represents the resistance from the cable shield tometer LO, and I G is the current through R G as a result of driving the shieldto the same potential as the input HI terminal. This current is supplied bythe guard buffer, not the voltage source. Since the voltage across R L is nowmany decades lower, the leakage current will be negligible in most cases.By definition, a guard is a low impedance point in the circuit that’s atnearly the same potential as the high impedance input terminal.In modern electrometers, the preamplifier output terminal is such apoint, and can be used to reduce the effect of cable leakage, as shown inFigure 2-5. An additional benefit is that the effective cable capacitance is2-6 SECTION 2
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 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: cause significant loading errors. I
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
Page 75 and 76: FIGURE 2-27: Feedback Coulombmeter
Page 77 and 78: Advantages of Using a Coulombmeter
Page 79 and 80: FIGURE 2-30: Constant-Voltage Metho
Page 81 and 82: In addition to the voltage drop lim
Page 83 and 84: FIGURE 2-34a: Effects of Cable Resi
Page 85 and 86: Such devices require extreme care i
Page 87 and 88: charge will be lost through the zer
Page 89 and 90: FIGURE 2-40: Proper ConnectionCurre
Page 91 and 92: Figure 2-43 shows an example of AC
Page 93 and 94: fering voltage or current. A guard
Page 95 and 96: 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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Page 209 and 210:
Page 211 and 212:
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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