Low Level Measurements Handbook

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FIGURE 1-6: Common Mode NoiseMeasuringInstrumentHILOR imbalanceSignal(usually 1kΩ)NoiseAlthough the effects of common mode noise are usually less severe thannormal mode noise, this type of noise can still be a factor in sensitive measurementsituations. To minimize common mode noise, connect shieldsonly to a single point in the test system.Noise SpecificationsBoth NMRR and CMRR are generally specified in dB at 50 and 60Hz, whichare the interference frequencies of greatest interest. (CMRR is often specifiedat DC as well.) Typical values for NMRR and CMRR are >80dB and>120dB respectively.Each 20dB increase in noise rejection ratio reduces noise voltage or currentby a factor of 10. For example, a rejection ratio of 80dB indicates noisereduction by a factor of 10 4 , while a ratio of 120dB shows that the commonmode noise would be reduced by a factor of 10 6 . Thus, a 1V noise signalwould be reduced to 100µV with an 80dB rejection ratio and down to 1µVwith a 120dB rejection ratio.1.4.5 SpeedInstrument measurement speed is often important in many test situations.When specified, measurement speed is usually stated as a specific numberof readings per second for given instrument operating conditions. Certainfactors such as integration period and the amount of filtering may affectoverall instrument measurement speed. However, changing these operatingmodes may also alter resolution and accuracy, so there is often a tradeoffbetween measurement speed and accuracy.Instrument speed is most often a consideration when making lowimpedance measurements. At higher impedance levels, circuit settling timesbecome more important and are usually the overriding factor in determiningoverall measurement speed. Section 2.6.4 discusses circuit settling timeconsiderations in more detail.<strong>Low</strong> <strong>Level</strong> DC Measuring Instruments 1-15
1.5 Circuit Design BasicsCircuits used in the design of many low level measuring instruments,whether a voltmeter, ammeter, ohmmeter, or coulombmeter, generally usecircuits that can be understood as operational amplifiers. Figure 1-7 showsa basic operational amplifier. The output voltage is given by:V O = A (V 1 – V 2 )FIGURE 1-7: Basic Operational AmplifierV 1+–V 2AV OCOMMONV O = A (V 1 – V 2 )The gain (A) of the amplifier is very large, a minimum of 10 4 to 10 5 , andoften 10 6 . The amplifier has a power supply (not shown) referenced to thecommon lead.Current into the op amp inputs is ideally zero. The effect of feedbackproperly applied is to reduce the input voltage difference (V 1 – V 2 ) to zero.1.5.1 Voltmeter CircuitsElectrometer VoltmeterThe operational amplifier becomes a voltage amplifier when connected asshown in Figure 1-8. The offset current is low, so the current flowingthrough R A and R B is the same. Assuming the gain (A) is very high, the voltagegain of the circuit is defined as:V O = V 2 (1 + R A /R B )Thus, the output voltage (V O ) is determined both by the input voltage(V 2 ), and amplifier gain set by resistors R A and R B . Given that V 2 is appliedto the amplifier input lead, the high input resistance of the operationalamplifier is the only load on V 2 , and the only current drawn from the sourceis the very low input offset current of the operational amplifier. In manyelectrometer voltmeters, R A is shorted and R B is open, resulting inunity gain.1-16 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 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
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
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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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