Low Level Measurements Handbook

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Transfer StabilityA special case of relative accuracy is the transfer stability, which definesinstrument accuracy relative to a secondary reference standard over a veryshort time span and narrow ambient temperature range (typically withinfive minutes and ±1°C). The transfer stability specification is useful in situationswhere highly accurate measurements must be made in reference to aknown secondary standard.Calculating Error Terms from Accuracy SpecificationsTo illustrate how to calculate measurement errors from instrument specifications,assume the following measurement parameters:Accuracy: ±(25ppm of reading + 5ppm of range)Range: 2VInput signal: 1.5VThe error is calculated as:Error = 1.5(25 × 10 –6 ) + 2(5 × 10 –6 )= (37.5 × 10 –6 ) + (10 × 10 –6 )= 47.5 × 10 –6Thus, the reading could fall anywhere within the range of 1.5V ±47.5µV, an error of ±0.003%.1.4.3 DeratingsAccuracy specifications are subject to deratings for temperature and timedrift, as discussed in the following paragraphs.Temperature CoefficientThe temperature of the operating environment can affect accuracy. For thisreason, instrument specifications are usually given over a defined temperaturerange. Keithley accuracy specifications on newer electrometers, nanovoltmeters,DMMs, and SMUs are usually given over the range of 18°C to28°C. For temperatures outside of this range, a temperature coefficient suchas ±(0.005 % + 0.1 count)/°C or ±(5ppm of reading + 1ppm of range)/°Cis specified. As with the accuracy specification, this value is given as a percentageof reading plus a number of counts of the least significant digit (oras a ppm of reading plus ppm of range) for digital instruments. If the instrumentis operated outside the 18°C to 28°C temperature range, this figuremust be taken into account, and errors can be calculated in the mannerdescribed previously for every degree less than 18°C or greater than 28°C.Time DriftMost electronic instruments, including electrometers, picoammeters, nanovoltmeters,DMMs, SMUs, and SourceMeter instruments, are subject tochanges in accuracy and other parameters over a long period of time,whether or not the equipment is operating. Because of these changes,instrument specifications usually include a time period beyond which the<strong>Low</strong> <strong>Level</strong> DC Measuring Instruments 1-13
instrument’s accuracy cannot be guaranteed. The time period is stated inthe specifications, and is typically over specific increments such as 90 daysor one year. As noted previously, transfer stability specifications are definedfor a much shorter period of time—typically five or 10 minutes.1.4.4 Noise and Noise RejectionNoise is often a consideration when making virtually any type of electronicmeasurement, but noise problems can be particularly severe when makinglow level measurements. Thus, it’s important that noise specifications andterms are well understood when evaluating the performance of an instrument.Normal Mode Rejection RatioNormal mode rejection ratio (NMRR) defines how well the instrumentrejects or attenuates noise that appears between the HI and LO input terminals.Noise rejection is accomplished by using the integrating A/D converterto attenuate noise at specific frequencies (usually 50 and 60Hz) whilepassing low frequency or DC normal mode signals. As shown in Figure1-5, normal mode noise is an error signal that adds to the desired inputsignal. Normal mode noise is detected as a peak noise or deviation in a DCsignal. The ratio is calculated as:peak normal mode noiseNMRR = 20 log _______________________________[ peak measurement deviation ]FIGURE 1-5: Normal Mode NoiseMeasuringInstrumentHILONoiseSignalNormal mode noise can seriously affect measurements unless steps aretaken to minimize the amount added to the desired signal. Careful shieldingwill usually attenuate normal mode noise, and many instruments haveinternal filtering to reduce the effects of such noise even further.Common Mode Rejection RatioCommon mode rejection ratio (CMRR) specifies how well an instrumentrejects noise signals that appear between both input high and input low andchassis ground, as shown in Figure 1-6. CMRR is usually measured with a1kΩ resistor imbalance in one of the input leads.1-14 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 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
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,
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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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Page 209 and 210:
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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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