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Using Electrometers & Picoammeters for
Low-Level Current Measurements
Measuring DC Amps Where the DMM Can’t
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Low-Level Current Measurement Applications
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n
High-resistance Measurements of Materials
– Measuring high-resistance of materials by sourcing high voltage and measuring low
current with a Model 6517B Electrometer and a Model 8009 Surface Resistivity Box
n
Leakage Measurement of Capacitors
– Measuring capacitor leakage by sourcing high voltage and measuring leakage
current with a Model 6517B Electrometer
n
Simplified Insulation Resistance Measurements
– Measuring insulation resistance by sourcing high voltage and measuring low current
with a Model 6517B Electrometer or a Model 6487 Picoammeter
n
Testing Breakdown Voltages and Resistances
– Powering devices and measuring breakdown voltages and resistances by sourcing
voltage and measuring low currents with a Model 6487 Picoammeter
n
Simplified Two-Channel, Powered Measurements
– Aligning and monitoring ion beams and optics, testing multiple devices, and testing
multi-pin devices with a Model 6482 Dual-channel Picoammeter
What is an Electrometer?
Like digital multimeters, (DMM,) Keithley electrometers are
instruments that measure charge, currents, voltages, and
resistances. However, electrometers measure beyond the
capabilities of standard DMMs by measuring charges with 10fC
resolution, currents with 100aA resolution, and resistances up to 200TΩ.
Electrometers are used where there is a need for extreme sensitivity or where
there is a need for multiple types of sensitive electronic measurements.
What is a Picoammeter?
Measuring low DC currents often demands a lot more than a DMM
can deliver. Generally, DMMs lack the sensitivity required to measure
currents less than 100nA. Even at higher currents, a DMM’s input
voltage drop (voltage burden) of hundreds of millivolts can make
accurate current measurements impossible. The low voltage burden
of a picoammeter makes it function much more like an ideal ammeter
than a DMM, so it can make current measurements with high
accuracy, even in circuits with very low source voltages. Keithley
picoammeters combine the economy and ease of use of a DMM with
low current sensitivity near that of an electrometer.
2
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Simplified High Resistance Measurements of Materials Using a Keithley Electrometer
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High Resistance Measurements
Resistance is most often measured with a DMM, which can make measurements up to about
200MΩ. However, in some cases, resistances in the gigohm and higher ranges must be
measured accurately. These cases include such applications as characterizing high megohm
and gigohm resistors, determining the resistivity of insulators, and measuring the insulation
resistance of printed circuit boards. These measurements are made by using an electrometer,
which can measure both very low current and high impedance voltage.
Common High Resistance Measurements
n
Insulation Resistance – Insulation resistance (IR) is the ratio of a DC applied voltage between two electrodes and
the total current between them. Examples of insulation resistance measurements include measuring the leakage
between traces on a printed circuit board or the resistance between conductors in a multi-conductor cable.
n
Volume Resistivity Measurements – Volume resistivity is the electrical resistance through a one centimeter cube
of insulating material and is expressed in ohm-centimeters.
n
Surface Resistivity Measurements – Surace resistivity is the electrical resistance between two electrodes on
the surface of an insulating material and is expressed in ohms (usually stated as ohms per square for clarity).
1
Both volume and surface resistivity measurements are obtained by taking wresistance measurements and then converting them to
resistivity values by taking geometric considerations into account. Resistivity measurement setups are shown below in Figure 1. Both
volume and surface resistivity measurements can be improved by using test fixtures like the Keithley Model 8009 (shown in Figure 2)
and the Model 6517B Electrometer (shown in Figure 3).
1
1
Figure 1. Resistivity measurements
Figure 2. Model 8009 Resistivity Test Fixture
3
Figure 3. Model 6517B Electrometer/High Resistance Meter
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Simplified Capacitor Leakage Measurements Using a Keithley Electrometer
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Capacitor Leakage Measurements
Capacitors are very important in all areas of electronics. From timing circuits to sample and hold applications, we depend on capacitors to
act in a nearly ideal fashion. In many cases, however, complex electrochemical interactions cause capacitors to fall short of perfect. One of
the less ideal properties that a capacitor has is leakage, or insulation resistance (IR).
Capacitor leakage can either be expressed as insulation resistance, expressed in megohm-microfarads (computed by dividing the resistance
value by the capacitance) or by leakage current at a specific voltage. The Model 6517B Electrometer is particularly useful for this application
because it can display either resistance or leakage current and will source up to 1000V DC.
Capacitor leakage is measured by applying a fixed voltage to the capacitor under test and measuring the resulting current. The leakage
current will decay exponentially with time, so it’s usually necessary to apply the voltage for a known period (the “soak” time) before
measuring the current. Improved performance will result if a forward-biased diode (D) is included in the circuit, as shown in Figure 4.
The diode acts like a variable resistance, low when the charging current to the capacitor is high then increasing in value as the current
decreases with time. The series resistor can be much smaller since it is only needed to prevent overload of the voltage source and damage
to the diode if the capacitor becomes short-circuited.
For statistical purposes, a quantity of capacitors is
often tested to produce useful data. Obviously, it is
impractical to perform these tests manually, so some
sort of automated test system is required. Figure 5
illustrates such a system, which employs a Model 6517B
Electrometer/High Resistance Meter and switching cards
that are installed in a switching mainframe. The Model
6517B is particularly useful for this application because it
can display either resistance or leakage current and will
source up to 1000V DC.
Figure 5. Capacitor leakage test systemFigure 4. Capacitor leakage test circuit with diode
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Simplified Insulation Resistance Measurements Using a Keithley Electrometer
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High Resistance Measurements
Resistance is most often measured with a DMM, which can make measurements up to about
200MΩ. However, in some cases, resistances in the gigohm and higher ranges must be
measured accurately. These cases include applications such as characterizing high megohm
resistors, determining the resistivity of insulators, and measuring the insulation resistance of
printed circuit boards. These measurements are made by using an electrometer, which can
measure both very low current and high impedance voltage.
Common Insulation Leakage Measurements
n
Connector insulation resistance – Given today’s ever-shrinking circuit geometry and the higher frequencies of
electronic signals, isolation is an important consideration for reliability and crosstalk. Isolation is typically tested by
applying a voltage across two pins in a connector and measuring the resulting current that flows between them. The
corresponding resistance from the test is compared to a predetermined threshold value. If the resistance level is too low,
the connector is rejected. Figure 7 shows the electrical equivalent of a connector; the isolation resistance is identified as
Riso. When testing very high ohmic devices, the measured resistance may change significantly in response to a change
in the applied voltage, an effect known as the voltage coefficient of resistance.
n
PCB surface insulation resistance – Low surface insulation resistance (SIR) of a printed circuit board (PCB) can
degrade the performance of the circuits on the board considerably. Factors that affect a board’s surface insulation
resistance include the board material used, the presence of coatings such as solder masking or conformal coatings,
board cleanliness, and relative humidity. The measured insulation resistance typically ranges from 107Ω to 1016Ω.
Figure 6 shows a test setup for measuring PCB surface resistances.
Figure 7. Electrical
equivalent of a connector
showing pin continuity and
isolation resistance
Figure 6. PCB insulation resistance measurement
5
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What are the Keithley Model 6514 and 6517B Electrometers?
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What is an Electrometer?
Like DMMs, Keithley electrometers are instruments that measure charge, currents, voltages,
and resistances. However, they go beyond the capabilities of standard DMMs by measuring
charges with 10fC resolution, currents with 100aA resolution, and resistances up to 200TΩ
(using the 6517B’s voltage source).
The Keithley Model 6514 and 6517B Programmable Electrometers are the industry’s leading
electrometers. Their superior sensitivity and performance increase utility and boost productivity
in a variety of applications from high resistance measurements, to physics and chemistry
research, to new product development. The Model 6517B’s programmable voltage source
adds to instrument density and ease of control by combining both measurement and sourcing
capabilities.
Keithley Model 6514 and 6517B Electrometers
Key Specifications
Features 6514 Electrometer 6517B Electrometer/High Resistance Meter
Resistance Measurement
Max/Min
Charge Measurement
Max/Min
Current Measurement
Max/Min
Voltage Measurement
Max/Min
200 GΩ / 10 Ω 10 PΩ (1016 Ω) / 100Ω
20µC / 10 fC 2µC / 10 fC
20mA / 100aA
200V / 10µV
Voltage Sourcing N/A 1000V / 5mV resolution
Please visit the Keithley website at www.keithley.com for detailed
Max readings / sec 1,200 450
specifications for the Models 6514 and 6517B
Computer Interface GPIB, RS-232 GPIB, RS-232
n Application Note: Low Current Measurements
n White Paper: Optimizing Low-Current Measurements
Connectors/ Cabling Triax
and Instruments
For other information, please contact your local applications engineer.
System-level automation Digital I/O, TriggerLink, Analog out Digital I/O, TriggerLink, Analog out
6
Triax (measurement), Banana (voltage source),
Thermocouple Input
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Keithley Electrometer Features
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DMM-Like Operation
The Models 6514 and 6517B Electrometers feature easy, DMM-like operation via the front panel, with singlebutton control of important functions such as resistance measurement. They can also be controlled via a built-in
IEEE-488 interface, which makes it possible to program all functions over the bus through a computer controller.
Unlike a DMM, however, electrometers have lower offset currents and input burdens. Typically, offset currents in
DMMs are tens or hundreds of picoamps, which severely limit their low current measuring capabilities compared
to the Model 6514 or 6517B, which each have <3fA input bias currents.
The Model 6514’s and 6517B’s feedback ammeter designs minimize voltage offsets in the input circuitry, which
can affect current measurement accuracy. The instruments also allow active cancellation of its input voltage and
current offsets, either manually via the front panel controls or over the bus with IEEE-488 commands.
Scanner Cards (6517B)
Two scanner cards are available to simplify scanning multiple signals., such
as in production testing of capacitors or other circuitry. Either card can
be easily inserted in the option slot of the instrument’s back panel.
The Model 6521 Scanner Card offers ten channels of low-level current
scanning. The Model 6522 Scanner Card provides ten channels of high
impedance voltage switching or low current switching.
Alternating Polarity Resistance/Resistivity (6517B)
The Model 6517B uses the alternating polarity method for resistance/resistivity measurements, which virtually
eliminates the effect of any background currents in the sample. First and second order drifts of the background
currents are also canceled out. By alternating the polarity of an applied voltage, this method typically produces a
highly repeatable, accurate measurement of resistance (or resistivity).
7
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Testing Breakdown Voltages and Resistances Using a Keithley Picoammeter
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High Resistance Measurements
Resistance is most often measured with a DMM, which can make measurements up to about 200MΩ. However,
in some cases, resistances in the gigohm and higher ranges must be measured accurately. These cases include
such applications as characterizing high megohm resistors, determining the resistivity of insulators and measuring
the insulation resistance of printed circuit boards. These measurements can be made with a picoammeter/voltage
source, utilizing the precision high voltage source and picoamp measurement capabilities to make accurate
resistance measurements.
High resistance measurements often use the constant voltage method. The basic configuration of the constant
voltage method using a picoammeter is shown in Figure 8.
Why would I need voltage source with my picoammeter?
n
To power devices – Built-in voltage sources allow devices under test to be powered without the use of
additional external equipment, saving money, time, and frustration.
n
To test breakdown voltages – Devices and insulation can be biased to a high voltage to test at what
point the insulator or device becomes electrically conductive (important in manufacturing quality testing
and in device and materials research).
n
To test high resistances – Higher voltages allow greater resistances to be measured.
Figure 8. Constant voltage method
for resistance measurements
In this method, a constant voltage source (V) is placed in series with the unknown resistor (R) and an ammeter
(A). Since the voltage drop across the ammeter is negligible, essentially all the test voltage appears across R. The
resulting current is measured by the ammeter, and the resistance is calculated using Ohm’s Law (R= V/I). High
resistance is often a function of the applied voltage, which makes the constant voltage method preferable to the
constant current method used on DMMs.
8
Figure 9. Model 6487 used to make
high resistance measurements
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Simplified Two-Channel, Powered Measurements Using a Keithley Picoammeter
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Ratio and Delta Modes for Beam Alignment Applications
Ratio and delta measurements, which show the ratio and difference, respectively, between two channels, can significantly
improve efficiency in beam alignment applications by performing these calculations on-instrument. Important in many
different fields from high energy particle beam monitoring to ion beam monitoring and fiber optics, the beam alignment
procedure involves measuring a signal from a detector and adjusting the position of the source to align it to the sensor such
that it matches a calibrated source/detector pair. Ratio and delta measurements can help to provide immediate feedback
about how far out of alignment these beams are, which helps the operator more quickly adjust the source. The Model 6482
Dual Channel Picoammeter, as shown in Figure 10, not only has these measurement capabilities, but also has dual voltage
biases, which allow these sensors to be powered without the need for additional equipment.
Common Powered Two-Channel Measurements
Ion Beam Monitoring – Focused ion beam systems are often used for nanometer-scale imaging, micromachining, and
mapping. Careful monitoring of the magnitude of the beam current with an ion detector is critical. These sensors accept
and put out a small current. Oftentimes, multiple ion beams and sensors are operated concurrently and require powered
sensors. The Keithley Model 6482 can power these sensors while taking picoamp measurements, saving space, reducing
the number of instruments needed, and simplifying control.
Beam alignment – Laser diodes, fiber optics, high-energy particle beams, and ion beams all need to be aligned in many
applications. The ratio and delta modes in the Keithley Model 6482 simplify these alignment procedures by doing these
measurements in real time and reporting these values to the operator, who can then adjust the beams quickly and easily.
Multiple Device Testing – Multi-channel devices increase throughput and reduce maintenance burden by increasing the
number of channels in the same form factor. The Keithley Model 6482 can power and measure two devices concurrently,
reducing the number of instruments needed.
Multi-pin Device Testing – Multi-pin devices, such as dual diodes, ICs, and other components often need multi-channel,
simultaneous current testing. Multiple channels allow one instrument to take measurements on multiple pins, opening up
new possibilities for tests and increasing throughput.
9
Figure 10. Two-channel ion beam comparison
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What are Keithley’s Model 6482, 6485, and 6487 Picoammeters?
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What is a Picoammeter?
Measuring low DC currents often demands a lot more than a DMM can deliver. Generally, DMMs lack the
sensitivity required to measure currents less than 100nA. Even at higher currents, a DMM’s input voltage
drop (voltage burden) of hundreds of millivolts can make accurate current measurements impossible. The
low voltage burden of a picoammeter makes it function much more like an ideal ammeter than a DMM,
so it can make current measurements with high accuracy, even in circuits with very low source voltages.
Keithley picoammeters combine the economy and ease of use of a DMM with low current sensitivity near
that of an electrometer. Keithley offers three picoammeters, the Model 6485 Picoammeter, the Model 6487
Picoammeter/ Voltage source, and the Model 6482 Dual Picoammeter/Voltage Source.
What are Keithley Model 6482, 6485, and 6487 Picoammeters?
The Keithley Model 6482, 6485, and 6487 picoammeters are the industry’s leading picoammeters. Superior
sensitivity and performance increase utility and boost productivity in a variety of applications from sensor
monitoring, to leakage currents, to physics and chemistry research, to new product development. The Model
6487’s programmable 500V precision voltage source adds high-resistance and breakdown voltage testing
capabilities. The Model 6482 boasts two measurement channels and the ability to power devices under test
with its dual 30V bias sources.
Keithley Model 6482, 6485, and 6487 Picoammeters
Key Specifications
Features 6485 Picoammeter
Current Measurement
Max / Min
Resistance Measurement
Max/Min
Channel Count 1 1 1
Voltage Sourcing N/A 500 V / 200 µV resolution 30 V / 400 µV resolution
Max readings / sec (via GPIB) 900 900 900
Buffer size 2500 3000 3000 (per channel)
20 mA / 20 fA 200 GΩ / 10 Ω 20 mA / 1 fA
N/A 10 PΩ (10
Picoammeter/ Voltage Source
6487
Dual Picoammeter/ Voltage Source
16
Ω) N/A
6482
Figure 11. Front and rear panels of the Keithley Model 6482 Dual-Channel Picoammeter
10
Computer Interface GPIB, RS-232
Connectors/ Cabling BNC Triax
System-level automation TriggerLink, Analog out
(and BNC via included adapters)
Triax
Please visit the Keithley website at www.keithley.com for detailed
specifications for the Models 6482, 6485, and 6487
n Application Note: Low Current Measurements
n White Paper: Optimizing Low-Current Measurements
and Instruments
For other information, please contact your local applications engineer.
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Keithley Model 6482, 6485, and 6487 Picoammeters Features
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Common Picoammeter Measurements
Requiring a Voltage Source
High resistance components – The high voltage source present
in the Keithley Model 6487 Picoammeter allows high voltages to
be measured quickly and accurately by sourcing a voltage and
measuring the resulting current.
High resistance materials – Paired with the Keithley Model
8009 Resistivity Test Fixture, the Keithley Model 6487 can
measure surface and volume resistivities.
Breakdown voltage identication – The voltage source can
sweep voltage to determine when electrical breakdown occurs in
resistors or devices in component testing and materials research.
Resistor voltage coefcient – Very high ohmic value resistors
may exhibit a significant non-linear change in resistance with a
change in applied voltage. This effect is known as the voltage
coefficient. The voltage coefficient is the percent change in
resistance per unit change in applied voltage.
DMM-Like Operation
The Models 6482, 6485, and 6487 are designed for easy, DMMlike operation via the front panel, with single-button control of
important functions such as math functions and zeroing
measurement functions. They can also be controlled via built-in
IEEE-488 and RS-232 interfaces, which make it possible to
program all functions over the bus through a computer controller.
Advantages of two picoamp measurement
channels
Increase channel density – In research and manufacturing
applications where bench and rack space is at a premium, higher
channel density becomes very valuable. The Keithley Model 6482
Dual-Channel Picoammeter offers twice the channel density of
earlier Keithley picoammeters.
Increase instrument control and improve setup –
Fewer instruments to control and setup decreases setup and
programming time.
Quickly compare two signals – On-instrument delta and
ratio calculations in the Model 6482 help to quickly compare the
signals coming from multiple sensors or multiple-pin ICs,
components, and devices.
11
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Measuring DC Amps Where The DMM Can’t
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6430 Sub-Femtoamp SourceMeter® (Electrometer)
Electrometers
Picoammeters
Model 6430
Measure 10 aA to 100 mA
Source 50 aA to 100 mA
Model 6487
Measure 10 fA to 20 mA
Source 200µV to 500V
Model 6517B
Measure 1 fA to 20 mA
Source 5mV to 1000V
Model 6482
Measure 1 fA to 20mA (x2)
Bias 30V on each channel
Model 6514
Measure 100 aA to 20 mA
Model 6485
Measure 10 fA to 20mA
1 aA
6517B Electrometer/High Resistance Meter
6514 Electrometer
6482 Dual Picoammeter/Voltage Source
6485 Picoammeter
6487 Picoammeter/Voltage Source
Typical DMM
1fA 1 pA 1 nA 1 μA 1 mA 1A
Measured Current
12
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Keithley’s electrometers & picoammeters
Keithley Instruments hosts an online applications forum to encourage idea exchange,
discussions among users. Join the discussion today.
for low-level current measurements?
Contact us by phone, fax, mail, or email:
KEITHLEY CORPORATE HEADQUARTERS
Keithley Instruments, Inc.
28775 Aurora Road
Cleveland, Ohio 44139
Phone: 440-248-0400
Tol l-fr ee: 800 - 552-1115
Fax: 440-248-6168
info@keithley.com
To learn more about how Keithley’s Electrometers & Picoammeters for Low-Level Current
Measurements contact your local Keithley representative or ask us a question online.
Consult with a Keithley applications engineer and learn how
to get the most from your Keithley products
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