A
G R E A T E R
M E A S U R E
O F
C O N F I D E N C E
Low Current, High Resistance
Measurement Solutions
w w w. k e i t h l e y. c o m
High accuracy solutions for
W I D E
M E A S U R E M E N T
R A N G E S
low current applications
Measuring low DC currents often demands a lot more than a digital multimeter 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, as the figures
below indicate. Electrometers can measure low currents very accurately, but the circuitry needed to measure
an electrometer’s cost significantly. Keithley’s line of picoammeters combines the economy and ease of use of a
DMM with low current sensitivity near that of an electrometer.
A/D
V
BURDEN
Rs
I=VBURDEN/RS
BURDEN
A
A/D
+
Vo
I=VO /RS
VBURDEN ~ V /A ≤0.2mV
0
The shunt ammeter technique used in DMMs can introduce
hundreds of millivolts of voltage burden.
The feedback ammeter technique used in Keithey’s
picoammeters limits voltage burden to 200µV.
Low currents without the high cost
Typical picoammeter applications
Keithley brings more than half a century of expertise in designing
and manufacturing sensitive instrumentation to our new low
current measurement solutions. Our picoammeters combine
high accuracy with sensitivity as good as 10fA. They have always
provided exceptional voltage burden specifications and wide
measurement ranges; our newest instruments offer higher measurement speeds than ever before, up to 1000 readings per
second on the Model 6485. Our family of instruments offers economical alternatives to other low current measurement solutions,
such as optical power meters or user-designed instrumentation.
• Materials and components characterization, including resistivity
and leakage current measurements
• Insulation resistance measurements
• Device I-V characterization
• Photodiode current measurements
• Dark current (photodiode leakage)
• Fiber alignment
• Photomultiplier current measurements
• SEM beam current measurements
• Particle and beam monitoring
• Thermal luminescence glow current measurements
• Fluorometer and spectrometer measurements
• Circuit test and analysis
• Sensor characterization
• Teaching labs
S P E E D S
VBURDEN ≤200mV
V
Rs
-
M E A S U R E M E N T
I
I
H I G H E R
extremely low currents, combined with functions like voltage, resistance, and charge measurement, can increase
Model 6485 Picoammeter
Exceptional low current
Flexible interface options
S U P E R I O R
The Model 6485 is designed for easy connection to other instruments and voltage sources. For example, the built-in Trigger Link
interface allows synchronizing two or more instruments. It combines six independent selectable trigger lines on a single
connector for simple, direct control over all instruments in a system. A CE-approved IEEE-488 interface is included for controlling
the instrument via the GPIB bus with a PC. The Model 6485 also
offers an RS-232 interface.
L O W
C U R R E N T
The Model 6485 is Keithley’s most modern 5½-digit picoammeter. It employs the latest current measurement technology
but it is significantly less expensive than other instruments that
perform similar functions, such as optical power meters, competitive picoammeters, or user-designed solutions. With eight current
measurement ranges and high speed autoranging, the Model 6485
can measure currents from 20fA to 20mA. The Model 6485’s superior sensitivity allows characterizing low current phenomena, while
its 20mA range lets it measure currents high enough for 4-20mA
sensor loops.
S E N S I T I V I T Y
Low voltage burden
While DMMs typically employ shunt ammeter circuitry to measure
current, the Model 6485 is a feedback picoammeter. This design
reduces voltage burden by several orders of magnitude, resulting
in a voltage burden of less than 200µV on the lower measurement
ranges. The low voltage burden makes the Model 6485 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.
Just like before, only better
The Model 6485 expands on the capabilities of one of Keithley’s
most popular picoammeters, the Model 485, offering an additional
20mA measurement range, as well as dramatically higher measurement speeds. A built-in emulation mode simplifies upgrading
existing applications originally configured with a Model 485. With a
top speed of up to 1000 readings per second, the Model 6485 is
the fastest picoammeter Keithley has ever made. It offers ten times
greater resolution than the Model 485 on every range. A timestamped 2500-reading data buffer provides minimum, maximum,
and standard deviation statistics.
Low current measurements made easy
A variety of additional instrument features simplify measuring low
currents accurately. The Model 6485’s rear panel BNC inputs allow
the use of inexpensive, easy-to-use BNC cables. Its 220V overload
protection on all ranges and robust design let it withstand abusive
overflows. A REL function permits making relative readings with
respect to a baseline value; the LOG function displays the
logarithm of the absolute value of the measured current. The
Model 6485 can calculate resistance by dividing an externally
sourced voltage value by the measured current. The instrument
can be digitally calibrated using the front panel controls or via the
IEEE-488 bus. A scaled voltage analog output makes it simple to
transmit measurement results to devices like DMMs, data acquisition cards, oscilloscopes, or strip chart recorders. For research on
light-sensitive components, such as measuring the dark currents
of photodiodes, the front panel display can be switched off to
avoid introducing light that could significantly reduce the accuracy
of the results.
Powerful software options
To simplify integrating the Model 6485 into PC-based test systems,
the instrument comes with Keithley's ExceLINX, an add-in utility
for Microsoft® Excel. Within minutes of installing ExceLINX on a
PC, users can acquire data directly from the Model 6485 instrument, then employ Excel's graphics, charting and analysis
capabilities to turn that data into useful information. No programming is required to use ExceLINX—a few mouse clicks are all it
takes. Keithley also provides a 6485 instrument driver for use
with Application Development Environments such as LabVIEW™,
LabWindows™/CVI, Visual Basic, C/C++, and TestPoint. This
IVI-style driver (VISA based) supports all of the functionality of
the Model 6485. Numerous examples and an online help utility
are provided to help programmers get their applications "Up &
Running" quickly.
performance at an affordable price
Condensed Model 6485 specifications
ANALOG
RISE TIME
10% to 90%
NMRR1
(50 or
60Hz)
MAXIMUM
CONTINUOUS
INPUT VOLTAGE
2 nA
10 fA
0.4% + 400 fA
20 fA
8 ms
60 dB
220 VDC
20 nA
100 fA
0.4% + 1 pA
100 fA
8 ms
60 dB
220 VDC
200 nA
1 pA
0.2% + 10 pA
1 pA
500 µs
60 dB
220 VDC
2 µA
10 pA
0.15% + 100 pA
10 pA
500 µs
60 dB
220 VDC
20 µA
100 pA
0.1% + 1 nA
100 pA
500 µs
60 dB
220 VDC
200 µA
1 nA
0.1% + 10 nA
1 nA
500 µs
60 dB
220 VDC
2 mA
10 nA
0.1% + 100 nA
10 nA
500 µs
60 dB
220 VDC
20 mA
100 nA
0.1% + 1 µA
100 nA
500 µs
60 dB
220 VDC
INPUT VOLTAGE BURDEN: <200µV on all ranges except <1mV on 20mA range.
LANGUAGE EMULATION: Keithley Model 485 command emulation.
TEMPERATURE COEFFICIENT: 0°-18°C & 28°-50°C For each °C, add 0.1 of accuracy spec.
ANALOG OUTPUT: Scaled voltage output (inverting 2V full scale on all ranges) 3% ± 2mV, 1kΩ impedance.
1
2
Typical Model 6485 applications
• Beam monitoring and radiation monitoring
• Leakage current testing in insulators, switches, relays,
and other components
• Optoelectronic device testing and characterization
• Fiber alignment
• Circuit test and analysis in DCLF circuits
• Sensor characterization
• I-V measurements of semiconductors and other devices
E V E R
At 1 PLC – limited to 60 rdgs/sec under this condition
At 6 PLC – limited to 10 rdgs/sec under this condition
P I C O A M M E T E R
TYPICAL
RMS NOISE 2
F A S T E S T
ACCURACY1
±(%rdg + offset)
18-28°C, 0-50% RH
K E I T H L E Y S
RANGE
51⁄2 DIGIT
DEFAULT
RESOLUTION
Other low current
measurement solutions
Model 486 Picoammeter
Model 487 Picoammeter/Source
F A S T
R I S E
T I M E S
The Model 486 is a 5½-digit picoammeter with 10fA sensitivity.
It can measure currents from 10fA to 2mA. The Model 486 offers
a shorter rise time than the Model 6485 does on the higher current
ranges. It also has a triax connector that extends the range over
which the instrument can float. The Model 487 adds a built-in
±500V source to the Model 486’s capabilities. This combination
makes the Model 487 a powerful high resistance meter and fast
picoammeter in one instrument. Two voltage source ranges are
available: a 500V range with 10mV resolution and a 50V range with
1mV resolution. This source allows the Model 487 to measure
resistances from 5Ω to 1014Ω. For tests that require a preset soak
time, such as insulation resistance measurements, the Model 487
can be programmed with reading intervals from 10ms to 1000s.
Model 428 Current Amplifier
The Model 428 is designed to convert small current transients
into voltages that can be displayed on a scope or waveform analyzer. A feedback circuit design gives the Model 428 both fast rise
times and sub-picoamp noise. Rise times from 2µs to 300ms can
be selected. The amplifier gain can be adjusted in decade steps
3
11
from 10 V/A to 10 V/A. Like Keithley’s picoammeters, the Model
428 has zero check and offset functions to enhance the integrity
of signals with voltage offsets. Up to 5mA of current can be suppressed, which can be useful for nulling out a residual or offset
current present at the Model 428’s input. A choice of three display intensities–bright, dim, and off–makes it suitable for use in
light-sensitive environments.
Applications for the Model 428 include:
The Model 486 is a good choice for applications such as:
• Surface science studies
• PMT current measurements
- Amplifier for a Scanning Electron Microscope (SEM)
• Mass spectrometer current measurements
- Observing secondary electron emission, as in X-ray and beam line currents
• Probe current measurements in electrochemistry
• Laser and light measurements
• Plasma generated current measurements
- Amplifier for use with PMTs and photodiodes
• Ion chamber current measurements
- Analysis of fast photoconductive materials
The Model 487’s built-in voltage source
makes it ideal for:
- IR detector amplifier
• Resistivity measurements
• I-V characterization
• Transient phenomena
- Current DLTS studies
- Breakdown in devices and dielectrics
• Component leakage tests
• Front-end amplifier/converter for oscilloscopes or waveform digitizers
• Insulation resistance testing
• Fiber alignment
Common picoammeter
applications
Calibrated Light Source
Transistor
Under Test
Base is
open
Photodiode
Pads
Probe Needles
...
Probe Needles
VCEO
Model
487
pA
Wafer
Model 6485
Test
Fixture
Trigger Link Cable
Ion Beam
High Resistance Measurements
I
Metal
Shield
HI
Model 2400
Voltage
Source
6485
LO
The Model 487’s integrated voltage source is specifically designed for
making high resistance measurements. If paired with an external voltage
source, either the Model 6485 or Model 486 Picoammeter can be used to
make measure high resistances (>1GΩ) in applications such as insulation
resistance testing. A constant voltage source is placed in series with the
unknown resistance and the picoammeter. The voltage drop across the
picoammeter is negligible, so essentially all the voltage appears across the
unknown resistance. The resulting current is measured by the picoammeter and the resistance is calculated using Ohm’s Law (R=V/I). To prevent
generated current due to electrostatic interference, the unknown resistance is housed in a shielded test fixture.
A P P L I C A T I O N S
In semiconductor fabrication, 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. The ion detector generates a secondary current that’s proportional to the current of the primary ion beam. When this secondary
current is measured, it can be used to control the intensity of the primary beam. However, this secondary current is very low, often just a
few picoamps, so the instrumentation measuring it must provide high
measurement accuracy and repeatability, as well as sub-picoamp resolution. The Model 6485’s wide measurement range and 5½-digit
resolution make it ideal for this application. Signal connections to the
Model 6485 are made through the instrument’s BNC connector.
O F
R
R A N G E
Ion
Detector
B R O A D
6485
Picoammeter
A
Monitoring and control of focused ion beam currents
The Model 6485 Picoammeter can be paired with a triggerable voltage
source, such as the Model 2400 SourceMeter® instrument, a calibrated
light source, and a probing fixture to create a cost-effective photodiode
test system. Multiple Model 6485s can be connected to the DUT’s probe
pads to provide photocurrent readings or, with the addition of a switch
matrix, one or more picoammeters can take current measurements from
multiple pads. The Trigger Link interface available on both the Model
6485 and Model 2400 simplifies synchronizing these instruments, providing control over triggering voltage sourcing and photocurrent
measurement. In the first step of the measurement process, performed in
total darkness, the Model 2400 produces a voltage sweep and the Model
6485 measures the resulting dark current. In the second step, a voltage
bias is applied and the resulting photocurrent is measured while the light
level is increased in calibrated steps. The same basic test configuration
can be used for testing positive intrinsic negative (PIN) and avalanche
photodiodes (APDs).
F O R
Testing bipolar transistors typically involves two leakage current measurements: ICEO (collector-emitter with base open) and ICBO (collector-base
with emitter open). To measure the leakage current, a voltage potential
is placed across two terminals with the third terminal open, and the
resulting current is measured. The Model 487 is ideal for this application because of its integrated voltage source. Its low voltage burden
allows measuring small leakage currents without degrading the measurement. The instrument’s internal memory can store results as the test
runs, then the data can be recalled and plotted.
S O L U T I O N S
Model 2400
Bias Voltage
E C O N O M I C A L
Wafer-level photodiode testing
Transistor Leakage Current Testing
All the support you need
Informative reference materials
Service you can depend on
Visit our web site, www.keithley.com, to access our
reference library, including:
When you need help, contact us at www.keithley.com or call us at
1-888-KEITHLEY (534-8453). Whatever your application, Keithley’s
application engineers are ready to help you meet its challenges,
before and after the sale. You can rely on us to suggest the most
effective system configurations and to provide prompt, reliable
applications support once your system is set up.
Low Level Measurements
This informative handbook describes
theoretical and practical considerations
involved in the measurement of low
DC currents, high resistances, low
DC voltages, and low resistances.
Application Notes
These helpful notes include practical, real-world answers to
many application questions related to low level
measurements.
Lab Exercises
Keithley's suite of lab exercises is designed to help students
learn about making electrical and physical measurements
through hands-on experience. Many lab exercises are available for free downloading from our website. For example:
The next time you’re faced with a challenging application, give us a
call. We’ll offer you a cost-effective solution that will help you
improve your product quality, throughput, and yield.
A greater measure of confidence
With more than a half-century of expertise in making demanding low level
measurements, Keithley offers its customers a greater measure of testing
confidence on the production floor, in the QA lab, and in R&D. For more information on how Keithley test solutions can help you keep pace with changing
technologies, call your local Keithley sales engineer or visit our website.
• Energy Gap in a Semiconductor
• Digital Multimeter and Oscilloscope
• Linear Op Amp Circuits
• Step Response of Passive Circuits
• The Electric Field
• Field Effect Transistors
• Non-Linear Op Amp Circuits
• Frequency Domain Analysis of Signals
Specifications are subject to change without notice.
All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.
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© Copyright 2001 Keithley Instruments, Inc.
Printed in the U.S.A.
No. 2357
100125KOP
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