Optimizing Low Current Measurements with the Model 4200

Optimizing Low Current Measurements with the Model 4200
Number 2959
Application Note
Se­ries
Optimizing Low Current Measurements
with the Model 4200-SCS
Semiconductor Characterization System
Introduction
Many critical applications demand the ability to measure very
low currents—such as picoamps or less. These applications
include determining the gate leakage current of FETs, testing
sensitive nano-electronic devices, and measuring leakage current
of insulators and capacitors.
The Model 4200-SCS Semiconductor Characterization System,
when configured with the optional Model 4200-PA Remote
Preamp, offers exceptional low current measurement capability
with a resolution of 1E–16A. Making low current measurements
successfully depends not only on using a very sensitive ammeter
like the Model 4200-SCS’s, but on choosing the proper settings
in the system’s Keithley Interactive Test Environment (KITE)
software, using low noise fixtures and cabling, allowing sufficient
settling time, and using techniques that prevent unwanted
currents from reducing measurement accuracy. This application
note describes Keithley’s best-known methods recommendations
for optimizing low current measurements using the Model
4200-SCS.
Measuring the Offset Current of the System
One of the first steps in setting up a system for making ultralow current measurements is to determine the offset and
leakage current of the entire measurement system, which
includes the 4200-SCS itself, the connecting cables, switch
matrix, test fixtures, and probes. This identifies the noise floor
limit of the entire system and sets a starting point for making
improvements to the system, if possible. Start by measuring
the offset of the Source-Measure Units (SMUs) and then
continue to add components of the measurement circuit until
everything is connected except the device under test (DUT).
The measurements are made directly by the 4200-SMU with
the 4200-PA Remote PreAmp using the KITE software.
Internal Offsets
The ideal ammeter should read zero current when its input
terminals are left open. Practical ammeters, however, do have
some small current that flows when the input is open. This
current, which is known as the input offset current, is generated
by bias currents of active devices and by leakage currents
through insulators within the instrument. The offset current
generated within the SMU is included in the Model 4200-SCS’s
specifications. As shown in Figure 1, the input offset current
adds to the measured current so the meter measures the sum
of the two currents:
IS
FORCE HI
RS
IM
VS
IOFFSET
COMMON
Current Source
SMU or PreAmp
IM = IS + IOFFSET
Figure 1. Input Offset Current of SMU
The offset of each 4200-SMU with the 4200-PA preamp is
measured with nothing connected to the Force HI and Sense
HI terminals except metal caps. These three-lug metal caps are
included with the system. Before taking any measurements, the
SMUs should be warmed up with the metal caps connected to
the Force HI and Sense HI terminals of the preamps for at least
one hour. If the system has KTEI software version 7.1 or later
installed, the offset current can be measured using the project
called “LowCurrent” located on the system in this directory:
C:\S4200\kiuser\Projects\LowCurrent
Open this project and select the SMU1offset ITM. Click on the
graph tab and run the test. The results should be similar to the
graph shown in Figure 2. It may be necessary to use the Auto
Scale function to scale the curve appropriately. The Auto Scale
function can be found by right-clicking on the graph. With the
4200-PA preamp connected to the SMU, the offset current should
be in the femtoamp range. The current offset can be positive
or negative. Verify these results with the published ammeter
specifications for the Model 4200-SCS.
This test should be repeated using a separate ITM for each
SMU in the system. The LowCurrent project has ITMs for
performing offset current measurements on four SMUs with
preamps.
It is also easy to measure the offset current for systems
running versions of the KTEI software earlier than 7.1. Follow
these steps to create a test to perform this measurement on
SMU1:
1. Within a project that has already been created, open
a new Device Plan for a generic 2-terminal device.
2. Create a new ITM called SMU1Offset. Select SMU1 for
terminal A and the GNDU for terminal B.
External Offsets
Figure 2. Offset Current Measurement of SMU1
3. Set up the following in the Definition Tab:
Once the offset current of the ammeter has been determined,
verify the offset of the rest of the system by repeating (using
the Append Run button shown in Figure 3) the current (at zero
volts) vs. time graph after adding each piece of the test circuit.
Finally, make a measurement to the end of the probe in the
“up” position or to the test fixture with no device connected.
This process will help determine any trouble spots, such as
a shorted cable or instability in the measurement circuit.
However, be aware that connecting and disconnecting cables
generates current in the circuit. For making ultra-low current
measurements, it may be necessary to wait from a few minutes
to hours for the spurious currents to decay after changing
connections in the test circuit. Figure 4 illustrates a graph
showing the offset of 1) just the SMU with a capped Force HI
terminal, 2) with only a triax cable on the preamp, and finally
3) through the Keithley 7174A Low Current Switch Matrix to a
probe station with a probe in the “up” position.
SMU Force/Measure Configuration: Voltage bias 0V,
10pA fixed current range.
Timing Menu: Quiet Speed, Sampling Mode, 0s Interval,
20 Samples, 1s Hold Time, Timestamp Enabled checked
Formulator: Create a formula to measure noise using the
standard deviation function, NOISE=STDDEV(A1).
Figure 3. Append button
Also create a formula to measure the average offset
current: AVGCURRENT=AVG(A1).
4. Set up the following in the Graph Tab (right-click on
graph):
Define Graph: X-axis: Time
Y1-axis: Current (A1)
Data Variables: Select NOISE to appear on graph. Select
AVGCURRENT to appear on graph.
Once the test is configured, save the test and run it. The
results should be similar to those shown in Figure 2. Repeat the
test for all the SMUs in the system.
The input offset current specification can be optimized by
performing an auto calibration procedure in KITE. To perform
an SMU auto calibration, go to the KITE Tools menu and click on
SMU Auto Calibration. Before performing the auto calibration,
allow the system to warm up for at least 60 minutes after
power-up. Nothing should be connected to the SMU Force HI
and Sense HI terminals except a metal cap. The auto calibration
routine adjusts the current and voltage offsets for all source and
measurement functions of all SMUs in the system. This should
not be confused with a full system calibration, which should be
performed once per year at the Keithley factory.
The offset current measurement can be repeated once the
SMU auto calibration has been performed.
Figure 4. Offset Current Measurement of Entire Test System
This test should be repeated to determine any leakage circuit
in the measurement circuit by applying a test voltage when
generating the current vs. time graph. Rather than applying
a zero volt bias, use the test voltage that will be used in the
actual measurements of the DUT. Any leakage current in the test
fixtures and cables will now be measured and graphed. If the
leakage appears to be too high, adjustments can be made to the
measurement circuit to reduce the leakage current. Refer to the
section titled “Leakage Current and Guarding,” which describes
ways to reduce leakage current.
Sources of Measurement Errors
and Ways to Reduce Them
An example for determining the settling time can be
calculated as follows, if CSHUNT = 10pF and RS = 1TW, then:
Once the current offsets, leakage current, and any instability
have been determined, taking steps to reduce measurement
errors will help improve measurement accuracy. These sources
of error include insufficient settling time, electrostatic inference,
leakage current, triboelectric effects, piezoelectric effects,
contamination, humidity, ground loops, light, and source
impedance. Figure 5 summarizes the magnitudes of some of the
generated currents discussed in this section.
τ = 10pF × 1TW = 10 seconds
Therefore, a settling time of five τ, or 50 seconds, would be
required for the reading to settle to within 1% of final value!
Figure 7 shows the exponential response of a step voltage into
an RC circuit. After one time constant (τ = RC), the voltage rises
to within 63% of final value.
10—8
Standard
cable
10—9
10—10
Low
noise
cable
10—14
10—15
Triboelectric
Effects
Clean
surface
Ceramics
10—13
63
Epoxy
board
10—11
10—12
Percent
of Final
Value
Dirty
surface
Teflon
Typical Current Generated (A)
99
10—7
Mechanical
Stress
Effects
0
109Ω
1012Ω
Electrochemical
Resistor
Effects
noise in 1Hz
bandwidth
Current-Generating Phenomena
Figure 5. Typical Magnitudes of Generated Currents
Settling Time and Timing Menu Settings
The settling time of the measurement circuit is particularly
important when making low current and high resistance
measurements. The settling time is the time that a measurement
takes to stabilize after the current or voltage is applied or
changed. Factors affecting the settling time of the measurement
circuit include the shunt capacitance (CSHUNT ) and the source
resistance (RS). The shunt capacitance is due to the connecting
cables, test fixtures, switches, and probes. The higher the
source resistance of the DUT, the longer the settling time. The
shunt capacitance and source resistance are illustrated in the
measurement circuit in Figure 6.
Time
0
1.0
2.0
3.0
4.0
5.0
τ = RS CSHUNT
Figure 7. Exponential Response of Stepped Voltage Across RC Circuit
To make successful low current measurements, it’s important
to add sufficient time for each measurement, particularly when
sweeping voltage. This settling time can be added in the Timing
menu in the Sweep Delay field for the Sweeping Mode or the
Interval time field for the Sampling Mode. To verify how much
interval time to add, measure the settling time of the DUT by
plotting the current vs. time to a stepped voltage. The stepped
voltage should be the bias voltage that will be used in the actual
measurement of the DUT. The ITMs in the LowCurrent project
can be used to perform the settling time measurement. The
#Samples in the Timing Menu will probably need to be increased
to ensure settled readings will be displayed on the graph. When
making low current measurements, use the Quiet Speed Mode or
add extra filtering in the Timing Menu. Keep in mind that there
is a noise/speed trade-off. With more filtering and delays, there
will be less noise but a slower measurement speed.
Electrostatic Interference and Shielding
RS
Unknown
Resistance
of DUT
CSHUNT
VM
IS
4200 SMU
τ = RS CSHUNT
Figure 6. SMU Measurement Circuit Including C SHUNT and R S
The settling time is the result of the RC time constant, or τ,
where:
τ = RSCSHUNT
Electrostatic coupling or interference occurs when an electrically
charged object approaches the circuit under test. At low
impedance levels, the effects of the interference aren’t noticeable
because the charge dissipates rapidly. However, high resistance
materials don’t allow the charge to decay quickly, which may
result in unstable, noisy measurements. Typically, electrostatic
interference is an issue when making current measurements
≤1nA or resistance measurements ≥1GW.
To reduce the effects of the fields, the circuit being measured
can be enclosed in an electrostatic shield. Figure 8 illustrates
the dramatic difference between an unshielded and a shielded
measurement of a 100GW resistor. The unshielded measurements
are much noisier than the shielded measurements.
The guard is a conductor driven by a low impedance source
whose output is at or near the same potential as the high
impedance terminal. The guard terminal is used to guard test
fixture and cable insulation resistance and capacitance. The
guard is the inside shield of the triax connector/cable illustrated
in Figure 10.
Center Conductor: Force HI
Inner Shield: Guard
Outer Shield: Force LO
Figure 10. Conductors of 4200 Triax Connector/Cable
Figure 8. Shielded vs. Unshielded Measurements on a 100GW Resistor
The shield can be just a simple metal box or meshed screen
that encloses the test circuit. Commercial probe stations often
enclose the sensitive circuitry within an electrostatic shield. The
shield is connected to the measurement circuit LO terminal,
which is not necessarily earth ground. In the case of the Model
4200-SCS, the shield is connected to the Force LO terminal as
shown in Figure 9.
4200-SMU
Force HI
Conductive
Shield
A
Force LO
Guarding should not be confused with shielding. Shielding
usually implies the use of a metallic enclosure to prevent
electrostatic interference from affecting a high impedance circuit.
Guarding implies the use of an added low impedance conductor,
maintained at the same potential as the high impedance circuit,
which will intercept any interfering voltage or current. A guard
doesn’t necessarily provide shielding. The following paragraphs
outline two examples of guarding: 1) using guarding to reduce
the leakage due to a test fixture and 2) using guarding to reduce
leakage currents due to cabling.
Figure 11 shows how the guard can eliminate the leakage
current that may flow through the stand-off insulators in a test
fixture. In Figure 11a, the leakage current (IL) flows through
the stand off insulators (R L). This leakage current is added to
the current from the DUT (IDUT ) and is measured by the SMU
ammeter (IM), adversely affecting the accuracy of the low current
measurement.
a) Unguarded Circuit
Conductive shield
is connected to the
Force LO terminal
Standoff
Insulators
IM
×1
RL
IL
Z
Metal Mounting Plate
V
Force/Output LO
SMU
b) Guarded Circuit
Metal Shielded Test Fixture
IDUT
Force/Output HI
• Avoid movement and vibration near the test area.
IM = IDUT
RDUT
IM
Leakage Current and Guarding
Leakage current is an error current that flows (leaks) through
insulation resistance when a voltage is applied. This error
current becomes a problem when the impedance of the DUT is
comparable to that of the insulators in the test circuit. To reduce
leakage currents, use good quality insulators in the test circuit,
reduce humidity in the test lab, and use guarding.
IM = IDUT + IL
RDUT
RL
Guard
To minimize error currents due to electrostatic coupling:
• Keep all charged objects (including people) and conductors
away from sensitive areas of test circuit.
IDUT
Force/Output HI
Figure 9. Shielding a High Impedance Device
• Shield the DUT and connect the enclosure electrically
to the test circuit common, the Force LO terminal of the
4200-SCS.
Metal Shielded Test Fixture
0V
Guard
×1
Z
V
RL
RL
IL = 0
Metal Mounting Plate
Force/Output LO
SMU
Figure 11. Using Guarding to Reduce Leakage in a Test Fixture
In Figure 11b, the metal mounting plate is connected to the
guard terminal of the SMU. The voltages at the top and bottom
of the stand off insulator are nearly at the same potential (0V
drop), so no leakage current will flow through the standoffs to
affect the measurement accuracy. For safety purposes, the metal
shield must be connected to earth ground because the metal
mounting plate will be at the guard potential.
the measurement in two ways: 1) it reduces the effective
cable capacitance and thus decreases the RC time constant or
settling time of the measurement, and 2) it prevents the leakage
resistance of the cable from degrading the measurement accuracy.
Guarding can also be used to reduce leakage currents in
cabling. Figure 12 illustrates how a driven guard prevents
the leakage resistance of a cable from degrading low current
measurements. In the unguarded configuration, the leakage
resistance of the coax cable is in parallel with the DUT (R DUT ),
creating an unwanted leakage current (IL). This leakage current
will degrade very low current measurements.
In the guarded circuit, the inside shield of the triax cable is
connected to the guard terminal of the SMU. Now this shield
is driven by a unity-gain, low impedance amplifier (guard).
The difference in potential between the Force HI terminal and
the Guard terminal is nearly 0V, so the leakage current (IL) is
eliminated.
a) Unguarded Circuit
RL
IM
Guard
×1
As you can see from the graph in Figure 13, using triax cables
with guarding resulted in measured currents that had lower
leakage (a few picoamps lower) and had a faster settling time
(about ten times faster).
IDUT
Force/Output HI
Coax
Cable
IL
Z
RDUT
V
Force/Output LO
RL = Coax Cable Leakage Resistance
SMU
In addition to using shielding and guarding when making
connections to the DUT, it is very important to connect the
appropriate terminal of the 4200-SCS to the appropriate terminal
of the device. Improper connections of the SMU Force HI and
Force LO terminals can cause current offsets and unstable
measurements. These errors are due to common mode current.
IM = IDUT + IL
b) Guarded Circuit
IM
Guard
IDUT
×1
Z
0V
RL1
Triax
Cable
RDUT
RL2
V
Force/Output LO
SMU
If the SMUs must be connected to a test fixture with BNC
connectors, use Keithley triax cables from the SMUs to the test
fixture, and then BNC to triax adaptors (with guard removed) to
attach the cables to the test fixture.
SMU Connections to DUT
IL = Leakage Current
RDUT = Resistance of Device Under Test
Force/Output HI
Figure 13. Results of Using a Coax Cable and a Triax Cable when Measuring a
High Resistance
RL1 = Triax Cable Inside Shield Leakage Resistance
RL2 = Leakage Resistance Between Shields
RDUT = Resistance of Device Under Test
IM = IDUT
Figure 12. Using Guarding to Reduce Leakage Currents in Cabling
To see the results of using triax cable vs. coax cable when
making a very high resistance measurement, Figure 13 shows
the results of measuring current vs. time of a 10V step function
into a 100GW resistor. The triax cable enables guard, improving
In general, always connect the high impedance terminal
(Force HI) of the SMU to the highest resistance point of the
circuit under test. Likewise, always connect the low impedance
terminal (Force LO) of the 4200-SCS to the lowest resistance
point of the circuit under test. The lowest resistance point may
be a common terminal or earth ground. If the Force HI terminal
is connected to the lowest resistance point, common mode
current can flow through the measurement circuit.
Figure 14 illustrates both a proper and an improper
measurement connection. Figure 14a indicates a proper
connection because the Force HI terminal of the 4200-SMU is
connected to the gate of the device on a wafer, and the Force LO
terminal is connected to the grounded chuck. The gate terminal
on the wafer is the highest impedance point and the grounded
chuck is the low impedance point, so this circuit is a proper
connection. Note that the common mode current flows from the
Force LO terminal of the SMU to the grounded chuck; however,
the current does not flow through the ammeter, and therefore
does not affect the measurement.
Frictional motion at
boundary due to
cable motion
I
I
+
+
Inner
conductor
Insulation
–
A. Proper Connection
–
Force HI
Outer
jacket
A
Wafer
Outer
shield
Coaxial
cable
SMU
Conductive
lubricant in
low noise cable
Chuck
Figure 15. Offset Current Generated by the Triboelectric Effect
Force LO
kept short, away from temperature changes (which would create
thermal expansion forces), and preferably supported by taping or
wiring the cable to a non-vibrating surface such as a wall, bench,
or rigid structure.
I
Other techniques should also be employed to minimize
movement and vibration problems:
B. Improper Connection
Force LO
• Remove or mechanically decouple vibration sources such as
motors, pumps, and other electromechanical devices.
• Securely mount or tie down electronic components, wires,
and cables.
Wafer
SMU
A
Chuck
I
• Mount the preamps as close as possible to the DUT.
Piezoelectric and Stored Charge Effects
Force HI
Figure 14. Making SMU Connections to a Device on a Grounded Chuck
Figure 14b illustrates an improper connection with the Force
LO terminal of the SMU connected to the high impedance gate
terminal and the Force HI terminal of the SMU connected to the
grounded chuck. In this case, the common mode current will
flow through the SMU as well as the DUT. This will result in
inaccurate, even unstable measurements.
Piezoelectric currents are generated when mechanical stress is
applied to certain crystalline materials when used for insulated
terminals and interconnecting hardware. In some plastics,
pockets of stored charge cause the material to behave in a
manner similar to piezoelectric materials. An example of a
terminal with a piezoelectric insulator is shown in Figure 16.
Applied
force
Metal
terminal
I
+
I
Triboelectric Effects
Triboelectric currents are generated by charges created between
a conductor and an insulator due to friction. Here, free electrons
rub off the conductor and create a charge imbalance that causes
the current flow. This noise current can be in the range of tens
of nanoamps. Figure 15 illustrates the flow of triboelectric
current.
The triax cables supplied with the 4200-SCS greatly reduce
this effect by using graphite-impregnated insulation beneath the
outer shield. The graphite provides lubrication and a conducting
cylinder to equalize charges and minimize charge generated by
frictional effects of cable movement. However, even this type of
triax cable creates some noise when subjected to vibration and
expansion or contraction. Therefore, all connections should be
–
Piezoelectric
insulator
–
+
Conductive plate
Figure 16. Current Generated by Piezoelectric Effects
To minimize these effects, remove mechanical stresses
from the insulator and use insulating materials with minimal
piezoelectric and stored charge.
Contamination and Humidity Effects
The insulation resistance of test fixtures can be dramatically
reduced by high humidity or ionic contamination. High humidity
conditions occur with condensation or water absorption, while
ionic contamination may be the result of body oils, salts, or
solder flux. A reduction in insulation resistance can have a
serious effect on high impedance measurements. In addition,
humidity or moisture can combine with any contaminants
present to create electrochemical effects that can produce offset
currents. For example, commonly used epoxy printed circuit
boards, when not thoroughly cleaned of etching solution, flux, or
other contamination, can generate currents of a few nanoamps
between conductors (see Figure 17).
Printed
Wiring
Epoxy Printed
Circuit Board
4200-SCS
SMU
Force HI
Ground
Link
Installed
Signal Path
Ground Unit
Common
Chassis
DUT
Ground loop causes
current flow in
Common lead
DUT LO
Grounded
Ground Bus
Figure 18. Ground Loops
4200-SCS
I
Flux or
other chemical
“track” and
moisture
+
–
SMU
Force HI
Ground
Link
Removed
Signal Path
Ground Unit
Common
Chassis
DUT
DUT LO
Grounded
I
Ground Bus
Figure 17. Current Generated from Contamination and Humidity
Figure 19. Eliminating Ground Loops
To avoid the effects of contamination and humidity, select
insulators that resist water absorption, and keep humidity to
moderate levels (ideally <50%). Also, be sure all components
and test fixturing in the test system are kept clean and free of
contamination.
If a ground loop is suspected, unplug the suspect instrument
from the AC power and try making a sensitive current measure­
ment to verify the problem is gone. To eliminate ground loops,
make as few grounds as possible, preferably, no more than one.
Ground Loops
Some components such as diodes and transistors are excellent
light detectors. Consequently, these components must be tested
in a light-free environment. To ensure measurement accuracy,
check the test fixture for light leaks at door hinges, tubing entry
points, and connectors or connector panels.
Ground loops can generate spurious signals that may be a DC
offset or an AC signal (usually line frequency or multiples of line
frequency). Ground loops are caused by multiple grounds in
the test circuit. A typical example of a ground loop can be seen
when a number of instruments are plugged into power strips on
different instrument racks. Frequently, there is a small difference
in potential between the ground points, which can cause large
currents to circulate and create unexpected voltage drops.
The configuration shown in Figure 18 shows a ground loop
that is created by connecting both the 4200 signal common
(Force LO) and DUT LO to earth ground. A large ground current
flowing in the loop will encounter small resistances, either in
the conductors or at the connecting points. This small resistance
results in voltage drops that can affect performance.
To prevent ground loops, the test system should be
connected to ground at only a single point. If it is not possible
to remove the DUT ground, the ground link between the
4200 GNDU COMMON terminal and chassis ground should be
removed, as shown in Figure 19.
Light
Noise and Source Impedance
Noise can seriously affect sensitive current measurements. Both
the source resistance and the source capacitance of the DUT can
affect the noise performance of the SMU.
The source resistance of the DUT will affect the noise
performance of the SMU’s feedback ammeter. As the source
resistance is reduced, the noise gain of the ammeter will
increase. Figure 20 shows a simplified model of a feedback
ammeter.
In this circuit:
RS = source resistance
CS = source capacitance
VS = source voltage
V NOISE = noise voltage of the ammeter
The source capacitance of the DUT will also affect the noise
performance of the SMU. In general, as source capacitance
increases, so does the noise gain. Although there is a limit as
to the maximum source capacitance value, it’s usually possible
to measure at higher source capacitance values by connecting a
resistor or a forward-biased diode in series with the DUT. The
diode acts like a variable resistance, low when the charging
current to the source capacitance is high, then increasing in
value as the current decreases with time.
ZF
CF
ZS
RF
CS
—
RS
+
VO
VNOISE
VS
Current Source
Feedback Ammeter
Figure 20. Simplified Model of a Feedback Ammeter
R F = feedback resistor
CF = feedback capacitance
The noise gain of the circuit can be given by this equation:
RF
Output VNOISE = Input VNOISE (1 +____)
RS
Note that as the source resistance (RS) decreases, the output
noise increases. Because decreasing the source resistance can
have a detrimental effect on noise performance, there are
minimum recommended source resistance values based on the
current measurement range, which are summarized in Table 1.
Table 1. Minimum Recommended Source Resistance Values
Range
1pA to 100pA
1nA to 100nA
1µA to 100µA
1mA to 100mA
Minimum Recommended
Source Resistance
1GW to 100GW
1MW to 100MW
1kW to 100kW
1W to 100W
Compensating for Offsets
After external errors have been determined and reduced, if
possible, the internal and external offsets of the test system can be
subtracted from future measurements. First, perform the SMU auto
calibration with the capped input as described. Then, determine
the offsets for each SMU to the probe tip. This average offset
current can be subtracted from subsequent current measurements
in other projects using the Formulator tool in the software. For
making very low current measurements, the average offset current
should be remeasured periodically (at least monthly).
Conclusion
When configured with the optional Model 4200-PA Remote
PreAmps, the Model 4200-SCS Semiconductor Characterization
System can measure accurately currents of picoamps or less.
The offset current of the entire measurement system should
be measured to determine the system’s limitations, so it can
be adjusted if necessary. Sources of measurement errors can
be reduced by using techniques such as shielding, guarding,
and proper grounding of instruments, and by choosing
appropriate settings in the KITE software, including allowing
sufficient settling time. Keithley’s Low Level Measurements
Handbook provides further information on optimal low current
measurement techniques.
For Further Reading
Keithley Instruments, Model 4200-SCS Reference Manual, Section 5 (Included as
part of the system software)
Keithley Instruments, Low Level Measurements Handbook, 6th edition, 2004.
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.
A
Keithley Instruments, Inc.
■
G R E A T E R
28775 Aurora Road
© Copyright 2008 Keithley Instruments, Inc.
■
M E A S U R E
Cleveland, Ohio 44139-1891 ■
Printed in the U.S.A.
O F
C O N F I D E N C E
440-248-0400
■
Fax: 440-248-6168
No. 2959
■
1-888-KEITHLEY
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www.keithley.com
05.28.08
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