Guarding - Chroma ATE, Inc.
 APPLICATION NOTE Guarding
Guard Shield used for Low Current (nano-Amp) Measurement
Key Words: Guarding, Guard Shield, SMU, Settling Time, Force, Low Current
Measurement, nano-Amp range, Triaxial Cable, Leakage Current
Product Family: Model 52400 series SMU (Source Measure Unit)
June 20, 2013
Chroma ATE Inc.
Scope
Guarding- A technique that reduces leakage current
error and decreases settling time. It keeps the potential
flowing through the DUT and the leakage current, rather
than just the current flowing through the device.
of the guard connector at the same potential as the force
Triaxial cable prevents the Current
conductor, so current does not flow between the force
Leakage by keeping Guard same potential
and guard conductors. It also eliminates the cable
as Force
capacitance between source measure unit (SMU) and
DUT for faster and accurate measurements. This
A technique called “Guarding” can eliminate the effects
document describes how to prevent leakage current
of leakage currents flowing through the insulator. A
error for very-low current measurement. For more
guard is a low-impedance point in the circuit that’s at the
information about charging delay (settling time), please
same potential as the high-impedance lead in the circuit.
refer to the related document: AN-52400-02V1.00.
In a guarded measurement, the shield is driven to the
A guard is a low-impedance point in the circuit that’s at
the same potential as the high-impedance lead in the
circuit. In a guarded measurement, because the shield is
driven to the same potential as the force high terminal of
same potential as the Force Hi output terminal of the
SMU using a unity-gain, low-impedance amplifier
(guard). Therefore, no leakage current flows through the
insulation resistor.
the SMU (Source Measure Unit), no current flows
Guarding is a very effective way to reduce leakage
through the insulation medium; therefore, eliminates
currents. The Guard shield of a triaxial cable can solve
leakage current effects.
the current leakage problem. As it is shown in Figure-2,
the core (known as the Force) and the inner shield
Low-Current Measurement can be affected
(known as the Guard) are kept at the same electrical
by Cable Leakage
potential, thus the leakage current between them is zero
Leakage currents are currents flowing through paths
other than the intended signal path, for example,
for all practical purposes, despite the imperfections of
the insulation. This eliminates the current leakage that
would otherwise limit low current measurements.
current flowing through the interconnect insulator. This
leakage current can be a problem when the device under
test (DUT) impedance is similar to that of various
insulators in the circuit.
Figure-2. Triaxial Cable with Outer and Inner Shield
Figure-1. Leakage current flows through the cable’s
insulation resistance
When the SMU (Source Measure Unit) is connected to
the DUT using coaxial cable (see Figure-1.), a leakage
current flow through the cable insulation resistor. This
Connection for Guarding
The SMU port at the back of instrument has two pair of
independent output for both Source (Force) and Sense.
causes the SMU to measure the sum of the current
Chroma ATE Inc. | 2
One should also note that each output is a Triaxial cable
Other features, such as hardware sequencer for precise
(not BNC). In a Triaxial cable center conductor is
output profile control, two isolated channels in one
surrounded by circular cable (called guard) which has
compact PXI slot are capable to meet most demanding
the same voltage as center conductor thus avoiding any
test requirements
leakage current between center conductor and
outermost ground.
Model 52400 series SMU For more detailed information about Chroma 52400
series SMU & other Chroma solutions, please visit
Chroma’s website at: www.chromaate.com
Figure-3. Two pairs of Force/Sense
This is called guarding and used for very high impedance
devices or for ultra low current measurement. If you are
not doing ultra low current measurement, you can
simply ignore the sense output.
Figure-4. Connection Suggestion for Low current- Local
Sense, Guarded
Chroma 52400 Series SMU
The Chroma 52400 family has two fully isolated SMUs
allowing testing or measure; such as, FET or BJT IV
sweeping. No additional piece of equipment is necessary.
Kelvin remote sense connections for precise
voltage/sense at the DUT as well as +/-Guard signal to
suppress unwanted leakage current are available.
www.pxisa.org Specifications and descriptions in this document are subject to change without notice. ©Chroma ATE Inc. 2013 All Rights Reserved. Chroma ATE Inc. | 3
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