Model 4200A-SCS Source-Measure Unit (SMU) User's Manual
Section 5: Source-measure concepts
4200A-SMU-900-01 Rev. A December 2020
5-3
Guarding concepts
Guarding is especially important with high-impedance circuits. Consider the comparison of the
unguarded and guarded circuits shown in the following figures. In both cases, FORCE is connected to
DUT HI and COMMON is connected to DUT LO.
In the unguarded circuit in the following figure, the cable leakage resistance, R
L
, is effectively in
parallel with the DUT, creating an unwanted leakage current, I
L
. This leakage current may seriously
affect readings, particularly at low current levels.
Figure 69: Unguarded circuit
In the guarded circuit of the following figure, the cable shield is driven by a unity-gain, low-impedance
amplifier (GUARD). Since the voltage across R
L
is nearly 0 V, the leakage current is effectively
eliminated. Current through any leakage resistance (R
G
) between the shield and COMMON may be
considerable, but it is of little consequence because it is supplied by the unity-gain amplifier rather
than the FORCE terminal of the SMU or preamplifier.
Figure 70: Guarded circuit