Specifications

3.4 Practical guarding techniques
3.4.1 Measurement error due to stray capacitances
When the DUT is located near a conductor (for example, a metallic desktop) and a measurement
signal is applied to the DUT, a voltage difference will appear between the DUT and the nearby
conductor. This creates stray capacitances and allows the measurement signal to leak towards the
conductor as shown in Figure 3-15 (a). Unshielded portions of test leads also have stray capaci-
tances to the conductor.
Signal leakage through the stray capacitance on the High side of the DUT will bypass the DUT by
flowing through the conductor and the stray capacitance on the Low side. The ammeter (I-V convert-
er) on the Lc side measures the sum of the DUT current and the additional leakage current caused
by the stray capacitances. Thus, the effect of stray capacitances results in measurement error. The
stray capacitances produce greater measurement error for higher impedance of DUT and at higher
measurement frequencies.
Figure 3-15. Guarding technique (1)
H
p
H
c
L
c
L
p
V
Null
detector
DU T
Conductor
(e.g. desktop)
Stray capacitance
Leakage
current
Leakage
current
(a) Stray capacitance and leakage current
H
p
H
c
L
c
L
p
V
Null
detector
DUT
Conductor
(e.g. desktop)
No stray
capacitance!
Insert a
shielding plate
(b) Removing the stray capacitance
Connect to
guard
3-15