Keithley 6514

Amps Measurements 4-5
High impedance measurement techniques
Significant leakage could occur across a high impedance (≥1GΩ) DUT through the insulators
as shown in Figure 4-3A where R
L1
and R
L2
represent the leakage resistance. Instead of measur-
ing just the current (I
R
) through R, you are also measuring the leakage current (I
L
). The current
measured by the ammeter is I
R
+ I
L
.
By connecting ammeter input LO to the metal mounting (guard) plate as shown in Figure
4-2B, the leakage current (I
L
) is shunted to ammeter input LO and is not measured by the amme-
ter. Therefore, the ammeter only measures I
R
.
I
M
= I
R
+ I
L
HI
6514
LO
R
L1
E
R*
A
I
R
Metal Mounting Plate
Insulators
R
L2
I
L
A. Unguarded
I
M
= I
R
HI
6514
LO
R
L1
E
R*
A
I
R
Metal Guard Plate
I
L
*R = ≥1GΩ
B. Guarded
Figure 4-2
High impedance
current measurements