Keithley 2520, User Manual, Page: 290 / 329

F-6 Measurement Considerations Model 2520 User’s Manual
Exposed loop area
The open loop area (Figure F-4) also affects the rise time of the measurements because of
added inductance. With the relatively large open loop area in
Figure F-4 A, the shields are
not carried through to the DUT, and the exposed signal lines form a current loop. This loop
effectively increases the inductance and affects the rise time in the same manner as cable
inductance outlined above. To minimize the exposed loop area, carry all shields as close to
the DUT as possible, as shown in
Figure F-4 B. Doing so will reduce the distributed induc-
tance and decrease the rise time of the current pulse.
Inductance for a loop of wire X inches by Y inches with a diameter D inches can be calcu-
lated as follows:
Thus, a typical 1 inch square loop has an inductance of about 100nH, while a typical 2
inch square loop has an inductance on the order of 170nH.
Figure F-4
Effects of open loop area
L 10.16 10
9
–
X 2
Y
D
----
 
 
Y 2
X
D
----
 
 
ln
+
ln
×
=
DUT
Connecting
Cables
Large
Exposed
Loop Area
A. Large exposed loop area.
DUT
Connecting
Cables
Small
Exposed
Loop Area
B. Small exposed loop area.
Voltage Sense HI
Current Output HI
Current Output LO
Voltage Sense LO
Voltage Sense HI
Current Output HI
Current Output LO
Voltage Sense LO