Model 2651A High Power System SourceMeter® Instrument Reference Manual
Section 4: Theory of operation
2651A-901-01 Rev. A / March 2011
4-17
Figure 80: Contact check circuit configuration
Remote
SENSE HI
IN/OUT HI
Remote
IN/OUT LO
SENSE LO
Local
Local
x1
GUARD
+
–
1
2
1
2
3
3
Contact
resistances
Sense Output
Adjust V-Source
(Feedback)
V-source
NOTES: 1.
This represents a protection circuit that is very
high impedance until the voltage across it exceeds
approximately 3 V. Above 3 V, the protection turns
on and allows current to flow through it.
2.
Approximately 13 k
Ω
.
3.
High impedance differential amplifier.
+
-
300
μ
A
Current Source
300
μ
A
Current Source
I-meter
A
V
V-meter
Guard
WARNING
GUARD is at the same potential as output HI. Thus, if hazardous voltages are present at output
HI, they are also present at the GUARD terminal.
Guard overview
The driven guard (available at the rear panel GUARD terminals) is always enabled and provides a
buffered voltage that is at the same level as the input/output HI (or sense HI for remote sense)
voltage. The purpose of guarding is to eliminate the effects of leakage current (and capacitance) that
can exist between input/output high and low. In the absence of a driven guard, leakage in the external
test circuit could be high enough to adversely affect the performance of the High Power System
SourceMeter
®
Instrument.
Leakage current can occur through parasitic or nonparasitic leakage paths. An example of parasitic
resistance is the leakage path across the insulator in a coaxial or triaxial cable. An example of
nonparasitic resistance is the leakage path through a resistor that is connected in parallel to the
device under test (DUT).
