4-18
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2600AS-901-01 Rev. B / September 2008
Section 4: Source-Measure Concepts
Series 2600A System SourceMeter® Instruments Reference Manual
Figure 4-12
Contact check circuit configuration
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 SourceMeter
instrument.
Leakage current can occur through parasitic or non-parasitic leakage paths. An example of
parasitic resistance is the leakage path across the insulator in a coax or triax cable. An example of
non-parasitic resistance is the leakage path through a resistor that is connected in parallel to the
DUT.
Remote
SENSE HI
IN/OUT HI
Remote
I-Meter
IN/OUT LO
V-Meter
Sense Output
Adjust V-Source
(Feedback)
SENSE LO
Local
Local
x1
GUARD
+
V-Source
1
2
1
2
NOTES: 1. This represents a protection circuit that is very
high impedance until the voltage across it exceeds
approximately 3V. Above 3V, the protection turns
on and allows current to flow through it.
2. Approximately 13k
W
.
3. High impedance differential amplifier.
3
3
300
m
A
Current Source
Contact
Resistance(s)
300
m
A
Current Source
