6-38 Source-Measure Concepts 2400 Series SourceMeter
®
User’s Manual
Guard sense
When the GUARD-to-LO resistance path is less than 1kΩ, remote guard sensing should be
used to compensate for IR drop in the GUARD test lead and/or switch contacts on a
switching card.
Figure 6-19 was modified to create Figure 6-20A by changing the value of R3 to 100Ω and
showing the 1Ω resistance (R
TL
) of the GUARD test lead. Since the resistance path from
GUARD-to-LO is less than 1kΩ, the IR drop in the guard test lead (R
TL
) becomes signifi-
cant. The guard voltage applied to the bottom of R2 is now significantly lower than the In/
Out HI voltage of the SourceMeter. As a result, leakage current (I
L
) flows through R2,
adversely affecting the resistance measurement of R1.
The guard test lead IR drop is compensated for by connecting GUARD SENSE as shown
in Figure 6-20B. Sensing allows the guard voltage to be sensed (measured) at the resistor
network for better guard voltage regulation. If the remotely sensed guard voltage is less
than the output voltage of the SourceMeter, the guard voltage will be increased until the
sensed guard voltage equals the output HI voltage.
Note that in order to ensure that guard current (I
G
) in Figure 6-20 does not exceed 50mA,
output voltage from the SourceMeter must not exceed 5V (50mA × 100Ω = 5V).
NOTE Guard sense operation is automatic. There is no menu selection to enable or dis-
able guard sense.
For 6-wire ohms guard measurements, use the GUARD output-off state. For
details on the GUARD output-off state, see Section 13, “Output-off states.”
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