User Manual

6-32 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 6-18 was modified to create Figure 6-19A 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 significant. 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 measure-
ment of R1.
The guard test lead IR drop is compensated for by connecting GUARD SENSE as
shown in Figure 6-19B. Sensing allows the guard voltage to be sensed (mea-
sured) 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 6-19 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 disable 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.”