Keithley 6430 User Manual

5-24 Source-Measure Concepts
Ohms guard
The OHMS guard selection provides a low-impedance (<1Ω), high current (up to 50mA)
driven guard. This lets you perform in-circuit ohms measurements of the DUT where other par-
allel resistive paths are present. These measurements are typically performed in Delta or Wye
configurations.
NOTE Ohms guard is only available at the V,Ω banana jack on the mainframe. It is not
available at the Remote PreAmp. See also “6-wire ohms measurements” in Section 4
and “Ohms guard” in Section 2.
If you want to measure the resistance of a single resistor in the network, you must use the
ohms guard configuration. Figure 5-13 shows how to measure the resistance of R
1
. Since the
voltage on either side of R
2
is the same, no current can flow through it. Thus, all the pro-
grammed current (I
M
) from the SourceMeter will flow through R
1
. The voltage across R
1
is
then measured, and an accurate resistance measurement is calculated, in this case 20kΩ.
NOTE Ohms guard current (I
G
) must not exceed 50mA. If it does, the guard voltage drops
lower than the output voltage allowing leakage current. Thus, the guarded ohms
measurement becomes corrupted.
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.
In Figure 5-14A, Figure 5-13 was modified by changing the value of R
3
to 100Ω and show-
ing 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 R
2
is now significantly lower than the In/Out HI voltage of the
SourceMeter. As a result, leakage current (I
L
) flows through R
2
, adversely affecting the resis-
tance measurement of R
1
.
6430
x1
I-Source
V-Meter
In/Out LO
In/Out HI
REMOTE
PreAmp
Guard
Ohms Guard Selected
G
V,Ω GUARD
HI (In/Out)
Guard (Cable)
LO (In/Out)
20kΩ
10kΩ
100kΩ
(Ohms Guard)
I
0V
Resistor
Network
IN/OUT HIGH
1
R
2
R
3
R
G
I
3
R=
/
M
V
Figure 5-13
I
n-circuit ohms
measurements

Keithley 6430 Specifications

General

Brand	Keithley
Model	6430
Category	Measuring Instruments
Language	English