5-24
Source-Measure Concepts
Ohms guard
The OHMS guard selection provides a low-impedance (<1
Ω
), 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 20k
Ω
.
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 1k
Ω
, 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 100
Ω
and show-
ing the 1
Ω
resistance (R
TL
) of the GUARD test lead. Since the resistance path from GUARD-
to-LO is less than 1k
Ω
, 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)
20k
Ω
10k
Ω
100k
Ω
(Ohms Guard)
I
0V
Resistor
Network
IN/OUT HIGH
1
R
2
R
3
R
G
I
3
R
=
/
M
V
Figure 5-13
In-circuit ohms
measurements
Summary of Contents for 6430
Page 26: ......
Page 32: ......
Page 78: ...2 14 Connections ...
Page 98: ...3 20 Basic Source Measure Operation ...
Page 138: ...5 30 Source Measure Concepts ...
Page 156: ...6 18 Range Digits Speed and Filters ...
Page 168: ...7 12 Relative and Math ...
Page 176: ...8 8 Data Store ...
Page 202: ...9 26 Sweep Operation ...
Page 248: ...11 22 Limit Testing ...
Page 310: ...16 6 SCPI Signal Oriented Measurement Commands ...
Page 418: ...17 108 SCPI Command Reference ...
Page 450: ...18 32 Performance Verification ...
Page 477: ...A Specifications ...
Page 489: ...B StatusandErrorMessages ...
Page 498: ...B 10 Status and Error Messages ...
Page 499: ...C DataFlow ...
Page 503: ...D IEEE 488BusOverview ...
Page 518: ...D 16 IEEE 488 Bus Overview ...
Page 519: ...E IEEE 488andSCPI ConformanceInformation ...
Page 523: ...F MeasurementConsiderations ...
Page 539: ...G GPIB488 1Protocol ...
Page 557: ......