Front Panel Operation
2-32
2.7
Resistance and resistivity
measurements
The Model 6517A can make resistance measurements and
resistivity measurements (surface and volume). High resis-
tance measurements (above 1M
Ω
) may exhibit problematic
background currents and can be improved using the Alter-
nating Polarity Test Sequence (see paragraph 2.14).
Auto V-Source
The Model 6517A has an auto V-Source mode for resistance
and resistivity measurements. With AUTO V-Source select-
ed, the Model 6517A will automatically set the V-Source to
an optimum test voltage level; either 40V or 400V. The se-
lected test voltage and current measurement range depends
on which ohms measurement range is being used (see Table
2-11). With AUTO V-Source selected, the Model 6517A will
display the ohms measurement range and the V-Source val-
ue. Note that with AUTO V-Source selected, you will not be
able to manually adjust the V-Source or change the V-Source
range when in the ohms function.
NOTE
If AUTO V-source ohms is on, the voltage
limit of the V-source is <400V, an ohms
range that requires 400V is selected, an er-
ror message will occur, and the voltage
source will be turned off. See “Setting a
Voltage Limit” in paragraph 2.9.3 to
change the voltage limit value.
The published specifications for ohms only
apply for the specified AUTO V-Source
test voltages. If using the MANUAL V-
Source setting, you must add the V-Source
errors to the amps measurement range er-
rors to determine the total ohms errors.
With the MANUAL V-Source setting selected, you can set
the V-Source to any value and change the V-Source range
while in the ohms function. The Model 6517A will display
the amps range that is being used for the measurement and
the V-Source value.
WARNING
A hazardous voltage (400V) may automat-
ically be set for the ohms function when
AUTO V-Source is selected. Table 2-11
identifies the ohms ranges that use 400V.
The V-Source setting (AUTO or MANUAL) is selected from
the VSOURCE item of CONFIGURE OHMS menu (see
paragraph 2.7.3 for details).
Ohms Ranges
Each measurement range for the ohms function has a lower
reading limit that is one decade below the selected range.
For example, the 20M
Ω
range has a lower reading limit of
2M
Ω
. The reading ranges for the ohms function are listed in
Table 2-11.
Ohms measurements are performed by sourcing voltage and
measuring current. Thus, ohms ranges are actually current
ranges with ohms displayed.
When the resistance of the DUT (device under test) is too
low for the selected ohms range, the resultant current will ex-
ceed full scale and cause the UNDERFLOW message to be
displayed. This message indicates that the measured resis-
tance is below the lower reading limit of the selected range.
This problem can be resolved by manually selecting the next
lower range or by using AUTO range.
There are three ways you can be assured of optimum range
selection:
• Use AUTO range.
• Select the next lower range when UNDERFLOW is dis-
played.
• With the MANUAL V-Source selected, use the multiple
(NEXT) display that provides the actual measured cur-
rent. This allows you to check that the selected amps
range is the lowest range that can handle the measured
current.
Note that with AUTO range selected, the instrument cannot
go to the 2T
Ω
, 20T
Ω
, or 200T
Ω
ranges since a hazardous
voltage level (400V) may be selected by the instrument. You
must select these ranges manually. To speed up the auto
range process, you can set upper and/or lower range limits.
Eliminating ranges in the auto range search speeds up the
measurement process. See paragraph 2.7.3 (AUTORNG) for
details.
With AUTO V-Source selected, the Model 6517A will dis-
play the ohms measurement range and the V-Source value.
With MANUAL V-Source selected, the amps range for the
measurement and the V-Source value will be displayed.
NOTE
Since AUTO ohms uses the Source V,
Measure I measurement method, a cur-
rent measurement overflow will result in
an UNDERFLOW error. Conversely, a
0A measured current will result in an
OVERFLOW error. To avoid confusion,
use NEXT to show the measured current
on the secondary display.
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