Operator`s manual

connector pins. Violating this limit may result in personal injury and/or permanent damage to the
DMM.
Example: Assume a 30 k resistor is in parallel with two resistors, a 510  and a 220 , which are
connected in series with each other. In a normal resistance measurement, the 510  and 220  would
“swamp” the measurement shunting most of the DMM Ohms source current. By sensing the voltage at the
top of the 30 k, and then applying this same voltage to the junction of the 510  and 220 , there is no
current flow through the shunting path. With this “guarding”, the SMU2064 accurately measures the 30
k resistor.
Figure 4-4. 6-wire guarded in-circuit ohms measurement configuration.
The current compliance of the Guard Force is limited to a maximum of 20 mA and is short circuit
protected. The resistor connected between the low of the 4-wire terminals and the guard point is the
burden resistor, or R
b
. Due to the limited guard source current, this resistor can not be lower than R
bmin
:
R
bmin
= I
o
* R
x
/ 0.02, where I
o
is the ohms source current for the selected range, and R
x
is the resistance
being measured. For example, selecting the 240  range and measuring a 220  resistor imposes a limit
on R
b
of at least 15  or greater. Since the top burden resistor, R
a
, does not have this limit imposed on it,
selecting the measurement polarity, R
a
can become R
b
and vise versa. For cases where this limit is a
problem, simply set the measurement polarity such that R
a
is the higher of the two burden resistors.
To measure values greater than 240 k using the 6-wire guarded method, it is necessary to select the 2-
wire ohms function, and maintain the 6-wire connection as in Figure 4-4 above.
4.3.5 Extended Resistance Measurements (SMU2064)
The Extended Resistance measurement function complements the standard resistance measurement.
While the standard resistance measurement forces a constant current, this function forces a variable
voltage. It is ratiometric in its operation, meaning it is using internal precision resistors to establish
references for the various ranges. The maximum test current is defined by the selected range. A negative
Over-Range is reached when the test current exceeds this limit. Positive Over-Range is declared when the
current is lower than 0.04% of the current limit. The test current is equal to the set test voltage divided by
the measured resistance value.
Ranges are defined in terms of their current limit rather than resistance. The lowest range’s current limit is
set at 24µA, therefore the lowest resistance it can measure with the test voltage programmed to 10V, is
about 400k. With the test voltage set to 0.1V the minimum value is about 4k. The next range’s limit is
39 Signametrics