Operator`s manual

Figure 4-4. 6-wire guarded in-circuit ohms measurement configuration.
r, or R
. Due to
e current for the selected range, and R
x
is the resistance being measured. For example, selecting the 330
range and measuring a 300 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 330 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 (SM2064)
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. 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 2.4µA which corresponds with
4M at 10V and 40k with 0.1V. The highest range current is limited to 240nA, which implies that the lowest
resistance it can measure with 10V source is 40M and the lowest resistance it can measure with 0.1V is 400k. The
highest range practical measurement limit is as high as 10G. The connection topology with optional active
guarding is depicted in Figure 4-7.
Set the test voltage using the DMMSetDCVSource() function. Due to the availability of a higher test voltages than
is available with the normal resistance function, as well as the ratiometric method, this measurement function is best
ng a specific test voltage is to prevent
rning on of semiconductor junctions while testing high value resistors. The combined ability to limit both voltage
and current is significant in test applications where the destruction of a delicate sensor is a concern. The built-in
d +10V. Also consider that with lower voltages, there is increase in
g 10Meg resistor with 0.1V is noisier than using 1V.
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 resisto
b
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 sourc
for high value resistors such as measuring leaky cables. Further benefit in setti
tu
voltage source can be set between -10V an
easurement noise. For instance measurinm
37 Signametrics