2.4µA which corresponds with 4M
Ω
at 10V and 40k
Ω
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
Ω
and the lowest
resistance it can measure with 0.1V is 400k
Ω
. The highest range practical measurement limit is as high as
10G
Ω
. The connection topology with optional active guarding is depicted in Figure 4-5.
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 for high value resistors such as measuring leaky cables. Further benefit in
setting a specific test voltage is to prevent turning 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 voltage source can be set between -10V and
+10V. Also consider that with lower voltages, there is increase in measurement noise. For instance
measuring 10Meg resistor with 0.1V is noisier than using 1V.
Additional applications include testing high value resistive elements such as cables, transformers, and
other leaky objects such as printed circuit boards
,
connectors and semiconductors.
Range
Range Code
Measurement range
Resolution
Voltage Range
Current Limit
400k
0
1k
to 100M
10
±0.02V to ±10.0V
25µA
4M
1
10k
to 1G
100
±0.02V to ±10.0V
2.5µA
40M
2
100k
to 10G
1k
±0.02V to ±10.0V
250nA
Figure 4-5. Guarding improves accuracy when measuring high value resistors using the Extended
Resistance measurement method.
4.3.6 Effects of Thermo-Voltaic Offset
Signametrics
40
Resistance measurements are sensitive to Thermo-Voltaic (Thermal EMF) errors. These error voltages
can be caused by poor test leads, relay contacts and other elements in the measurement path. They affect
all measurement methods, including 2-Wire, 4-Wire, 6-Wire and 3-Wire (guarded 2-Wire ohms). To
quantify this error, consider a system in which signals are routed to the DMM via a relay multiplexing
system. Many vendors of switching products do not provide Thermal EMF specification, and it is not
uncommon to find switches having more than 100
V. With several relay contacts in the path, the error
compounds, which could be much worst in matrix type switches. This error can be measured using the
SMU2060 240mV DC range. To do this, close a channel which is shorted on the application side. Wait
for about 2 minutes, than measure the voltage on the DMM side of the multiplexer. Make sure to short the
DMM leads and set ‘relative’ to clear the DMM offset prior to the measurement. To calculate worst-case
error, count all relay contacts, which are in series with the measurement (
V,
+, V,
-
terminals in 2-
Wire, and
I+, I-
terminals in 4-Wire mode). Multiply this count by the Thermal EMF voltage. The
SMU2064 can source ten times the test current of most DMMs, resulting in ten fold reduction in error. At 1
V the
