Keithley 2750 User Manual page 115

3-24 Basic DMM Operation
NOTE
Dry circuit ohms measurement considerations
Dry circuit ohms uses a constant current source with voltage monitoring that is used to
clamp the current source voltage. The current source will remain constant as long as the
monitoring voltage is <20mV. When the voltage exceeds 20mV, the current source shunts
current internal to the DMM until 20mV is maintained at the DUT.
The voltage is monitored at the Sense HI terminal. This allows the Model 2750 to accom-
modate an additional 80% of range of path resistance per lead. So when measuring a 1Ω
resistor through a switching module, an additional 0.8Ω per lead of path resistance can be
tolerated. Lead resistance of several 100mΩ is common with a switching module, espe-
cially when using ribbon cable wire.
There is a secondary voltage monitoring circuit on Input HI. This is needed if Sense HI or
Input HI have different voltages. This could occur if measuring 4-wire resistance and
Input HI becomes an open circuit. In this condition, Sense HI would indicate no voltage
and the secondary voltage monitor would clamp the current source to <50mV.
When using dry circuit ohms, the DUT is shunted by 100kΩ and 0.9µF. This allows the
current source to have minimal overshoot voltage under transient conditions. When used
with a switching system, the overshoot is <70mV in 25µsec.
The states (on or off) of dry circuit ohms and offset-compensated ohms are
"remembered" by the Ω4 function after you select a different measurement func-
tion (i.e., DCV). When Ω4 is again selected, the previous states of dry circuit
ohms and offset-compensated ohms will be restored.
The accuracy specifications for dry circuit ohms is for offset-compensated ohms
and line synchronization enabled.
Model 2750 Multimeter/Switch System User's Manual