Keithley 2700, User Manual, Page: 120 / 500

Model 2700 Multimeter/Switch System User’s Manual Basic DMM Operation 3-29
Open test lead detection is illustrated in Figure 3-12 for an
Ω
4 measurement of a 100
Ω
resistor using the 100
Ω
range. For an
Ω
2 measurement, sense circuity is not used. With the
test leads properly connected, as shown in Figure 3-12 A, 1mA is sourced through the
100
Ω
DUT. The 100mV drop across the DUT appears on the Input Hi terminal. Resistance
is then calculated (100mV / 1mA = 100
Ω
) and displayed by the Model 2700.
Open input lead detection
100
Ω
through 1M
Ω
ranges
– For the lower ohms ranges, a hardware detector is used to
detect an open input lead. The hardware detector uses a comparator circuit to monitor the
voltage on the Input Hi terminal. Open circuit voltage on the Input Hi terminal is either
6.6V or 12.8V, depending on the selected measurement range (see resistance
specifications in Appendix A). When an input lead (Hi or Lo) is open, as shown in
Figure 3-12 B, voltage rises to the open-circuit level, which trips the “OVRFLW” message.
10M
Ω
and 100M
Ω
ranges
– For the two highest ohm ranges, open input lead detection is
implemented in software. Open circuit voltage for the 10M
Ω
and 100M
Ω
ranges is 7V. For
the 10M
Ω
range, the “OVRFLW” message trips when the open circuit voltage rises to
approximately 3.5V. For the 100M
Ω
range, the “OVRFLW” message trips when the open
circuit voltage rises to approximately 6.5V.
Open sense lead detection
100
Ω
through 1M
Ω
ranges
– For the
Ω
4 function, the sense leads must be connected to
the DUT. As shown in Figure 3-10 B, the sense leads connect the voltmeter of the
Model 2700 to the DUT. In general, if a test lead for the voltmeter is open, the reading on
the Model 2700 will randomly drift due to the high impedance circuitry of the voltmeter. If
this were allowed to happen for the
Ω
4 function, erroneous ohm readings would be
displayed.
To prevent erroneous ohms readings caused by an open sense lead, the Model 2700
implements software to detect an open sense lead. As shown in
Figure 3-12 A, with all test
leads properly connected, voltage on Sense Hi is at virtually the same potential as Input
Hi, and Sense Lo is at virtually 0V. When a sense lead (Hi or Lo) opens, that terminal will
drift to -15mV and it will trip the “OVRFLW” message.
Figure 3-12 C shows detection for
an open Sense Hi lead.