Keithley 2701 User Manual Download

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3-30

Basic DMM Operation

Model 2701 User’s Manual

Effects of open test leads on ohms readings

The Model 2701 will display readings up to 120% of range. Readings above 120% of
range will cause the “OVRFLW” message to be displayed. For example, on the 100

range readings up to 120

will be displayed. Above 120

, the “OVRFLW” message is

displayed.

The Model 2701 will also display the “OVRFLW” message if a test lead is open during an
ohms measurement. A hardware (H/W) detection circuit or software (S/W) detection is
used to detect an open input lead. For an

4 measurement, a software (S/W) detection rou-

tine is used to detect an open sense lead.

Open test lead detection is illustrated in Figure 3-12 for an

4 measurement of a 100

resistor using the 100

range. For an

2 measurement, sense circuity is not used. With the

test leads properly connected, as shown in Figure 3-12A, 1mA is sourced through the
100

DUT. The 100mV drop across the DUT appears on the Input Hi terminal. Resistance

is then calculated (100mV / 1mA = 100

) and displayed by the Model 2701.

Open input lead detection

100

through 1M

ranges

– For the lower ohms ranges, a hardware detector is used to

detect an ope n 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 3-12B, voltage rises to the open-circuit level, which trips the “OVRFLW” message.

10M

and 100M

ranges – For the two highest ohms ranges, open input lead detection is

implemented in software. Open circuit voltage for the 10M

and 100M

ranges is 7V. For

the 10M

range, the “OVRFLW” message trips when the open circuit voltage rises to

approximately 3.5V. For the 100M

range, the “OVRFLW” message trips when the open

circuit voltage rises to approximately 6.5V.

Open sense lead detection

100

through 1M

ranges – For the

4 function, the sense leads must be connected to the

DUT. As shown in Figure 3-10B, the sense leads connect the voltmeter of the Model 2701
to the DUT. In general, if a test lead for the voltmeter is open, the reading on the Model
2701 will randomly drift due to the high impedance circuitry of the voltmeter. If this were
allowed to happen for the

4 function, erroneous ohms readings would be displayed.

To prevent erroneous ohms readings caused by an open sense lead, the Model 2701
implements software to detect an open sense lead. As shown in Figure 3-12A, 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 3-12C shows detection for
an open Sense Hi lead.

Keithley 2701, User Manual

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