4-Wire Resistance Tests - Keithley 7074 Instruction Manual

SECTION 3
Applications
DMM zero feature, and leave zero enabled while making
measuements.
3.2.2
4-Wire Resistance Tests
More precise measurements over a wider range of sys-
tem and DUT conditions can be obtained by using the
4-wire measurement scheme shown in Figure 3-2. Here,
separatesenseleadsfrom theModell96 DMMarerouted
through the multiplexer to the resistor under test. The ex-
tra set of sense leads minimizes the effects of voltage
drops across the test leads. Note, however, that an extra
two poles of switching are required for each resistor to be
tested. For that reason, only 48 resistors per card can be
tested using this configuration.
Although the 4-wire connection scheme does minimize
problems caused by voltage drops, there is one other po-
tentially troublesomeareaassociated with low resistance
measurements: thermal EMFs caused by the relay con-
tacts. In order to compensate for thermal EMFs, the off-
set-compensated ohms feature of the Model 196 DMM
should be used. To use this feature, short the HI and LO
terminals of one of the bank inputs, (at the card BANK
connector) then close the relay. Enable zero on the Model
196, then select offset-compensated ohms.
For even lower resistance measurements, use the Model
580 Micro-ohmmeter. This instrument can make resis-
tance measurements with lo&? resolution.
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Figure 3-2. 4.Wire Resistance Testing
3-2