720A Operation and Maintenance Manual – Rev S – January 25, 2019
Page 16
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the wire is through the hole in the post, you can then re-tighten the plastic by screwing it
clockwise, which secures your electrical connection.
The OP-300 48” gold-plated Kelvin clip lead set and the OP-403 48” Kelvin mini spear
probe set that come standard with the 720A meter both plug directly into the banana
jack terminals on the front panel. The spacing of the jacks only allows horizontal
connection of the dual banana jack test leads to prevent misconnection.
D-5.1 4-wire Kelvin Information
In many resistance measurement applications, the connection contact resistance can
exceed the value of the test resistance by several orders of magnitude. However, the 4
terminal configuration of the AMPTEC 720A meter eliminates measurement errors
typically caused by in series test lead or test lead contact resistance.
Figure 7
below
and the rest of this section provide a detailed description of how this is achieved.
Figure 7
illustrates the 4-wire Kelvin principle which eliminates lead wire length and test
lead connection contact resistances as potential sources of measurement errors. The
internal constant current source inherently overcomes all in series resistance (within
voltage compliance limits) and delivers a precise constant current. Separate DC to DC
isolation circuitry provides independent circuit voltage supplies for both polarities of the
constant current source circuit. The internal high impedance digital voltmeter then
senses the voltage drop across the test resistance.
Fundamentally, the 4-wire Kelvin resistance method avoids measurement errors
induced from the resistance of the test lead/probes and the contact resistance of the
lead/probes to the device under test. It doesn’t matter if the current loop is longer or if
there is some contact resistance with the connection under test, because the constant
current source’s compliance voltage works to maintain constant current flowing in the
loop.
Figure 7
4-wire Kelvin Diagram
