620UK-B Operation and Maintenance Manual – Rev J – January 2020
Page 8
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and “pin G” of the test lead connector (see
diagram on right). The unit’s chassis ground can
also be plugged into an earth ground connection
(for example, at the test site to preclude any static
voltages from building up, if desired). To prevent
damaging the gold pins and sockets, only use
AMPTEC test leads that are model 620UK-B
compatible. When using AMPTEC test leads or
probes, the notched connector end plugs directly
into the single access notched main connector.
Turn the connector collar clockwise until you feel
the click of the mating connector.
D-2.2 Alternate Connections and 4-Wire Kelvin Information
The AMPTEC 620UK-B meter is
compatible with most AMPTEC 630
series accessories. 620UK-B
compatible test lead/probes use a
Trident connector on the front panel as
the main connection for lead/probe
sets. The diagram on the right also
provides insight on how the 620UK-B
operates. The Trident connection
provides the same pin socket arrangement, 4-Wire current high/low, and voltage sense
high/low routed to the end of the leads, and the 4-Wire Kelvin wires then terminate in a
variety of configurations depending upon the accessory. The 620UK-B keyed access
connector also makes it virtually impossible during normal operation to incorrectly connect
lead/probe sets.
The 620UK-B meter measures DC resistance with a 4-Wire Kelvin, or 4 terminal ohms,
technique. 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. Two of the wires, current high and current low,
pass through the bond resistance under test in a current loop as part of a constant
current source of 100 mA. 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 100 mA flowing in the loop. The other 2
wires, voltage high and voltage low, sense the voltage drop across the resistance under
test. The voltage measurement is high impedance. Meaning, virtually no current is
drawn in either voltage sense lead wire.
In conclusion, with 4-Wire Kelvin measurements, long distance test leads do not create
a measurement error as long as you have enough compliance voltage remaining in the
constant current loop. If test resistance is too high for the range, the meter will flash
“1 - - - -” and “>>>OR” to indicate the meter is over range.
