Voltech AT360, User Manual

Page: 242 / 470

F
IXTURES
P
AGE
5-18 V
OLTECH
AT
I
U
SER
M
ANUAL
5.5 K
ELVIN
C
ONNECTIONS
‘Kelvin’ or 4–terminal connections are commonly used in precision
measurement systems to obtain accurate measurements of impedance. Each
of the ATi’s 20 measurement nodes (including the BNC connectors) consists of
two terminals (Power and Sense) to allow complete 4-terminal testing.
Why use Kelvin connections?
Consider measuring a perfect 1.000 ohm resistor to check that it is within its
1% tolerance at dc.
In a standard 2 terminal measurement, there are TWO sources of error:
The resistance of the connecting leads.
The resistance of the contact between the connection and the lead of the
resistor.
R
L
, the resistance of the leads, is fixed and can be compensated for. Let’s
assume it is 2m
Ω
for good quality leads.
R
C
, the connection or contact resistance is a variable and cannot be
compensated for. R
C
will vary with the cleanliness of the leads and connector,
the pressure applied and the contact surface area--all of which can be difficult
to control. Let’s assume R
C
varies from a few m
Ω
to tens of m
Ω
, but is typically
5 m
Ω
.