Voltech AT360, User Manual

Page: 243 / 470

F
IXTURES
V
OLTECH
AT
I
U
SER
M
ANUAL
P
AGE
5-19
2-wire measurement circuit
The measurement system applies 100.0mV dc to the leads, measures the
current flowing, and divides V by I to obtain R the dc resistance. R = V / I
In this case, the total resistance of the circuit is 2 + 5 + 1000 + 5 + 2 = 1014
m
Ω
.
I = 100 / 1014 = 98.62mA
2-wire R = V / I = 100 / 98.62 = 1.014
Ω
Ω Ω
Ω
2 wire R= 1.014
Ω Ω Ω Ω
This shows that the resistor is +1.4% and out of tolerance when it is really
exactly 1.000
Ω
Notice also, that the voltage applied to the resistor is NOT exactly 100mV
because of the voltage drops across R
L
and R
C
.
In a ‘Kelvin’ or 4-terminal connection, the voltage is applied in exactly the same
way through a pair of ‘Power’ leads, but the voltage is MEASURED using an
extra pair of ‘Sense’ leads. The voltage measuring circuit is designed to have a
very high input impedance (10M
Ω
on the ATi) and so negligible current flows in
the Sense leads, there being a negligible error in the voltage measurement.
R = 1.000
Ω
V = 100mV
R
L
R
L
R
C
R
C
2m
Ω
5m
Ω
5m
Ω