Voltech ATi User Manual page 243

F
IXTURES
V = 100mV
R R
2mΩ
L L
R R
5mΩ
5mΩ
C C
R = 1.000Ω
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
mΩ.
I = 100 / 1014 = 98.62mA
2-wire R = V / I = 100 / 98.62 = 1.014 Ω Ω Ω Ω
2 wire R= 1.014Ω 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 and R .
L 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 (10MΩ on the ATi) and so negligible current flows in
the Sense leads, there being a negligible error in the voltage measurement.
V AT U M P 5-19
OLTECH I SER ANUAL AGE