Fluke 8508A - Measuring DC Voltage; Making Input Connections

Icm
Loop
3
Making Measurements
Using the Measurement Functions
Measuring DC Voltage
Making Input Connections
Simple Lead Connection
For the majority of applications the simple lead connection without external guard will be
adequate. See Figure 3-1.The disadvantage of this simple arrangement is that the
connecting leads can form a loop. If a stray alternating magnetic field (e.g., from the line
transformer of a neighboring instrument) passes through the loop, it will behave as a
single-turn secondary winding inducing unwanted AC voltages into the measuring
circuit. Use of a twisted-pair will reduce the loop area and adjacent twists will cancel any
induced voltages. If problems with stray pick-up are encountered, it is recommended that
a shielded twisted-pair cable be used with the screen connected to the
Lo terminal at the
source as shown in Figure 3-2.
HI
LO
(Internal
Guard)
GUARD
adj059f.eps
Figure 3-1. Simple Lead Connections
Hi
Lo
Guard
Vcm
Gu
Lo
Hi
adj060f.eps
Figure 3-2. Twisted-Pair Cable Connections
Common Mode Rejection - Use of External Guard Connection
Use the
Guard terminal with External Guard selected when the source to be measured
presents an unbalanced impedance to the measuring terminals, and common mode
voltages are present. Regardless of how the
Hi and Lo terminals are connected, the Guard
terminal should be referred to the source of common mode voltage, as shown in Figure
3-3. This ensures that errors caused by common mode currents in the measuring circuit
are minimized by providing a separate common mode current path.
Lo
Gu
Vcm
2
Vcm
1
C1 R1
C2
R2
C3 R3
Case
Guard
Hi
R1, C1 = Input impedance
R2, C2 = Input-to-Guard
leakage impedance
R3, C3 = Guard-to-Case
leakage impedance
Vcm1, = Common Mode
Vcm2 Voltages
Icm = Common Mode
Current
Icm
Loop
adj061f.eps
Figure 3-3. External Guard Connections
3-7