Measuring Dc Voltage; Making Input Connections - Fluke 8508A User Manual

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
source as shown in Figure 3-2.
Common Mode Rejection - Use of External Guard Connection
Use the
Guard
presents an unbalanced impedance to the measuring terminals, and common mode
voltages are present. Regardless of how the
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.
Case
Guard
C1 R1
C2
R2
C3 R3
HI
LO
(Internal
Guard)
GUARD
Figure 3-1. Simple Lead Connections
Hi
Lo
Guard
Figure 3-2. Twisted-Pair Cable Connections
terminal with
External Guard
Hi
Lo
Icm
Gu
Loop
Vcm
2
Figure 3-3. External Guard Connections
Using the Measurement Functions
Hi
Lo
Gu
Vcm
selected when the source to be measured
and terminals are connected, the
Hi Lo
R1, C1 = Input impedance
R2, C2 = Input-to-Guard
leakage impedance
R3, C3 = Guard-to-Case
leakage impedance
Vcm1, = Common Mode
Vcm2 Voltages
I
cm = Common Mode
Vcm
1
Current
Making Measurements
terminal at the
Lo
adj059f.eps
adj060f.eps
Guard
adj061f.eps
3
3-7