Common Mode Rejection - Use Of External Guard Connection; Ac Voltage - Fluke 8588A Operator's Manual

8588A/8558A
Operators Manual

Common Mode Rejection - Use of External Guard Connection

When the source presents an unbalanced impedance to the measuring
terminals, and common mode voltages are present, use the GUARD terminal
with External Guard selected. Use  and then  (Ext. Guard) to
activate the GUARD terminal. See Input Terminal Selection (INPUTS).
Regardless of how the INPUT HI and LO terminals are connected, the GUARD
terminal should be referred to the source of common mode voltage, see Figure 4.
This minimizes errors caused by common mode currents in the measuring circuit
by providing a separate common mode current path.

AC Voltage

The AC Voltage function provides 2-wire measurements using the V INPUT HI
and LO terminals. Push  to use the AC Voltage (ACV) function. The
Product makes true-rms ac voltage or ac+dc voltage measurements using a
proprietary sampling method with up to 10 MHz bandwidth. These ranges are
available:
10 mV to 1000 V, where the 10 mV to 100 V ranges provide 121.2 % overrange.
Full scale is 121.2 % of range for these ranges. For example, the 1 V range can
show up to 1.212 V. The 1000 V range can measure up to 1050 V rms.
Input impedance is selectable from 10 MΩ, 1 MΩ, or Auto when dc coupled. Auto
selects the highest impedance available.
20
Icm
Loop
Figure 4. External Guard Connections
R1, C1 = Input impedance
R2, C2 = Input-to-Guard
leakage impedance
R3, C3 = Guard-to-Case
leakage impedance
Vcm1, = Common Mode
Vcm2 Voltages
cm = Common Mode
Current
adj061f.emf