Keysight 34401A, Service Manual

Page: 101 / 165

Front/Rear Selection
Referring to the front/rear schematic on page 9-8, the purpose of this
circuitry is to select either the front terminals or the rear terminals
(via switch
S1
). The output of
S1
is connected to the Function Switching
schematic (see page 9-9). Input protection circuitry designed to protect
the measuring circuits from high-energy transients such as electrostatic
discharge or power-line transients is also shown.
The double fused (
F101
,
F102
) current input is designed to protect
against potential catastrophic damage caused by accidental input
connection across high VA sources beyond the 250 V rating of the rear-
panel current input fuse.
U110
bootstraps out the offset current errors
caused by the leakage current of
CR100
.
CR100
clamps the current input
to protect
R120
and
R121
from excessive overloads.
Caution
Only the front panel or rear panel current input is fused with
F101 and F102 as selected by the front/rear switch S1. The unselected
input is not fused through F101 and F102.
Chapter 5 Theory of Operation
Front/Rear Selection
94