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5.3
THEORY
OF OPERATION BY
BLOCK
5.3.1
Channel
A/B
Block
5.3.1. 1
Functional Description
5.
3.1.1.
1
Channel
A/B
block processes
the signals applied
at the
respective
A/B
inputs to
produce
differential pairs of
signals
which
are fed
to
the
measurement
block.
A
block
diagram
is
shown
in
Figure
5.2.
5. 3.
1.1. 2
Each channel
includes relay-contr oiled circuits
which
allow selection of
50f2
/1MH
input
impedance,
AC/DC
coupling,
and xl/xlO
attenuation.
The
COM(mon)
A
configuration
(Channel
B
signal
disconnected and
Channel
A
signal
connected
to
both
amplifiers
in
parallel)
can be
selected.
5. 3.
1.1.
3
The channel
amplifiers feature
separate
high
frequency and low frequency
paths.
The
crossover
frequency
is
nominally
5
kHz.
Signal
filtering
can be introduced,
in
Channel
A
only,
by disconnecting
the
high-frequency
amplifier
path and increasing the
bandwidth
of
the
low
frequency path
to
50
kHz
nominal.
The
signals
from
the high
and
low frequency paths are
combined and
drive
a
Schmitt
trigger
output
stage.
5.3.1. 1.4
The
trigger levels
for
the
two
channels
are
derived independently
in
the
digital-to-analog
converter
(DAC)
using
data
supplied
from
the
microprocessor.
5.3.
1.1.5
Control
signals
for
the
system
relays
are
supplied
from
the
microprocessor.
Figure
5.2
-
Channel
A/B
Block
Diagram
5-3
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