Fluke 8600A - 3-12. A;D Converter

8600A
INPUT SIGNAL
CONDITIONERS
v/f2m
v/£2lo
ma HI
MA
LO
Figure
3-1.
OVERALL
FUNCTIONAL BLOCK
DIAGRAM
across
the known resistance is
representative of the
un-
known current. In the case
of
dc
current inputs the rep-
resentative dc
voltage
is
applied directly
to
the A/D con-
verter, while in the case of ac current
inputs the repre-
sentative ac voltage is directed to the
ac converter first
and then to the
A/D
converter as an
equivalent
dc
voltage.
3-12. A/D Converter
3-13.
The A/D
Converter receives the dc voltage
output
from one
of
the Input Signal
Conditioners and integrates
it for
100
mS.
Figure
3-2
is
an
illustration of the output
of the
integrator.
The
slope of the
integrator output
volt-
age
during the
Integrate Period
is
directly
proportional but
opposite in
polarity
to
the A/D Converter input.
At the
end
of
the integrate period the
signal conditioner
out-
put
is
disconnected from the A/D
input
and a
dc
ref-
erence
voltage
is
connected to the input. The
A/D converter
then
integrates the reference voltage,
of
opposite polarity,
which
results
in
a constant slope returning the integrator
output toward
zero (Read Period). Since the read period
slope is held constant
the
time
required for the A/D
in-
tegrator
output
voltage to return to zero is proportional
to the
instrument
input.
3-14.
The
digital representation
of the
input is ob-
tained by
counting
the number
of cycles
of
a
clock
fre-
quency that
occur from
the
start
of
the
read period
to
the point where the
A/D integrator output
voltage re-
turns to the zero
detect
level.
The A/D
Converter sup-
plies the Control and
Display section with a
compare
signal at the end of the read
period. The
compare signal
stops the
counting of the clock oscillator
pulses so that
the analog
value of the instrument input is
now digitally
represented
by
the number of oscillator
pulses
counted in
the
4'A
digit counter.
3-15. Control and
Display
3-16.
The Control and Display section
provides
the
properly timed
signals that direct the correct
Input Sig-
nal Conditioner
output to the
A/D
Converter
during
the
integrate period.
At the end
of
integrate time
period the
Control and
Display section connects the
appropriate
ref-
erence supply to the A/D
Converter input for the
read
period.
The output of a 1 MHz
oscillator is
used to
main-
tain the
proper timing of the
control signals
as
well
as
provide the base
frequency
from which
the 100
kHz clock
signal
for the read
period is produced.
3-2