Fluke 8600A - 3-20. CIRCUIT DESCRIPTION

8600A
INTEGRATE
PERIOD
SLOPE
is
proportional
to
the
instrument
input
signal
A/D
INTEGRATOR
OUTPUT
VOLTAGE
CLOCK
OSCILLATOR
SIGNAL
100ms
READ
PERIOD
TIME is
proportional
to the
instrument
input
signal
ZERO
LEVEL
Figure
3-2.
TIMING
DIAGRAM
FOR
DUAL-SLOPE A/D
CONVERSION
3-17.
The
clock
oscillator
pulse
count,
accumulated
during
the read
period,
is
applied
to
the
LED
display
to
produce
the digital
readout of
the
instrument
input sig-
nal.
The range
information
from
the selected
range
switch
positions the
decimal
point
and
illuminates
the
proper
display
annunciator.
3-18.
Data
Output
Unit (DOU)
3-19.
The DOU
provides
the
display data
as a rear
panel
card-edge
output.
The bed
information
presented
to
the
instrument
display
(polarity,
overload,
range
code,
and
digit)
is also
applied
to
the DOU
input.
A
busy
flag
is
provided
so
that
the output
data may
not
be used
while
the 8600A
is feeding
a new input
measurement
to
the DOU.
A register
pulse train
plus one
of the
data
strobe
signals
time
the data
transfer
to insure
that all
the
data supplied
to
the
DOU output
is stable
and not
taken
during
transition
periods.
For
detailed information
refer
to Section
6.
3-20.
CIRCUIT
DESCRIPTION
3-21
. The
following
circuit
descriptions
are keyed
to the
functional
blocks
defined
in the overall
functional
block
diagram
of
Figure
3-1
. Corresponding
functional
blocks are
defined
in
more
detail
using either
simplified
block
dia-
grams
or
by referring
to the
schematics.
Schematics for
the
8600
A
circuits are
provided
in
Section
8.
3-22.
Input
Signal
Conditioners
3-23.
INPUT
DIVIDER
3-24.
The Input
Divider
comprises
a
series
connected
resistor
network
(R3,
R4, R5,
R6, R7,
and R8) totaling
approximately
ten
megohms.
This network
is tapped
at
three
points
to provided
division
ratios
of 10:
1
,
100:
1 ,
and
1000:
1 .
Relays
K2, K3, and
K4 each
select
one
of
the
division
ratios; as directed
by the range
switches.
3-25.
OHMS
CONVERTER
3-26
The
Ohms
Converter
produces a known
amount of
current that, when
applied to
an unknown resistance
(Rx)
connected across the VJ2
terminals,
will develop a voltage
(Vx)
proportional to the
value
of Rx. Producing the known
amount
of current
is accomplished
with a current follower,
Ul, two feedback
loops, and
a current
source (U3 and
Q3).
One
feedback loop
is
from
the Vf2
HI terminal to the non-
inverting
input of Ul (Pin
3).
The voltage
of this loop is
applied to the A/D Converter.
The other feedback loop is
from the output
of Ul through
R20
and R90 to the inver-
ting
input
of Ul
(Pin
2).
The current
source
(U3 and
Q3)
1/77
3-3