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conditioner
input, the External
Reference
HI
input
and
the
External Reference
LO
input
are
multiplexed
to the
A/D
Converter.
Samples
are
taken of
each
input
and
arithmet-
ically
manipulated
by
the
Controller
to arrive at a reading.
Outputs from
die
optional
AC
converter
modules
are
applied
to
the
Active
Filter
module, bypassing
the
DC
Signid
Conditioner.
3-26.
Fast
A/D
Converter
3-27.
The
Fast
A/D
Converter
employs
Fluke's
patented
recirculating
remainder
(R^)
technique
for
con-
verting
a
dc
input
signal into a
binary,
bit-serial
data stream,
The R^
technique
has
been modified
for
microprocessor
control.
Obtaining
a
sample
is
a
five-step
process.
Each
step consists
of
a
decision period
of
five
decisions
and
a
subtraction period. Set-up
of
the converter, decisions,
and
reset are initialized
by
the
Controller addressing the
A/D
Converter. Figure
3-8
is
a
block
diagram of
the
A/D
Converter.
3-28.
During
the
first
step,
the
input
signal
is
applied
to
the
Summing
Node. The
polarity
of
the
input
is
detected,
and
the resulting
bit
of information
is
transmitted
to
the
Controller.
On
the
basis
of
the
returned
polarity, the
A/D
module
selects
which
reference polarity
is
required;
positive
for negative inputs, negative
for positive inputs.
The
first,
of
five
precision currents
is
switched
into the
Summing
Node
and
a polarity
bit
returned.
If
the polarity
is
changed,
the
first
current
is
switched
off.
If
not,
it is
left
on. I'hen
the
next current
is
switched
into
the
Summing Node
and
another
polarity bit
returned.
Another
decision
is
made
and
so
on
until
all
five
currents
have been switched
into the
Summing Node
and
five
decisions
liave
been made.
This
completes
the
first
decision period.
The
five
precision
currents are related
by
powers
of two.
The
fifth
current has
a
resolution
of
thirty-two.
3-29.
Following
the
decision
period
is
a
subtraction
period.
Feedback through
the
Remainder
Storage
nulls
whatever remainder
was
left
after
the
five
currents
have
been switched
into
the
summing
node.
The
remainder
is
amplified
by
16
in
the
Remainder
Amp
and
is
stored
on
a
capacitor
in
Remainder
Storage. This
completes
the
first
step.
The
input
is
now
switched out of
the
Summing
Node
and
the amplified
remainder switched
in
for
the
next
step.
There
are
two
remainder
channels
in
Remainder
Storage
and
they
are alternated
in
the four
subsequent
steps.
Since the
fifth
current has
a
resolution
of thirty-two
and
the
Remainder
Amp
has
a gain
of
16,
the
first
bit
of
a
step
has the
same
significance
as
the
last
bit
of
the
pre-
vious
step.
3-30,
Of
the
five
steps required to
complete
a
sample,
the
first
uses the
input
signal for
decision
and
subtraction
periods.
The
four
subsequent
steps alternate
remainder
channels to use the amplified
remainder
of
the
preceding
step
for
decision
and
subtraction
periods. Polarity
bits
PRECISION
POLARITY
VOLTAGE
SWITCH
REFERENCE
1C 1-4
IDO-3
ID7
ACK
^
RT5
>
FROM
FILTER
MODULE
Figure
3-8.
A/D
Converter Block
Diagram
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