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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