Table of Contents
Advertisement
THEORY OF OPERATION
INSTRUCTION MANUAL
Digital
MultImeter
Moclet, 112A.
173A
c.
Basic
Operation
of
the
A/D Converter.
The four
basic
operating
modes of
the
A/D
r
converter
are
described
as follows:
I)
AZ-l.
The buffer
input
is
connected
to
reference
low
(signal
LO in V and A
functions),
or
summing
junction
of
the
ohms amplifier
in R function.
The switching
rate
for
zero
input
is
forced
into
the
current
switch
drivers.
The comparator
output
applies
a dc correction
voltage
to auto-zero
storage.
This
voltage
is
stored
and applied
to
the
active
integrator
forming
a negative
feedback
loop
which
quickly
stabilizes.
2)
Auto-Calibrate.
In this
mode,
the
buffer
input
is connected
to
the
reference
(3.3444
Vdc in V and A functions,
or
the
ohms reference
in 0).
Auto-zero
storage
continues
to apply
AZ-l
correction
voltage
to
the
active
integrator.
The current
switch
drivers
are
forced
to switch
at
the
appropriate
rate
for
reference
input,
and
a dc correction
voltage
from
the
comparator
is applied
to auto-Cal
storage.
This
correction
voltage
is
stored
and continually
applied
to
the
reference
current
source
which
changes
the
reference
current
removed
from
the
active
integrator,
completing
another
negative
feedback
loop.
3)
AZ-2.
The same procedure
as AZ-l
is
followed,
with
the
exception
that
the
buffer
is
connected
to
input
LO (zero
circuit
for
dc V and A functions,
ac/dc
converter
for
ac V and A, or
input
HI for
n).
4)
Signal
Integrate.
The buffer
is
connected
to signal
input
(dc attenuator
output
for
dc V and A, ac/dc
converter
output
in ac V and A, or
input
LO in 0).
AZ-2
and ACAL
voltages
are
still
applied,
which
provide
compensation
for
zero
and gain
errors.
The
A/D conversion
is now made,
and displayed
during
the
next
conversion
cycle.
d.
A/D Buffer.
The buffer
(shown
in Simplified
Schematic,
Figure
5-13)
performs
two
,
functions.
It
has four
multiplexing
JFET switches
(Sl
through
S4)
that
connect
the
necessary
inputs
to
the
A/D converter
to provide
.the
four
basic
modes of
operation.
The
driving
signals
(Zl,
Z2,
R and NS) for
the
multiplexing
switches
come from
LSI circuit
ugo1.
The second
function
of
the
buffer
is
to provide
two gains
(xl
and x10)
which
give
3V and 0.3V
ranges,
respectively.
Buffer
gain
during
AZ-l
and ACAL is xl
for
V, A and
every
other
R range.
Buffer
gain
during
AZ-Z
is
the
same as signal
integrate,
which
is
xl0
on 300mV,
3OV, LDfi,
300mA on
l72A
and all
current
ranges
of
l73A.
A gain
of
xl
is
used
elsewhere.
LSI chip
U901 controls
S5 and 56 (Q7O2 and Q7Ol)
via
the
GIO line.
INPUTS
1
A/D CAL o
'3@Q7ll
SIG.
INT.
AZ-l
AZ-2
S2
0
Q708
OUTPUT
-
-
Yk
Ik
*
I
FIGURE 5-13.
Simplified
Schematic
of
A/D Buffer.
5-16
Advertisement
Table of Contents
