Analog To Digital - Datron 1062 Service Handbook

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FIG. 3.23
SIMPLIFIED
DIAGRAM
OF THE AC PREAMPLIFIER SCALING
J
3.3.3
RMS
Converter (430402
sheet
2)
The
RMS
converter takes the scaled
AC
signal
from
the preamplifier
and
converts
it
to an equivalent
DC
signal
suitable for Analog-to-Digital conversion.
The
conversion
technique
is
electronic true
RMS
sensing as
shown
in
the
simplified
block diagram
Fig.
3.25.
M8
and
M9
form
a
summing
type,
full
wave
recti-
fier.
The
output
of
M8,
a precision
half-wave
rectifier
inverter,
is
summed
with the non-inverted
signal
with
a
weighting
of 2
;
1
at
the input of M9. This forces
a
full-
wave
rectified
current to
flow
in
RMS
module M6.
Potent-
iometer
R50
balances the
rectifier
to provide the
same
output for non-inverted or inverted
asymmetric waveforms.
The
output current
from
the
RMS
module
passes
into filter-buffer
Ml
and
is
converted
to a
nominal 5
volts
for
a full
range
signal.
Q1 and
Q2
switch
in
additional
capacitors
when FILTER
is
selected, to
operate
down
to
45Hz.
M2
is
a
voltage to current converter providing
a
feedback current to the
RMS
module proportional
to
the
output
voltage.
R90
is
the zero
adjustment
for the half
wave
rectifier
M8
and
R35
is
the high
crest
factor gain
adjustment.
R75
is
adjusted for
optimum
linearity.
The output
of
M1
(TP2)
is
fed to
a resistor
chain
R1
-
R7,
to provide an output of 3.14 volts
by
the selection
of resistors
R2
-
R5.
Q3
is
turned
on
when
AC
is
selected
and
switches the
output
of the
AC
converter into the
Analog-to-Digital
Converter (Drawing No,
430328
sheets 3
and
4).