Option 12 Ac Assembly; Generalprinciples; Preamplifier And Scaling A27 - Datron 1061 Calibration And Servicing Handbook

A27
LOW
PASS
FILTER
COMPUTATIONAL
BLOCK
Y2
V1
PRECISION
FULL WAVE
RECTI
FIER
FREOUENCY
GAIN
NETWORK
CONTROL
FIG.
A3.22 SIMPLIFIED DIAGRAM OF AC CONVERTER
RMS CONVERTER
I
I
L
Vf
H¡
Loo-
ü
I
I
I
To
A-D
Converler
PREAMPLIFIER.scALtNG
AND
FREOUENcyNsATtoN
43.3
OPTIOTT
Ð
AC
ASSEMBLY
(Circuit
Drawing No.
430552)
(For
OPTION
10
see
page 27)
43.3.1
General Princíples
The preamplifier
buffers and
ranges
the
signal in
order
to
present
0.9 volts
full
range
to
the AC
to
DC
converter
section.
Once converted
to
an
equivalent
DC signal,
it
is
applied
to
the
analog
to
digital converter on the
main
analog assemblY.
The
conversion
technique
is
electronic
true
RMS
sensing
as shown
in
the
simplified block diagram
Fig.
A3.22.
The Datron
RMS
module can
be
best considered
as
functional block
consisting
of circuitry
which
accepts
two
inputs,
V
and V1, computes
V2/Vf
and
has an
output of
V¡
which
is
then
filtered
so
that
all
the
AC
components
are
removed.
The
output of the block
is
fed
back
to
V1,
thus
closing
the loop
around the
whole circuitry.
MathematicallY:
W
=
Vo
but
Y
*=
Y2¡Y,
l2/v¡
=
Vs,
but Vo
=
Vf
Ç2
=yo2
:
vo=J v2
43.3.2 Preamplifier
and Scaling
(430552
sheets 1
&
2)
When the
AC option is
selected,
the
AC
preamplifier
is
connected
in
parallel
with the
1000
Volt
range
of
the
DC
isolator.
The resultant
impedance presented
at the
input
terminals is a resistance
of
1MS),
shunted
by
150pF.
Relay
RL2 is
energized
on selection
of AC,
directly
connecting the
Hiterminal
to
the
input of the AC
assembly.
lf
DC and AC are
selected
together,
the AC
assembly
becomes
DC
coupled
by
energizing
RL3,
causing
C77,
the AC coupling capacitor,
to
be by-passed'
The signal
is
then
fed
to
the
switched gain
inverting
preamplifier
whose
full
range
output
is 0.9 volts
r.m.s.
A
simplified
diagram
of
this
arrangement
is
shown
in
Fig.
43.23. The
frequency response
is
held flat,
to
within
*.1o/o,
by
controlling
the gain defining component
time
constants,
io
a
similar order
of
accuracy.
Residual
errors
are
removed by the frequency compensation
stage.
(See
section
3.3.4).
The main amplifier M22
responds
to
signals from
DC
to
above
lMHz. lts input buffer 036
reduces
bias
current errors.
A
chopper-stabilized
amplifier M21
nulls
the offset
of
036. Filter
components
R123 and
C90
eliminate
the
effects
of current 'kickback' from M21 to
the main
signal
path. M22
output
(Test link TLK)
is fed
directly to
the
unity
gain
frequency compensation
stage.
t.e.