Datron 1061 - Generalprinciples; Option 10 Ac Assembly; Preamplifier and Scaling

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#
3.3
OPTION
1O
AC ASSEMBLY
(Circuit
Drawing
No. 430402)
(For
OPTION
12 see
page
A27)
3.3.1 GeneralPrinciples
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
ma¡n
analog
assembly.
MathematicallY
but
t.e.
The
conversion
technique
is electronic
true
RMS
sensing
as
shown
in the simplified
block
diagram
Fig.
3.22.
The
Datron
RMS
module
can
be
best
considered
as
a
functional
block
consisting of
circuitry
which
accepts
two
inputs,
V
and
V1, computes
V¿/V¡
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.
Vx =Vo
V*
=
V2lV¡
V¿lVf
=
Ve,
but
Vo
=
Vf
12
=
voz
Vo
=
'/Y2
3.3.2
Preamplifier and Scaling
(43O4O2
sheet
1)
Relay RL2
is energised on selection
of AC, directly
connecting the
Hi terminal to the
input
of
the AC assem-
bly. lf DC and AC
are selected
together, the AC assembly
becomes DC
coupled by energising
RL3, causing C57, 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
ís
shown
in
Fig.
3.23.
The
frequency response
is held
flat,
to
within
+'l
%,
by controlling the
gain
defining
component time
constants,
to a
similar
order of
accuracy.
Residual
errors are removed
by
the
freQuenc,¡r
compensation
stage.
(See
section
3.3.4).
The
preamplifier
has a stable
DC
path provided
by
a
dual transistor
pair
O33 and
a fast
AC
path
by dual
F.E.T.'s O32
and
O34. Further
gain
is
provided
by the
following long-tail
pair
cascade
of O20,
O21.422
and O23,
which is loaded
by a
current mirror,
O24. O15 and
016
with bias components
O17 and
O18
form
a conventional
class AB
output
stage. R121
compensates for the
bias
current of
O33, whileRl
12 trims
the
offset
voltage
to
zero.
The unity
gain
frequency compensation
amplifier
consists of a
stable DC
path, provide
by
M11, and a
fast
AC
path
provided
by
O25 to
O29. The bootstrap
circuit
of
019
presents
the varicap
diode, D11,
with
a high imped-
ance,
thus
ensuring that
the
varicap is
not shunted
to
ground.
27
t2
l,
LOW
PASS
FILTER
V1
COMPUTATIONAL
BLOCK
PRECISION
FULL
WAVE
RECTI
FIER
FREOUENCY
MP
GAIN
NETWORK
CONTROL
F|G.3.22
SIMPL¡FIED
DIAGRAM OF
AC CONVERTER
RMS CONVERTER
I
I
L
Vx
Vf
Loo'_,
+
I
I
I
To
A-D
Converter
PREAMPLIFIER,
scALtNc
AND FREoSSM?YENSAT|oN