Datron 1061 - Option 12 Ac Assembly; Generalprinciples; Preamplifier and Scaling A27

A27
LOW
PASS
FILTER
COMPUTATIONAL
BLOCK
Y2
V1
PRECISION
FULL
WAVE
RECTI
FIER
FREOUENCYGAIN
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.