Rms Converter - Datron 1061 Calibration And Servicing Handbook

--
A28
CBB and C89
decouole
R160 and
R162
except
on
the
100mV
range,
when oäg
ano
o34
are
switched
off
to
provide
greater
open loop
gain.
To
ensure
stability at
the
higher feedback
levels
requ-ired
for the
10V, 100V,
and
1000v
ranges;
c73
is switàhed
in
by
o32 to
decouple M22
non-inverting input,
further
reducing
the open
loop
gain.
The unity
gain
f
requency-compensation
amplif ier
includes
a
stable DC
path M20, and a fast AC path
O28
and O29. The
capacitance of
varicap diode
Dl4
is
deter.
mined
by the
bias voltage
at
J1-1,|.
The
bootstrap
circuit
of
A17/A21
ensures
that both
halves
of
the
varicap are
subjected
to
the
same AC
signal, removing
the non-linearity
of the
voltage-capacitance characteristic.
RL4
R142
Fig. A3.23
SIMPLIFIED DIAGRAM
OF THE AC PREAMPLIFIER SCALING
R121
R122
To
Frequency
R146
Rt 43 RL5
1KV
10v
c77
Compensation
Section
BLl
10OmV
RL6
100v
RL3
RL3
<Þ
Lo
\--<2
Hi
c--{
RL2
o23
100mV
M21,M22
031-036
R167
to
R170
43.3.3
RMS Converter
(430552
Sheets 2
&
3l
log
v
vx
Vt
Fig.43.25 BLOCK
DIAGRAM
OF
RMS CONVERSION
TECHNIOUE
The RMS
converter takes
the
scaled AC
signal
and
converts
it
to
an
equívalent DC
signal suitable for
Analog-
to-Digital
conversion.
The technique used
is
Electronic
true
Rlvls Sensing
as
shown
in the simplified block
diagram
Fis.43.25'
M13 and M14
form a
summing full'wave rectifier'
The
output of precision half-wave
rectifier
M13
is
summed
with the non-inverted signal
at the input
of
M14,
with
a
weighting
of
2:1
.
This
forces an accurately
rectified
full'
wave
current
to flow in
RMS
module
M11.
Potentiometer
R62 adjusts the rectifier symmetry
to
provide the
same
output
for
signals
of
either polarity.
The output current
from the
RMS module
drives the
low
pass burrent-to-voltage
converter M10/M13,
which
generates a nominal
0.5 Volts
for. a
full
range
signal.
(Note
that M10,
M9 and
M4
are
chopper-stabilized amplifiers
to
handle
the low signal voltages).
M16
is the
active element
of
a
switched
3-pole
Bessel
filter.
M15 and M17 switch the time
constants,
extending
the
overall
low
frequency
response
down to
10Hz (See
Fig.
43.24), when
'Filter'
is selected.
The high
impedance
output
from the
3-pole
filter
is
buffered
by
M9/M2, and
the other half of M2 provides
a
bootstrap
for
M9
input.
D26 and
D16
prevent the
voltage
on
TL
A
from
exceeding
the +5V power rail,
providing
overload protection.