A28
C88
and
C89
decouple R160 and R162 except on
the
lOOmV range, when Q33 and Q34
are switched off
to
provide greater open
loop gain. To ensure
stability
at the
higher
feedback levels required
for
the 10V, 100V, and
1000V
ranges;
C73
is
switched
in by
Q32 to
decouple
M22
non-inverting
input, further reducing the open
loop gain.
The
unity gain frequency-compensation
amplifier
includes
a
stable DC path
M20,
and
a
fast
AC path Q28
and Q29. The
capacitance
of
varicap
diode
D14
is deter-
mined by the bias voltage at J1-11. The bootstrap circuit
of
Q17/Q21 ensures that both
halves of
the varicap are
subjected to the same
AC
signal, removing the non-linearity
of the voltage-capacitance characteristic.
A3.
3. 3
RMS
Converter
(430552
Sheets
2 &
3)
The RMS converter takes the scaled
AC
signal
and
converts
it
to an
equivalent
DC
signal suitable for
Analog-
to-Digital
conversion. The
technique
used
is Electronic
True
RMS
Sensing as shown in the simplified block diagram
Fig.
A3. 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 Ml
4,
with
a
weighting
of 2:1. This
forces an
accurately
rectified
full-
wave
current to flow
in RMS
module Mil.
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
current-to-voltage
converter
Ml
0/M
13,
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).
Ml
6
is
the
active element of a
switched 3-pole
Bessel filter.
Ml 5
and Ml 7
switch the time constants,
extending the overall
low-frequency
response down
to
10Hz
(See Fig. A3.
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
-i-5V
power rail, providing
overload protection.
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