Fluke 93 Service Manual page 51

CIRCUIT
DESCRIPTIONS 3-17
The
signal Sr4i>
operates the
switch,
which
is
used
to
ground
the
L
F.
part of the input signal during
offset calibration.
This
is
done
automatically
to
prevent
drift.
The
offset
DAC
oircuitry
(see
figure 3.7)
provides the
offset
voltage
for
operational amplifier
N2201
The
offset
compensation
is
done
automatically
by
means
of
the
signals
So
10b...
So
14b,
coming
from
the
D-ASIC.
L.R Calibration
S95a S94s
R2229
-TMHI
—
^
^
1
W
0
1
R2233
R2234 R223R
M
3k83
hJ
^
7kS
M
7B0EI—
tv
R2231
lA
R2232
/
Sg7a f Sg6a
R2237
215k
LF and
HP
attenuation
ok
increase
LF
stgnal part
decrease
LF
signal part
Figure 3.5
Automatic ad/ustmenf
of
the
LF.
attenuation
Rne
adjustment
of the
L.F.
path
attenuation
Is
completed
dunng
calibration of the
H.F.
path
attenuation.
This
is
done
by
means
of
a simple
4-bits
D>to-A converter, consisting
of resistors
R222Q,
R2231
,
R2232. R2233, R2234. R2236.
and
switches
D2202.
These
switches are operated by
sign^s
Sg4a, SgSa,
8960,
and Sg7a, see
figure 3.6.
Resistors
R2229, R2231 end R2232
divide the output
signal of
the attenuator
section.
Resistors
R2233, R2234.
and R2236
increase the input resistance
of
the Inverting amplifier of the regulating
loop.
Feedback
loop
The
output signal
of
the
impedance
converter
is
fed
back
to
the Input of operations!
amplifier
N2201
with the signal
com^g
from the LF.
calibration
section
(via
R2237) and a
DC
position
voltage
(5V
via
R2246), proportional with the
MOVEment
of
the
trace (via R2248). Transistor
V2210
is
used
to
enlarge the dynamic
range:
when
D-POSCHA
is
active,
R2270
is
incorporated
in
the
circuitry.
The
feedback loop operates
as
follows.
If,
for
example,
the output signal of the
L.F.
path
is
too small,
the correction
amplifier
N2201
will
drive
V2207
via
V2208.
In this
way
the amplitude
of
the
L.F.
path
and
the
position
voltage are increased (compensation).
Input
pfoteethn
The
input protection
safeguards
the
ScopeUeter
against overvoltage.
The
Input protection
circuit
consists
Ot
C2203 and V2206A/2204 (damp
HF
attenuator)
and R2219
and V2212A^221 3 (clamp
LF
attenuator).