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 So10b... So14b, 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—
L
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).
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