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CIRCUIT
DESCRIPTIONS
3-19
Ohm
measurement
circuitry
n«i9»
Figure 3.11
Ohm
measurement
circuitry
(principie
of
operation)
The
resistance
R*
to
be measured
is
connected as
a
feedback
resistor of
an
amplrfier
circuit
(opamps
N2761).
The
output voltage
of
this
measuring
amplifier
Is
proportional to resistance R,:
VoLj.
=
<Vret''Rrt''R«
The
different
ranges are
obtained by
selecting
different
values
for resistor
R,
,
This
can
be
done
with
ine
Ohm
range
selection
circuit
(D2750 and surroundmg
resistors),
which
Is
controlled
by
the
Analog
Control
circuitry
(circuit
dagram
A2a,
figure 10.5,
B-OFFSET
lines).
Table
3.5
Ohm
range
selection
circuit
control
lines.
RANGE
Sc15
Sc16
Set
7
Sole
300Q
1
0
1
1
3kii
1
1
0
30k£2
1
0
0
0
300W2
1
1
0
3MO
0
1
0
0
30MI2
0
1
0
Switches
D2751 choose between
the
mV DC
voltage
and
the voltage
from
the
Ohm
circuit.
The
outputs
of
these
sv/itches
are
connected
to
the
L.F.
part of
the
channel
A
attenuator
(circuit
diagram
A2a,
figure 10.5).
Diode
measurement
circuitry
While
In
DIODE
METER
mode,
the
ScopeMeter
uses
the
same
circuitry
as
In
OHM
mode.
WARNING:
The
BLACK
terminal
id
not
connected
to
the
BNC
grounds, while
in
OHM
or
DIODE
METER
mode!
While
in
OHM
er
DIODE
METER
mode,
the
ScopeMeter can
not
be grounded
via
the
BLACK
banana
terminal.
EXTernai
trlggerirjg
The
trigger signal
is
fed
to
the
A-AStC
on
A2a
(figure
1
0.6)
via
resistor
R2750 and
voltage
divider
R2763/R2754
(see
circuit
diagram A2b,
figure 10.6).
It
Is
also possible
to trigger
on
the
signal
made
by
frie
generator.
Then
the
trigger signal
Is
made
out
of
the
Signals
STIMUL
and
G-OUTP
by D2S60.
V2758, and
related
components.
Generator
signal
The
output
of the
generator (see paragraph
3.4,7)
is
sent
to
the
EXT
banana
terminals via
K2751
b.
K2750a and
R2760.
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