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--
Model
3484A
Section
IV
SECTION
IV
THEORY OF OPERATION
4-1
.
INTRODUCTION.
This
section gives a
brief description
of the
circuits and
methods
employed
in the
Model 3484A
to enable
the
3480A/B to
make
ac
and de voltage
measurements
of
100 mV to 1000 V
full scale, and resistance measure-
ments
of
J
00
n
to
1 O
Mn
full scale.
Figure
4-1
is
a
Simplified
Block
Diagram
of
the
3484A.
A
complete
Block Diagram is
shown in
Figure
7-2.
Tabl
e
4-2 gives
an explanation of
Logic
Symbols
used in
this manual.
4-2.
DC
CIRCUITS.
4-3.
INPUT ATTENUATOR.
In
de
voltage measurements, the
Input
Attenuator is
bypassed on
the
IOOmV, lOOOmV,
and
lOY
ranges.
Attenuation
on
the
100 V
and
1000 V
ranges
is
100
to
I
.
Input
resistance
is
greater than
1010
ohms on
the
three
lower
ranges,
and 10
megohms on
the
two higher
ranges.
The
Input Attenuator is
also
bypassed
in resis-
tance
and
ac voltage
measurements.
Attenuation
is con-
trolled by reed relays which
arc
driven by
signals from
the
Range
and
Function
Assembly
A3.
INPUT
LOW~
GUARO
>-
Al
TRUE RMS
AC
- -4---+- - I
CONVERTER
OPTION
CM3
A7
MA.N
0 TO+
1.S
V
REM
OHMSIOC
flllE
ll
SELECTION
A3
4-4.
FEEDBACK
ATTENUATOR.
The
gain
of
the
DC Amplifier is
adjusted
by varying
the
amount of feedback voltage. Since the
amount
of feed-
back
is inversely
proportional
to
the feedback
resistance,
the correct
resistance
for each range and
function
is
selected
by reed
relays.
These
relays are also controlled
by
signals from the
Range
and
Function Assembly.
Table
4-1
lists the
relays closed for each
de voltage
range, together
with
the
input
attenuation and amplifier
gain. In
both
ac
voltage
and ohms functions,
A2Kl
is
closed, giving the
DC
Amplifier
a
gain
of
10. Amplifier
output
is
0 to
±
15
V
on all
de voltage ranges,
and
0
to + 15
V
on
all ac
voltage
and ohms ranges.
voe
RANGE
100 mV
1000
mV
10
v
100
v
1000
v
Table
4-1.
DC Voltage Range Relays.
RELAYS
CLOSED
A1K1
A 1K 1, A2K1
A 1K
1,
A2K2
A1K2,
A1K3, A2K1
A 1 K2, A 1 K3,
A2K2
VARIABLE
~EE00ACK
INPUT
ATTEN.
1
/
1
1/1
1/1
100/1
100/1
S
f
NS(
AMP.
TOTAL
GAIN
GAI
N
100
100
10
10
1
1
10
0.1
1
O.Q1
rJ.'aoiiiB
- -
I
I
OTO+
15
V
'---+--+-__,f-+-----i
CONVCRTCR
1----------------+-~
OPTION
042
L __
_
MAN
AUTO
REM
A6
RANGE
SELE.CTI
ON
AJ
Figure 4-1. SimpHfied Block
Diagram.
4-1
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