HP 3490A - Page 71

Model3490A
Figure
4-26. Dielectric
Absorption
Compensation.
Amplifier
is shorted,
and its output
is only the
offset
present. This
offset voltage
is
inverted
by the
Offset
Amplifier and
subtracted
from the
Sample/Hold
sigral
voltage
at
the input to the
S/H Output Amplifier.
The
purposg
ald action of
the
Offset Amplifier
are also
mentioned in
Paragraph
4-220.
4-227.
Output Amplifier. This unity-gain
inverting
amplifier inverts
the
output from
Circuit
B
(less
the
Section
IV
Offset Amplifier
output)
and applies
this
voltage
to
the
Integrator (A
-
to
-
D
Conversion)
circuit
through
Kl,
as
shown
in Figure
4-31. If
Sample/Hold
operation
is
not
selected,
th9 DC
Input
Amplifier
output
signal
by-passes
the
Sample/Hold
circuits
through
K2.
4-228.
Sample/Hold
Logic Circuits.
4-229.
Fipre
44,7
is
a block
diagram
of the
Sample/
Hold logic
circuits.
The
function
of
each
block
is
discussed
in the
following
paragraphs.
The
sequence
of
the
main
timing
signals
must
be modified
for
Sample/
Hold
measurements
to
allow the
Sample/Hold circuits
to
track the
input
and
reference
signals
at
the proper
times.
The
modified
timing
also
grounds
the
input
to the
DC
Input
Amplifier
during
the
run-up
and run-down
peri-
ods.
The
Sample/Hold
logic circuits
also determine
the
proper
delay
for
Acquire/Hold
measurements
in
accor-
dance
with the
range selected.
The
logic
circuits
are
controlled
by
an Algorithmic
State Machine
(ASM).
A
brief explanation
of
an
ASM
will
be found
in
paragraph
4-90.
4-290.
Timing
and
Trigger
Circuits. The
block
diagam
in
Figure
4-28
details
the circuits
and
sigrals
used
in
the
timing
and trigger
circuits.
4-231.
Sample/Hold
State
Clock. The input
to
the
Sample/Hold
State
Clock comes
from
the 34904
crvs-
tal-controlled
Clock
Oscillator,
A1U3.
This
signal
is
divided
by 16, so
that
the
Sample/Hold
State
Clock
sigral
has
a frequency
of
250 kHz
in instruments
desigred
for
60 Hz
line
operation,
and
208.3
kHz
in
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SIGNALS
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E TEO FOR
THE
3490A TO
EERATE
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IO
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MAIN
LGIC
Flgure
4-27.
Sample/Hold
Logic Block Diagram.
+25
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