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HP 3457A - Ohms Precharge Amplifier Auto Calibration; Extended Ohms Range Auto Calibration

HP 3457A
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SUL—SIL_
@
10
sv_w
3v
Rance
al
TO
0.5V
ON
300m
RANGE
eal
Sn
ada
aaa
AC
INPUT
c
5
Pw
t0
mo
|
n
I
commcer_
|
'
FROM
7
KH
100
Low
wicroprocessor
___|
tt
&
xX
1924
N=
0
To
1023
34520
F.12.4
Figure
8-4.
Attenuator
Compensation
There
are
two
separate
compensated
input
attenuators
used
in
the
AC
section.
One
is
used
for
the
30mV
and
300mV
ranges
and
the
other
is
used
for
the
3V,
30V
and
300V
ranges.
The
attenuators
are
each
automatically
compensated
by
the
ACAL
function.
During
ACAL,
a
0.5
volt
and
a
5.0
volt
pulse
signal
are
generated
by
the
AC
Autocal
circuit
A2U402
located
on
the
A2
Input
Signal
Conditioning
assembly.
These
values
are
determined
by
resistive
divider
A2R301
and
R302.
Switch
A2U402
is
used
to
select
the
desired
voltage
level.
These
pulse
signals
are
applied
to
the
input
attenuators
of
the
AC
board
(A3)
and
measurements
are
made
to
determine
the
flatness
of
the
signal.
The
first
measurement
is
made
shortly
after
the
falling
edge
of
the
pulse
signal
and
a
second
is
made
at
a
later
time.
The
first
measurement
will
determine
the
amount
of
the
overshoot
or
undershoot
(see
Tl
on
Figure
8-4).
The
second
measurement
(made
at
T2
on
Figure
8-4)
will
be
the
reference
level
to
which
the
first
measurement
is
adjusted.
These
measure-
ments
are
then
nulled
via
the
circuit
composed
of
DAC
U311
and
the
capacitors
C304
or
C308.
These
capacitors
shunt
their
respective
attenuators.
Their
values
are
effectively
varied
from
0
to
|
times
their
value
in
1024
steps
by
the
control
lines
to
DAC
U311.
As
previously
stated,
the
values
of
C304
and
C308
are
being
changed,
thus
a
variable
capacitor
is
electronically
simulated.
This
is
done
by
monitoring
the
output
of
A3U302
with
A3U301
pin
5.
A3U301
is
a
buffer
whose
output
goes
to
DAC
U311.
The
DAC
is
not
configured
as
typically
seen
but
is
driven
in
a
voltage
mode
and
pin
15
of
the
DAC
becomes
the
input
signal
to
U301.
The
level
of
this
signal
will
vary
and
is
dependent
on
the
amount
of
compensation
required.
The
gain
of
U301
is
selec-
ted
by
the
combination
of
U313,
Q305
and
Q306.
For
all
ranges
except
the
300V
range,
Q305
is
on
and
this
sets
the
gain
of
U301
to
XI.
On
the
300V
range.
Q306
is
on
and
this
sets
the
gain
of
U301
to
N10.
The
gain
is
set
to
X10
for
the
300V
range
because
the
input
to
amplifier
Q307/U302
is
taken
from
the
junction
of
R307/R308
and
this
signal
level
is
a
factor
of
10
less
than
the
signal
at
the
junction
of
R305/R307.
The
output
of
U301
to
C304
and
C308
is
used
to
effectively
vary
the
value
of
the
capacitance
to
provide
frequency
compensation.
DAC
U311
controls
the
amount
of
feedback
to
capacitors
C304
and
C308.
The
amount
of
feedback
is
calculated
during
Autocal,
retained
in
memory
and
this
correction
value
is
used
to
control
DAC
U311
during
each
measurement.
The
DAC
receives
it
control
instructions
via
data
lines,
DO
-D9.
HP
3457A
Multimeter
8-31

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