HP 3457A - Page 208

+10V REF
U101
INPUT HYBRID
34820 F.12,25
Figure 8-8. Amps Sense Circuitry
Test
7
tests
the
DC
Offset
voltage
of
the
amplifier
circuitry
on
assembly
A3
to
determine
if
it’s
within
the
proper
limits.
The
amplifier
being
tested
consists
of
A3U302.
A3U304
and
A3Q307.
During
testing.
the
AC
amplifier
input
is
grounded
by
FET
switch
A3Q304.
Any
offset
voltage
from
A3Q307
and
A3U302
is
coupled
through
analog
switch
A3U303
to
amplifier
A3U304
which
amplifies
the
error
and
adds
any
it
may
have.
The
output
of
A3U304
is
coupled
through
switches
A3U306
(pins
3-
4)
and
A2U403
(pins
5
-
6)
to
input
hybrid
A2UI01.
The
Offset
DAC
(A2U217)
on
assembly
A2/I1
is
providing
offset
compensation
back
to
the
amplifier
on
the
AC
assembly
through
A3U307.
The
300V
range
is
selected
to
set
the
overall
gain
from
A3Q307
input
to
A3U304
output
to
X10,
a
measurement
is
made
and
the
Main
Controller
(ALU501)
performs
error
checking
to
DAC
(A2U217)
count
limits
of
512
+472.
This
limit
assures
being
within
the
calibration
range
of
the
DAC.
Figure
8-9
illustrates
some
of
the
circuitry
used
for
this
test.
On
error,
if
the
AUNERR?
command
is
executed,
the
number
128
will
be
displayed
on
the
control
panel
if
their
are
no
other
errors
(See
Table
8-2).
This
indicates
an
AC
Amplifier
DC
Offset
problem
that
could
possibly
be
corrected
by
performing
the
ACAL
command.
HP 3457A Multimeter
8-43