HP 3457A - 8-60. DC Offset Correction; 8-61. Frequency Comparator; 8-62. Automatic Calibration; Auto-zero

8-60. DC Offset Correction
When
measuring
using
the
ACV
function,
small
input
circuitry
offsets
occur
and
cause
measurements
errors.
To
reduce
this
error,
the
circuitry
around
U307
and
U308
is
used.
When
in
the
ACV
function,
U303
pin
5
-
6
are
closed
and
8
-
9
are
open.
U308
is
configured
as
an
integrator
to
measure
the
DC
offset
at
the
output
of
U304
and
this
offset
correction
is
applied
to
the
inverting
input
of
U304d.
When
using
the
ACDCY
function,
U303
pins
8
-
9
close
and
5-
6
open.
When
pins
8
-
9
close,
R338
is
switched
in
as
the
feedback
resistor
to
reduce
the
gain
of
U308
to
prevent
feedback
error
Q309
is
used
to
switch
capacitor
C320
into
the
circuit
when
the
AC
Normal
function
has
been
selected.
Additional
DC
offset
correction
is
discussed
in
a
later
paragraph
titled
Automatic
Calibration,
8-61. Frequency Comparator
Comparator
U310
is
selected
when
the
frequency
or
period
function
is
selected.
This
directs
the
AC
signal
to
the
A2
Inguard
Controller
for
measurement.
8-62. Automatic Calibration
When
the
Automatic
Calibration
function
(ACAL)
is
selected,
the
AC
converter
is
electronically
calibrated
to
provide
the
greatest
degree
of
accuracy
by
compensating
for
DC
offsets
and
frequency
response
errors.
This
is
done
using
the
Auto-zero
and
Frequency
Response
Compensation
circuitry
described
below,
Auto-zero
Auto-zero
is
performed
at
the
beginning
of
each
automatic
calibration
routine,
During
Auto-zero,
the
AC
amplifier
input
is
grounded
by
FET
switch
A3Q304.
Any
offset
voltage
from
amplifier
A3Q307/U302
is
coupled
through
analog
switch
A3U303
to
amplifier
A3U304
which
amplifies
the
of
f-
set
error
and
adds
any
error
U304
may
have.
The
output
of
A3U304
is
coupled
through
switches
A3U306
(pins
3
-
4)
and
A2U403
(pins
5
-
6)
to
input
hybrid
A2UI01.
The
error
signal
is
measured
and
the
results
used
by
the
inguard
controller
to
set
DAC
A2U217.
A2U217
controls
the
output
of
amplifier
A2U218
which
is
then
connected
back
to
the
AC
board
(A3).
This
signal
is
connected
through
buffer
A3U307
to
the
minus
input
of
amplifier
A3U304
to
cancel
the
offset
voltage.
Frequency Response Compensation
Many
voltmeters
make
AC
measurements
at
frequencies
up
to
100KHz
but
this
voltmeter
makes
measurements
to
MHz.
To
achieve
this,
compensated
RC
attenuators
are
used.
At
low
frequencies
the
attenuators
are
resistive
dividers,
while
at
high
frequencies
they
act
as
capacitive
dividers.
To
equalize
the
high
frequency
and
low
frequency
responses
of
such
an
attenuator,
a
manual
adjustment
of
one
of
the
capacitance
elements
is
often
used.
In
this
instrument,
that
adjustment
is
performed
electronically
with
the
use
of
the
"ACAL"
function.
By
adding
a
third
capacitor
to
the
attenuator
and
dri
ng
it
with
a
scaled
value
of
the
input
signal,
the
capacitive
attenuator
can
be
controlled
so
that
a
flat
frequency
response
is
produced.
The
signal
fed
back
to
the
capacitor
is
controlled
by
a
DAC
(digital
to
analog
converter)
operated
as
a
variable
attenuator.
A
simplified
block
diagram
is
shown
in
Figure
8-4.
A
more
detailed
discussion
follows.
HP 3457A Multimeter
8-30