EasyManua.ls Logo

HP 3457A - Page 178

HP 3457A
273 pages
Print Icon
To Next Page IconTo Next Page
To Next Page IconTo Next Page
To Previous Page IconTo Previous Page
To Previous Page IconTo Previous Page
Loading...
Transistor
Q211
allows
a
more
linear
operation
of
U212
(increased
loop
gain
since
it’s
not
required
to
output
a
voltage
so
close
to
its
limits).
Without
it,
U212
would
be
required
to
output
+12
volts
from
a
supply
of
only
+15
volts
and
linearity
would
not
be
as
good.
8-29.
Output
FET
Gate
Bias
Amplifier
Amplifier
U213
in
combination
with
the
output
MOSFET
in
U101
and
switch
amplifier
U212
comprise
a
voltage-to-current
Converter
which
is
the
Ohms
Current
Source.
The
noninverting
input
of
U213
is
driven
by
a
voltage
derived
from
the
Ohms
Voltage
Reference
Amplifier,
U2I1.
This
input
is
main-
tained
at
+8
volts
independent
of
the
output
of
U2I11
by
selecting
one
of
two
sense
points
in
the
gain
network
consisting
of
the
17.1K
and
2.9K
ohm
resistors
in
Ul0!.
UI0!
MOSFET
switch
11
will
be
on
when
the
input
to
U211
is
+8
volts
and
this
voltage
is
switched
to
the
input
of
U213.
When
the
input
to
U2I1
is
+5.6
volt,
U10l
MOSFET
switch
77
will
be
on.
This
will
select
the
+8
volts
at
the
junction
of
the
17.1K
and
2.9K
ohm
resistors
in
U101
and
apply
it
to
the
input
of
U213.
The
operation
of
U213
then
forces
the
inverting
input
of
U213
to
+8
volts
as
long
as
current
can
be
made
to
flow
through
one
of
the
range
resistors
(ULO1,
4K,
40K,
400K
or
4M
ohm).
With
a
path
for
current
to
flow,
the
range
resistor
will
have
either
4
volts
or
0.4
volts
across
it,
thus
generating
a
precise
output
current.
8-30.
Offset
Compensation
Switching
(OCOMP
command)
The
ohms
function
can
be
thought
of
as
simply
measuring
the
voltage
across
a
resistor
that
has
a
preci-
sion
current
passed
through
it.
However,
if
there
are
unwanted
voltages
in
the
path,
errors
will
be
in-
troduced.
For
example,
the
error
voltage
may
be
a
thermally
generated
offset
in
the
resistor
under
test.
If
this
unwanted
voltage
could
be
eliminated
from
the
desired
voltage,
then
potentially
large
sources
of
error
in
the
measurement
could
be
removed.
Offset
compensation
is
a
technique
which
removes
sources
of
errors.
While
in
the
ohms
mode.
a
measurement
is
periodically
made
with
the
ohms
current
source
off.
This
result
is
the
error
voltage
we
want
to
remove.
This
value
is
subsequently
subtracted
from
normal
ohms
readings
to
yield
an
error
free
result.
Analog
switch
U214
is
used
to
switch
the
ohms
current
source
on
and
off.
The
function
of
U213
is
to
control
the
gate
voltage
and
thus
the
current
flowing
to
UIOI
pin
15
from
the
output
MOSFET
in
U101.
When
the
gate
voltage
is
positive
the
current
source
is
off.
The
purpose
of
U214
is
to
toggle
be-
tween
current
source
on
(switch
from
pins
I-16
on)
and
current
source
off
(switch
from
pins
3-4
on),
Independent
of
offset
compensation,
the
network
composed
of
R218,
Q217
and
CR202
provides
an
al-
ternate
current
path
when
the
output
MOSFET
in
ULOI
is
of
f.
Without
this
network,
the
combination
of
the
range
resistors
(4K,
40K, 400k
and
4M
ohm)
in
ULOI
and
stray
capacitance
at
the
inverting
input
of
U213
can
cause
stability
problems
on
the
higher
ohms
ranges.
With
this
network,
when
the
output
of
U213
approaches
-7.5
volts,
the
diode
connected
low
leakage
JFET
Q217
forward
biases
and
output
current
is
diverted
away
from
the
ohms
output
MOSFET.
HP
3457A
Multimeter
8-12

Other manuals for HP 3457A

Preview: Operating Instructions
Operating Instructions362 pages

Related product manuals