HP 3455A - Page 27

Model
345SA
Section
Ml
with High Resolution turned
on or
off.
3-29. Aato-Cil.
3-30
The
purpose of the AUTO-CAL feature is to
eliminate offsets, gain non-linearity, and
drift which
maybe present in the analog measuring
circuits of the
34SSA. This is
accomplished
by
measuring the offset
and gain
errors and then mathematically
correcting the
measurement reading to exclude them.
Each of the gain
and error measurements, called
Auto-Cal constants, are
stored in the “memory” by the
3455A’s main con-
troller. These Auto-Cal
constants are usually taken be-
tween each sample of the instrument and are
updated
each time a
new
cal constant
measurement is made.
3-29.
The reading rate of the 34SSA increases
when the
Auto-Cal feature is turned off.
Table 3-1
gives
the
reading rate of the
various functions
with
Auto-Cal on
or off.
3-32.
The last set of constants are used to
correct
measurements, when the Auto-Cal mode is
turned off.
As long as the input
amplifier offsets, gain linearity
and drift do not
vary the 34S5A should remain
within
it’s
accuracy specifications. The time
period over which
these parameters
will
not
change may
vary
from instru-
ment to
instrument.
When
the Auto-Cal
function
is
disabled to
obtain faster reading rates, it is recommend-
ed to
periodically return the 34SSA to the
Auto-Cal
mode in
order to update the cal constants. This can be
done after a
block of readings have been taken or
when
the instrument is not in use. The instrument
will then
update the cal constants for
accurate measurements.
Allow
about 6 seconds for
updating the cal constants, if
the 34SSA is in the Hold mode.
3-33.
Trigger.
3-34.
The 3455A has three
trigger
modes.
INTERNAL,
EXTERNAL, and
HOLD/MANUAL. The
following
is
an explanation of each trigger mode.
a.
Internal Trigger: This trigger is generated inter-
nally and triggers the 34SSA to lake a reading,
after
the
previous operation is completed (a
reading
or
Auto-Cal
measurement). This trigger mode is
entered when
the in-
strument is turned on,
when
the Internal Trigger button
is
pressed, or a Device Clear message is remotely sent.
b. External
Trigger: When the 3455A is the External
Trigger mode,
the
user can trigger the instrument from
an external trigger pulse. This trigger pulse has to be
ap-
plied to the rear External Trigger
Connector and should
have a
negative
TTL edge and must be
at least 3 seconds
wide. The instrument
will
take a
measurement,
when
this trigger pulse is
received.
After
the measurement is
taken, the 34SSA can be
triggered again for a
new
reading. If the
instrument is triggered
while
making a
measurement, the
new
trigger is delayed.
After the first
measurement
cycle is completed, the delayed trigger
will
iniate a
second measurement cycle. Only one
trigger will
be delayed during any
given measurement cycle. Any ex-
tra triggers sent during this
cycle will
be
ignored.
c. Hold/Manual
Trigger: This trigger is similar to
the
External Trigger, except it can be executed by the
Hold/Manual button. The Hold/Manual button must
be pressed once in order to place the
3455A in the Hold
m<^e. After
pressing the Hold/Manual button the se-
cond time, a
measurement is taken.
When
the
measure-
ment
cycle is completed, the
Hold/Manual button can
be pressed
again for a
new
reading. It is
important to
remember that the Hold/Manual button
should
be
pushed
twice in order to take the first reading. If trig-
gered while a measurement is taken, the
trigger is
delayed until the
measurement cycle is complete. The
delayed
trigger will initiate a second measurement cycle,
when
the first one is
completed. Only one trigger will be
delayed
during any given measurement cycle. Any extra
triggers sent during this cycle
will
be ignored.
3-3S. Auto-Cal constants
measurements also depend on
the Trigger mode used. An
input reading and a cal cons-
tant
measurement will alternately be taken,
when the
34SSA is in
the Internal Trigger mode.
A typical se-
quence
would
be an input
reading, one cal constant
measurement,
another input reading, the next cal
cons-
tant
measurement, and so on. An
attempt of this se-
quence (input
reading/cal Constant
measurement) is
^so made
when
the
instrument is in the
Hold/Manual
or
External Trigger modes. If,
however,
a
trigger is
received
while
a cal constant
measurement is taken, this
measurement is
aborted and an input reading is taken.
After this
reading, the aborted cal constant measure-
ment is then retaken. If a
new trigger
is
received before
the cal constant
measurement is finished, the measure-
ment is again
aborted and a new input reading is taken.
The cal constant
measurement can be aborted a number
of times,
depending
on
the function of the instrument.
The
table below lists the number of times the cal cons-
tant measurements can be aborted. After
this number
has been reached, the
trigger will be delayed and the
Auto-Cal
constant measurement is then completed.
Functiun
Miximum Number of Cel
Conetant Termination
DC
126
DC (High Resolutiont 32
AC Fast
64
AC Normal 8
Ohms
64
Ohms <H»gh Resolution)
1
6
These numbers are
accumlative when Auto-Cal is on.
3-36.
Sample Rate
(Diaplay).
3-37.
The
SAMPLE RATE of the 3455A is set internal-
ly
and depends on the function selected, the
power line
3-5