Instrument Trigger Modes; Autozero Effects On 4-Wire Ohms Measurement - HP 3456A Operating And Service Manual

Operation
d. Fast Autorange Feature. Autoranging by the
3456A is done quickly in both the upranging and
down ranging operations.
I . Upranging. When a voltage applied to the
3456A's input is higher than the range used at that
time, an overload condition is detected. The
overload condition is detected before the input
measurement is complete. Upranging is done until
the overload condition disappears and the op­
timum range is reached. Since the total measure­
ment is never completed until the optimum range
is reached, upranging is fast.
2. Downranging. Downranging is also done quickly
by using a different method. When a non­
overload measurement is made, the 3456A takes a
complete measurement. Once the measuremenl is
completed, the instrument then calculates the op­
timum range from the reading. A maximum of
three ranges can be skipped at one time, enabling
the 3456A to downrange quickly.
3·46.
Instrument Trigger Modes.
3-47. The -hp- Model 3456A
modes: Internal Trigger, External Trigger, Single Trig­
ger, and Hold. A description of each mode is as follows.
a. inlernal Trigger. This
generated by the 3456A and is used to initiate a
measurement cycle. The instrument is placed into the
Internal Trigger mode by pressing the INT (Internal
Trigger) push bUll on located on the fronl panel. This
trigger is also automatically selected when the RESET
button is pressed and at turn on.
b. External Trigger. In the External Trigger mode,
the instrumenl can be triggered by an externally applied
trigger pulse. The pulse is applied to the External Trig­
ger Input connector located on the rear panel and
should be at least 500 nsec wide. The External Trigger
Input is TIL compatible with actual instrument trigger­
ing occurring on the falling (negative) edge. By applica­
tion of the pulse, the 3456A triggers and initiates a
measurement cycle. After this cycle is completed, the in­
strumenl can be triggered again for a new cycle. If any
triggering is done during the measurement cycle, the
trigger is ignored until the cycle is completed. To start a
new measurement, the 3456A has to be triggered again.
c. Single Trigger. The Single Trigger operation is
similar to the External Trigger operation with triggering
being accomplished by the front panel's SINGLE trig­
ger pushbutton. Depressing the button first places the
3456A into the Signal Trigger mode (if the 3456A is in
another trigger mode) and then triggers the instrument.
A measurement cycle is then initiated. If the pushbunon
3-8
has four trigger
trigger is inlernally
is pressed again during the cycle, a new measurement cy­
cle is started. The 3456A can be triggered again by
depressing the SINGLE trigger pushbutlon.
d. Hold. The Hold operation prevents the 3456A
from accepting locally generated trigger commands,
although an HP-IS trigger is accepted. Refer to the
Remote Operation paragraphs for additional informa­
tion on the HP-IS trigger.
3·48. Autozero.
3-49. The Autozero feature of the -hp- Model 3456A is
used to compensate offsets in the dc input amplifier cir­
cuit of the instrument. Its main purpose is to correct for
any zero drift in circuitry which may cause errors. The
method used is to short the amplifier's input to circuit
ground and take its offset reading. The reading is then
stored in the instrument's internal memory and is later
used to correct the following input measurement(s).
Once the offset reading is taken, the short is removed
and a regular input measurement is taken. As long as
the Autozero feature is enabled, the 3456A takes an
Autozero measurement and an input measurement.
When the feature is disabled, an Autozero measurement
is taken and is immediately stored into memory. No new
Autozero measurements are made for the successive
readings; only an input measurement is made. The
stored Autozero reading is subtracted from the input
measurement to correct the reading. Since only the in­
put measurement is made, the �456A's reading rate in­
creases. This also makes the instrument more suitable
for making measurements on high impedance circuitry,
since no input switching is done. The 3456A's long term
stability is affected (see Table )-1) with the disabled
feature, unless the Autozero reading is updated. Up­
dating is done when any change in instrument state
occurs. The only exception is that no updating is done
by triggering, Front Panel SRQ, and HP-IB Local com­
mands. The Autozero feature is enabled when the
3456A is first turned on and when pressing the RESET
button.
3·50. Autozero Effects on 4·Wire Ohms Measurement.
3-5 1 . The Autozero measurement is normally made
with the input amplifier shorted to circuit ground. In the
4-Wire Ohms mode, the input amplifier is shorted to the
4-WRO SENSE Low terminal for the AUlOzero Mea­
surement. With Autozero "ON", the Autozero reading
is updated for each measurement cycle. With Autozero
"OFF", the reading is not updated and causes an ohms
measurement error if the measuring lead's impedance
changes. To prevent this error, a new Autozero reading
should be taken by changing or updating instrument
state with the new measuring lead configuration. A
disabled Autozero is useful in ohms measurements for a
faster reading rate and where the 3456A's input swit­
ching may have affected the measurement.
Model 3456A
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