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Model3490A
3-162.
Sample/Hold
Trigger
(TTL).
Compatibility with
TTL
logic
circuits
requires
this
signal
go
be from HIGH
(>+
2.4
V)
to
LOW
((+
0.4
V)
for a minimum of
30
nanoseconds
in
order
to
initiate a Hold mode. The signal
must
be
HIGH
for
at
least
600
ps prior
to
going
LOW.
The
external
triggering
circuit must be capable of
sinking
I mA
at
(+
0.4
V.
3-163.
AC
Trigger.
The
leading
edge of a negative-going
pulse
at
least
30
nanoseconds wide and 2 V
to 200
V
in
amplitude
initiates
a
Hold mode. The sigral
must be
stable
for
at
least
2
ps
before
the negative-going transi-
tion.
3-164.
Termination
of Unused
S/H
Trigger
Input
Con-
nections.
When
operating in a
Sample/Hold
mode, some
precautions
are
necessary
to prevent
unwanted triggering
of
the
SampleiHold
circuits,
which can be caused by
cross
talk within
an external cable
attached
to the trigger
input
connector. Only one of the two trigger inputs
will
be
used at any
one time in
a
given
situation,
and
spurious
triggering
at
the
other input may
be
prevented
in
one of
two ways:
l. The unused
input
may
be
left open; that is,
with
no wire in
the
cable connected to this
pin
on
the
connector.
2.
The unused input
may be
terminated
at
the other
end of
the cable.
The TTL
Sample/Hold
Trigger
(dc
coupled)
input
line should be connected to
a HIGH
logic
level
()+
2.4V). The AC
Trigger
input line
should be
connected to
a LOW logic level
(<+
0.4 V).
3-165.
Initiating
a
Sample/Hold
Measurement.
3-166.
The
Sample/Hold option is
available
in three
different configurations
which
employ
various methods
of initiating
a
Sample/Hold
measurement.
SampleiHold
is
aväilable with the
BCD/Remote Expand
Option 020.
(Either
Data
Output
Option
021
or Remote Control
Option
022 or
both
may
also
be
installed
with Option
020.)
Sample/Hold is
also
available
with the General
Purpose
Interface
Bus I/O
Option 030.
In the
third
con{iguration,
Sample/Hold
is
available as Option 045,
without
any
of the other
options.
No data
output
or
remote control,
except
Sample/Hold
Trigger, is
available
in
Option
045. The
following
paragraphs
discuss
meth-
ods
of initiating
a measurement in instruments having
the
various
options.
Table 3-9 lists the methods of
externally
triggering
a
Sample/Hold measurement.
3-167.
Internal
Sample Control, Option
045
and Option
040
with
Option
020.
When the
iront
panel
Sample
Rate
control
is set
to
any
position
except Hold and
no
Sample/Hold
Trigger
command is
applied,
the instru-
ment
will
automatically
hold and read
the input signal
at
the
sample
rate selected.
However,
this
method of
operation provides
only
random sampling, since it is not
possible
to
s1'nchronize
the
measurement to
any
specific
point in the
input
signal.
Section III
Table
3-9.
Initiating
S/H
Measurements
by
External Triggering.
Sample/Hold
Option
Other Options
in
the
34904
Measurement
Initiated
By:
o40 o20
Separate
External
Encode
command
or
Stretched
Pulse
Output connected
to External
Encode input.
040
030
Sample/Hold
Trigger
045
neither
020
or O30
Sample/Hold
Trigger
3-168.
Internal
Sample
Control,
Option 040
with Op-
tion
030. The 3490A
with
the
GPIB
Option 030 may
be
programmed
for internal
sample control.
If so, the
instrument
will automatically
hold
and read the input
signal at the
sample
rate
selected.
Operating with
internal sample
control provides
only random sampling,
since the
measurement is not
synchronized
to
any
specific point
in the
input signal.
3-169. External
Triggering,
Option
045.
When
Sample/
Hold
is present
in
an instrument
with neither
BCD
Remote
Expand
Option
020 or GPIB
Option
030,
it is
designated
as
Option
045.
In this
case, either
Sample/
Hold
Trigger command
automatically
initiates
a mea-
surement.
3-170. External
Triggering,
Option 040
with Option
020. With this
combination
of
options,
a
Sample/Hold
measurement
may be initiated
by
external triggering in
two
ways. For either
method, the
34904
measurement
circuits
must be in
the
"Hold"
state.
That
is, the
Sample
Rate control
must be in the Hold
position,
or
if the
instrument
has
Remote
Control Option
022,
the lnter-
face
Hold line may be
held LOW. One method
of
initiating
a
Sample/Hold measurement requires
two
'separate
signal inputs,
a Sample/Hold Trigger command
,"
to initiate
a Hold mode,
and
an External
Encode
command
to initiate
a measurement. Both commands
may be
applied at the same time,
or
the
External Encode
command
may be
applied up to ll2 second after the
Sample/Hold
Trigger. In
the second method, the
Stretched Pulse
Output from the
Sample/Hold
trigger
circuits
(J7
pin
10) is
connected externally to the
External Encode
input
(J6
pin
46 or
J7 pin
24 or
28).
This
connection provides
an
External Encode signal to
initiate
a
measurement immediately
upon receipt of
a
Sample/Hold Trigger command. Neither signal
will have
any effect
if
applied while a measurement is in
progress.
If a
Sample/Hold Trigger command is
given
without an
External
Encode
command,
the Sample/Hold
circuits
will
be locked in the Hold
mode. This condition can
be
terminated
by applying
an
External
Encode command or
by setting the
Sample/Hold switch to
Off.
3-171. External
Triggering,
Option
040
with Option
030.
When
operating
Sample/Hold with General
Pur-
3-t7