Keithley 220 Instruction Manual page 20

IOmA
/ I /
smsec
Figure 2.4.lOmA to ZmA Square Wave
NOTE
In this example the Model 220 does not change
range. If the Model 220 is programmed to change
from one range into another (e.g. 1OmA to ImA),
the output drops to zero for approximately 2msec
between range changes.
Example 7 Uslng External Trigger (Input and Output) -
The external trigger input initiates the program mode (single,
continuous, or step) in the same manner as the START button.
To output any programmed values the program mode must be
selected, the OPERATE button enabled, and the instrument
must receive the external trigger input pulse.
The external trigger output is a pulse signifying the completion
of a programmed dwell time. The pulse is present at the etier-
nal trigger output in any of the three program modes. For the
single or continuous program modes there is an output pulse
at the end of every programmed dwell time. For the step pro-
gram mode, there is an output at the end of the programmed
dwell time for the one memory location. To go on to the next
step (memory location), and therefore output another external
trigger output pulse, another external trigger input pulse is
required.
Using the Model 619 in conjunction with the Model 220 can
help illustrate thls example. Like the Model 220, the Model 619
has external trigger input and output (electrometer complete
and external trigger) lines. Connect the two instruments es
shown in Figure 2-5. In this configuration and with the proper
programming, the Model 220 will output the programmed
current when the START button is pressed. At the end of the
specified dwell time the Model 220 outputs the external trigger
pulse. This pulse triggers the Model 619 to take a reading.
When the Model 619 measurement cycle is completed it
will output a trigger pulse (via the electrometer complete line)
to the Model 220. The trigger pulse from the Model 619 to the
Model 220, triggers the Model 220 to advance to the next
memory location and output the next programmed current.
NOTE
Only the Model 619's with electrometer complete
and external trigger can be used for this example.
At this point the cycle repeats itself and keeps on repeating
itself by advancing through the programmed memory loca-
tions of the Model 220. To stop the cycle, disconnect one of
the trigger lines.
NOTE
The Model 619 can be set to the talk-only mode
and connected to an IEEE compatible printer. The
Model 619 measurement data would then be
recorded by the printer automatically. This would
free the operator from recording the data manually.
Example 8 Uslng Guard - Leakage resistance between low
current conductors and nearby voltage sources can cause
significant error currents. For example, If a printed circuit
element has a leakage path with a resistance of 10% to a.
nearby 15V supply terminal, a current of 15nA will be
generated as shown in Figure 2-6a. In order to keep this
current below 1 pA, the leakage resistance would have to be
above 1.5 x 10%. This high resistance is difficult to maintain
in many situations. In order to eliminate such stringent
insulation resistance requirements, guarding techniques may
be used as shown in Figure Z-6b.
Guarding is surrounding the sensitive input with a conductor
(the guard) connected to a low impedance point which is at
(virtually) the same potential. The GUARD terminal located on
the rear panel provides an easy connection to a low imped-
ance voltage source which is equivalent to the output compli-
ance voltage.
The maximum load capacitance for the guard output is
O.OluF. The maximum load current which includes guard and
output is not to exceed 105mA. The accuracy of the guard
+lmV excluding output lead I*R voltage drop.
Example 9 Floating Opsrstlon - The Model 220 can be
floated off chassis ground. Guard or output common MUST
NOT be connected to chassis ground when floating the instru-
ment above chassis ground potential. Chassis ground is con-
nected to earth ground with the line power cord and an
appropriate grounded three-wire receptacle. The Model 220
can be floated up to 250Vrms above chassis ground.
TRlGGER
MOOEL 220 REAR PANEL
Flgurs 2-5. Using the Model 220 External Trlggsr
2-8