A Lim.it, V Limit And Preset; Delay Time - Keithley 595 Instruction Manual

OPERATION
time (plus 0.04sec) the voltage changes by STEP V. The out-
put voltage remains at this level for another delay time (plus
O.Okec) and then returns to the original Vl level. The out-
put voltage continues to repeat the above process. For
capacitance measurements, the reading voltage is reported
as V, +~ 1% V,, For current measurements the reading
is reported at the voltage where the measurement was
made.
Staircase is selected in the same way as square wave. The
timing~is also the same but at the end of the second delay
time the waveform does not return to VI. Instead, it in-
crements again by STEP V to VZ. This results in an output
voltage waveform that resembles a staircase-continually
increasing (or decreasing) uniformly in steps. When the
output voltage reaches the upper or lower limit, the out-
put voltage will switch to DC. To restart a staircase
waveform, set the voltage source output to the desired star-
tine voltage (the "ores&' feature makes this easv) and re-
se&t the&p waveform.
r---V2
t STAIRCASE
Figure 3-4. Voltage Output Waveform
VOLTAGE SOURCE Block
The Model 595 has a built-in voltage source that is used
to make capacitance measurements. It can alsa be used with
the current function for measuring IV characteristics of
devices. The output voltage may be set from -2OV to +ZOV
in IOmV increments. Limits may be set to prevent the out-
put from exceeding a selected value. The maximum~cur-
rent for which the output voltage is guaranteed is 2mA and
current limiting prevents the output current from exceeding
4mA even into a direct short.
The following controls are found in the VOLIAGE SOURCE
block of the Model 595 front panel. The indicator lights in-
dicate which parameters are displayed. Press to select a par-
tic&r parameter, press again to return to the measurement
display.
3.3.7 A LIMIT, v LIMIT, and PRESET
The A LIMIT and 'I LlMIT buttons set the maximum and
minimum voltage that the voltage source will output. These
limits may be set by the user to protect a device that could
be damaged by excess voltage. On power up the A LIMIT
is set to +u) volts and the 7 LIMIT to -20 volts. These
power up limits are the maximum and minimum voltages
that the limits can be set to.
When a LlMlT button is pressed, the display shows that
voltage limit. The ADJUST buttons are used to change that
limit in 1OmV increments.
An a- function of the voltage limit buttons is PRESET.
PRESET allows the user to quickly set the output voltage
value to the maximum or minimum limit without having
to use the ADJUST buttons. To use PRESET, press SHIFI,
then A LIMIT or v LIMIT.
3.3.8 STEP V
When a step waveform is selected, STEP V determines the
step size (the small incremental change in voltage) and
whether the waveform will be a square wave or a staiiase.
The .step size may be O.OlV, O.O2V, O.O5V, or O.lOV with a
positive or negative polarity. On power up, the step voltage
is set to +O.O5V square wave. Detail descriptions of the
voltage waveform may be found under WAVEFORM
BLOCK - STEP.
To select the step voltage, press STEP V. The indicator light
will be on while the step voltage is displayed. Use the
ADJUST buttons to select the step size. The * button may
also be used to change the polarity of the step voltage.
Press SHIFT then STEP V to toggle the waveform between
square wave and staircaseSquare wave and staircase are
shown pictorially next to the STEP V button.
3.3.9 DELAY TIME
The time from the voltage step to the
fmal
charge measure-
ment for a capacitance calculation, or from the step to the
measurement in current, is the delay time. Delay time is
used to allow the device under test to respond to the change
in voltage before measurement. The delay time is not usual-
ly critical when performing simple capacitance or IV
measurements. It is important when performing capaci-
tance and CV measurements on MOS capacitors.
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