Part 2-Making Cv, Iv Or Q/T-V Plots; Setting Up Measurement Parameters - Keithley 595 Instruction Manual

OPERATION
PAFIT 2
Making CV, IV or Q/t-V Plots
Fart 2 of Section 3 will show how to use the feature% and
fur&ions described in Part 1 of Section 3 to take data or
plot curves with analog or digital plotters. Part 2 will also
discuss listing data points with a printer, the effect of us-
ing a filter on curves, and measurement considerations. The
procedures that follow are applicable for Cv, IV 01' Q/t-V
plots.
3.6 SETTING UP THE TEST FIXTURE
If the voltage source is on, select the OFF waveform to pre-
vent any excessive voltages during the set up. Enable ZERO
CHECKto protect the meteI's input amplifier, then make
all input connections (but leave the device under test
disconnected at the device for now).
Now set the voltage source limits below the device damage
levels. For example, if a MOS device with a breakdown
voltage of 8V is to be measured, set the maximum voltage
limit at 6 or 7V to prevent possible damage to the device.
Be sure to set both maximum and minimum limits for pro-
per protection.
If current is to be measured, perform zero correction if
necessary at this time (see paragraph 3.3.5). This will COP
rect for any internal offsets that may be present in the Model
595.
To &me1 any external offsets present in the test future for
either capacitance or current measurement, suppress the
offsets by removing ZERO CHECK (with the device still
disconnected as close to the device being measured as
possible to cancel offsets) and enabling SUPPRESS.
3.7 SETTING UP MEASUREMENT
PARAMETERS
At this point, connect the device under test to the Model
595. In order to determine the appropriate set up to make
the plot, it is a good idea to "spot che&' the device under
test at various voltages.
As an example, consider setting up an IV measurement for
a $.3V zener diode. First, set the waveform to DC. Next,
usmg the voltage source block, apply different voltages to
the device to determine its "acliw!' range. For a zener diode,
the -nt will rapidly increase at its *everse zoner voltage.
Try to fmd the voltage just before the cm-rent rapidly in-
creases and the voltage where the current reaches a plateau.
(Note: the Model 595 voltage source -nt limit will pre-
vent more than 4mA born flowing through the device.) For
this example, the region of interest (or area on the curve
where the most activity occurs) falls between -6.5V and
-4.OV. These voltage levels become the lower and upper
knits that should be set for the plot.
Now that the voltage limits are
chosen,
assume an exam-
ple with 7.00 samples (measurements) are desired in the
curve. The voltage step size now needs to be determined
for the plot. Simply divide the voltage span (difference be-
tween the voltage limits) by the number of samples to get
the approximate step size. Using the example above gives
the following:
2.5V
= .025V/sample
100 samples
NOTE
For capacitance measurements, there are two steps
per sample.
In this case, the .OZV (2Omv) step should be used. If
capacitance was being measured, the .OlV (lomv) step:
should be used. Generally speaking, if the step voltage does
not exactly match the allowable !%%F' V setting of the Model
595, then the next dosest value should be used (in this case,
.02V). Remember--a smaller voltage step (STEP V) will in-
crease the resolution of the curve over the voltage range.
The number of samples in the measurement can also be
determined if the voltage span and the step size are known.
Divide the voltage span by the voltage step as illustrated
by the following example:
2.5V
= 125 samples
.02V/sample
The direction of the measurement should also be deter-
mined at this time. For a positive-going staircase, make the
STEP V positive, and for a negative-going staircase, make
STFP V negative.
The appropriate capacitance or current measurement range
must also be set to achieve maximum resolution. Usually
3-15