Hi,
I am testing what is called a "dark curve" for semi-conductor devices in solar panel aplications. Basically, you sweep current in logarithmic increments and record the voltage, then plot. I am using a Keithly 2602, and for the most part have tried to use the script language the instrument uses in the .vi versus labview constructs (i.e. sweeping commands such as SweepILogMeasureV through script fed in through VISA blocks in Labview versus for loops or other such constructions). Because in some semi-conductor setups there are high resistances and certain capacitance issues in cryostatic tests, when I sweep I actually sweep in two parts, a lower range (typically 1e-8 - 1e-5 amps) in which the user can define the time interval between measurements, and then an upper range (typically from 1e-5 to 1 amp) in which I preset data to be taken at 100 ms intervals. Note though that the user can define the minimum, maximum, and midpoint currents to their liking.
SO the actual problem: whenever I set the upper and lower ranges to be relatively similar in size, the sweeps run ok, and the resulting data from the sweeps is one continuous curve. However when I make one range considerably bigger than the other (for example, lower range 1e-8 - 1e-6 and then upper 1e-6 - 1 amp, 2 orders of magnitude vs. 6), there is a large discrepancy in the data points that overlap. So I might get that on the lower sweep at 1e-6 A the voltage is 0.4 V while when I measure the same 1e-6 but in the upper range command, the measured voltage is 0.6 V. Since the typical curve yields voltages from 0.2 V - 1.8 V, this kind of discrepancy is actually very significant.
I am aware that this problem might be more of a leakage current issue in the circuitry of the instrument instead of an outright problem in LabView programming but I am hoping someone might have some insight. Keithley does not have a good user forum to address this sort of issue. Thanks in advance for any help!
Regards,
-s
