Cable Capacitance; Test System Performance - Keithley 4200A-SCS Reference Manual

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Section 12: Maintenance

Cable capacitance

Without guarding, the effects of cable capacitance would adversely affect the settling time when

sourcing current. The rise time of the source depends on the total shunt capacitance seen at its

output. For a high-impedance load, even a small amount of cable capacitance can result in long rise

times. For example, cable capacitance of 100 pF and a load resistance of 1 GΩ will result in an R

time constant of approximately 100 ms. Guarding drastically reduces cable capacitance, resulting in

much faster rise times. With FORCE and GUARD at virtually the same potential, the cable

capacitance cannot charge, and rise time is not affected (refer to

When sourcing voltage, the rise time due to cable capacitance is usually insignificant. Because the

voltage source is low impedance (<1 Ω ), the R

When making a semiconductor I-V measurement, there is always a speed-noise trade-off. Even with

given measurement settings, changes to the system configuration (such as cable length or adding a

switch matrix) changes the measurement results. The 4200A-SCS has four settings to allow optimal I-

V measurements. There are three fixed settings, fast, normal, and quiet, and a custom setting.

To achieve a low-noise measurement, the quiet setting is recommended. The trade-off is that

measurement speed is slower in comparison to the fast and normal settings. To make a fast

measurement, the fast setting can be selected, though the noise will be higher. Typically, the normal

setting is used to balance the speed and low-noise requirements. To further fine-tune the

measurement, the custom setting can be used.

The fixed settings are tuned to the 4200A-SCS for standard cable lengths connected to the DUT. In

general, this should be sufficient to make good measurements. However, when extra long cables or a

switch matrix are used in the system, these settings may not be adequate. A typical phenomenon is

the appearance of a glitch or offset error. The magnitude of the error increases if the fast setting is

used to make the measurement. This is caused by insufficient settling time for the system. With

added load or capacitance (cables or matrix relays), it takes longer to let transient effects settle.

Using the measurement parameters optimized for short cables only may result in an erroneous

measurement.

The best way to minimize this effect is to allow extra settling time. The normal or quiet settings should

improve the measurement result. Custom can also be used to fine-tune the measurement settings;

this may be a trial and error process. Various combinations of parameters can be used to achieve the

best results. In general, longer cables or slower settling of switch relays require a larger delay factor.

Model 4200A-SCS Parameter Analyzer Reference Manual

seconds 1 Ω x 100 pF is

time constant of 10

4200A-901-01 Rev. C / February 2017

(on page 3-23)).