Noise And Source Impedance; Cable Capacitance; Source Resistance; Source Capacitance - Keithley 4200-SCS Reference Manual

Section 5: Source-Measure Concepts

Noise and source impedance

Noise can seriously affect sensitive current measurements. The following paragraphs discuss how
source resistance and source capacitance affect noise performance.

Source resistance

The source resistance of the DUT will affect the noise performance of the SMU or preamp. As the
source resistance decreases, the current noise increases. Because decreasing the source
resistance can have a detrimental effect on noise performance, there are usually minimum
recommended source resistance values based on measurement range.
minimum recommended source resistance values for various measurement ranges.
Table 5-2
Minimum recommended source resistance values

Source capacitance

DUT source capacitance will also affect the noise performance of the preamp. In general, as
source capacitance increases, the noise gain also increases. Although there is a limit to the
maximum source capacitance value, you can usually measure at higher source capacitance
values by connecting a resistor in series with the source. Note, however, that doing so will
increase the voltage burden. For example, the source resistance values listed in
result in a voltage burden between 1 mV and 1V.

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 RC 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
section).
When sourcing voltage, the rise time due to cable capacitance is usually insignificant. Because the
voltage source is low impedance ( <1 Ω), the RC time constant of 10
negligible.
5-24
Minimum recommended
Range
1 pA to 100 pA
1 nA to 100 mA
1 μA to 100 μA
1 mA to 100 mA
Return to Section Topics
source resistance
1 G Ω to 100 G Ω
1 M Ω to 100M Ω
1 k Ω to 100 k Ω
1 Ω to 100 Ω
Model 4200-SCS Reference Manual
Table 5-2 summarizes
Table 5-2 will
Guarding earlier in this
-10
seconds 1 Ω × 100 pF is
4200-901-01 Rev. S / May 2017