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3-6
DC Current Source Operation
For example, assume the Model 6221 is on the 2mA range and FAST response is
selected. For this configuration, the Settling Time is specified at 2µs (typical). Also
assume the load impedance is 1kΩ. With the analog filter enabled, the additional
settling time (five time constants) is calculated as follows:
Additional settling time = 5RC
Enabling the analog filter adds 0.165µs settling time, which is not a significant
increase in comparison to 2µs.
Now assume the load is 1MΩ. The additional settling time with the analog filter
enabled is 165µs (5 x 1MΩ x 33pF). The analog filter greatly increased the settling
time from 2µs to approximately 167µs.
NOTE The above example is only intended to show how load
For load impedances <10kΩ, the analog filter has little effect on the output
response. At the 10kΩ load impedance point, the filter capacitor across the load
creates a filter response of less than 1MHz, which is the maximum output band-
width of the Model 6221 for the higher current ranges.
For load impedances greater than 1MΩ, the reduced response of the filtered
output can significantly reduce overshoot, noise, and instability (oscillation).
With the analog filter disabled, the output capacitance of the Model 622x is <10pF.
If not sure about using the analog filter, experimentation may be the best way to
determine which analog filter state (enabled or disabled) provides the best results.
NOTE An external user-supplied filter can be effective at reduc-
= 5 x 1kΩ x 33pF
= 0.165µs
impedance affects settling time when using the analog
filter. Actual settling times will also depend on other im-
pedances present in the test circuit, such as capacitance
and leakage resistance in cabling and in the test fixture.
ing high frequency noise generated by the Model 622x
current source. For details, see "External user-supplied
filter" in Appendix E of the Reference Manual.
Return to
Section 3 topics
Model 6220/6221 User's Manual
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