3-6 DC Current Source Operation Model 6220/6221 Reference Manual
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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
=5 x 1kΩ x 33pF
= 0.165µs
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.
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 out-
put 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.
Response speed setting (6221 only)
The filter response speed of the Model 6221 can be set for FAST or SLOW. For
the SLOW setting, the output response of the Model 6221 is the same as the out-
put response of the Model 6220.
NOTE The above example is only intended to show how
load impedance affects settling time when using the
analog filter. Actual settling times will also depend
on other impedances present in the test circuit, such
as capacitance and leakage resistance in cabling
and in the test fixture.
NOTE An external user-supplied filter can be effective at re-
ducing high frequency noise generated by the Model
622x current source. For details, see “External user-
supplied filter,” on page E-7.
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