Model 6220/6221 Reference Manual
Applications
E-9
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Filter circuit considerations
The filter can be tailored around the specific DUT impedance levels and speed
response requirements. Almost any range of resistance (R) and capacitance (C)
can be used as long as the corresponding voltage drops are within the dynamic
range of the Model 622x.
Care should be taken when using inductors (L), since there are maximum limita-
tions of inductance that the Model 622x can tolerate (<10µH preferably). Because
of this limitation, it is best to use ferrite inductors that do not exhibit as much phase
shift for the same increase in impedance as regular inductors.
The flexibility of adding a wide range of resistor and capacitor values, in addition
to the ferrite, provides a wide range of options to aid in the overall reduction of
noise.
High current applications
For higher current applications, resistors may not be the best choice, and a single
capacitor may suffice. This capacitance will interact with the load impedance
(assuming a resistance of R) and provide the following single pole low pass filter
response:
F
3db
= 1/(2
π
RC)
Again, the only real limitation is the maximum settling time response that can be
tolerated, which in this case would be approximately five RC time constants:
ST = ~5RC
Where:
R is in Ohms, C is in Farads, and ST is the settling time (in seconds) to
approximately 1% of the final settled value.
For additional higher frequency attenuation of noise, a ferrite can be inserted in
the Hi and Lo sides that will act as a resistor at the higher noise frequencies to
attenuate the noise spikes in the 1MHz and higher range.
Low current applications
For the other extreme where high DUT impedances are used along with the nA
current ranges, the capacitance will have a large effect on settling time. Therefore,
capacitance in the filter circuit should be in the 10pF to 1nF range to achieve a
response time that is tolerable.
In this case, minimizing the capacitance is desired. To help improve the filtering
capability, the series resistors in the Hi and/or Lo sides will help reduce the overall
noise coming out of the 622x. These can be sized as large as needed and can be
tolerated up to the point of the maximum voltage drop that is desired.
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