Suggested Filter Circuit Components - Keithley 6220 Reference Manual

E-10 Applications Model 6220/6221 Reference Manual
For example, when sourcing 2nA, two 1MΩ resistors (one in the Hi side and one in
the Lo side) would contribute only 4mV of additional drop (2nA x 2MΩ = 4mV).
Two 1GΩ resistors would add only 4V of additional drop. The Model 622x will still
accurately drive the programmed current through the DUT.
The only difference required is that the voltage compliance would need to be set
to a higher value to accommodate the additional voltage drops. However, keep in
mind that in an open-load condition, the voltage on the DUT test leads will rise to
the higher set compliance voltage.
The flexibility of adding a wide range of resistor and capacitor values, in addition
to the ferrite, provides a wide range of options for a DUT and settling time require-
ments, to aid in the overall reduction of noise.

Suggested filter circuit components

Filter test box – Pomona Electronics manufactures several different connection
boxes suitable for the filter enclosure. The examples in this application use the
Model 2417 enclosure. Another alternative enclosure that comes with two BNC
(Coax) connectors is the Model 3231. This could be used for non-guarded configura-
tions where high impedances are not used. Use the Keithley Model 7078-TRX-GND
triax to BNC adapter to use this enclosure with the Model 622x.
Triax connectors – Keithley Model 7078-TRX-TBC bulkhead triax connectors
can be used with the 2417 enclosure. Of course, any suitable panel mount triax
connectors could be used.
Ferrite inductors – Fair-Rite Products Corp. manufactures a wide range of prod-
ucts suitable for this application. One specific part number used is Fair-Rite Part
Number 2743001112. This device has an impedance of about 40Ω at 20MHz,
making it suitable for higher current applications when using the mA ranges of the
Model 622x.
Resistors – Any suitable metal film resistor capable of handling the power
needed for the specific application will work well. A specific device that works well
for this application is the Vishay/Dale Model CMF55 series of resistors.
Capacitor – The capacitor is probably the most critical component since it shunts
the output of the current source. It must have extremely low leakage when sourc-
ing pA level currents. For reasonable time response, it should also have low
dielectric absorption. A specific device that works well for this application is the
Vishay/BC components KP46x series of polypropylene capacitors.
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