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Old 29th April 2008, 12:54 PM   #1
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Location: Charlotte, NC
Default Stray capacitance of CCS

Using a Supertex DN2540 CCS in the tail of a differential pair, with a negative supply for a little extra headroom. Input signal is ground referenced.

Would like to mount the CCS assembly to the aluminum top plate by the heatsink; so a small heatsink will conduct its heat to the top plate. I should be able to dissipate quite a bit of heat with this method, and it makes for ultra simple mounting.

Question is regarding the thermal pads used to isolate the drain of the FET from the heatsink. With a dielectric constant of 2.4, the capacitance of my pad is around 0.2249*2.4*0.375/0.009 = 22.5 pF.

Not a ton of capacitance, but more than I would have if the heatsink were not bonded to the chassis.

How do I quantify the affect this capacitance to ground has on the differential pair?

Thanks !
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Old 29th April 2008, 12:59 PM   #2
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Location: Budapest, Hungary
The capacitance will be formed between the tied cathodes (a low impedance point) and the ground. It will have negligible effect. Other thing if the CCS were in the anode.
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