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I have 2N3823 JFETs in TO-72 package, which has 4 leads. No problems with Gate, Drain, and Source, but there is also a Case pin that I have been ignoring. Now I am putting together a PCB for a design that includes one of these FETs and I don't know what the Case pin should be connected to. I'm guessing ground (?) but I'm not sure and I am having difficulty locating any documentation for other devices with Case pins.

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The datasheet should tell you definitively tell you whether or not the case is connected to anything on the chip, and if so, where. Sometimes the case is isolated, sometimes the case is connected internally to the chip's ground, etc.

In this case, Note 3, page 5 of the datasheet from Microsemi says that the case is isolated from the case.

If it's isolated, then connecting to ground is a good bet because leaving it unconnected would be like having a floating ground pour or shield which is not good for noise.

The reason you have difficulty locating other examples is because they tend to be specialized parts contained in these expensive TO packages. Things like exotic optical sensors and amplifiers.

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  • \$\begingroup\$ Just to be clear - if I tie it to Source I shouldn't run into any issues? I saw it said the active components are insulated from the case, but then I was wondering why they would bother with the pin at all if it makes no difference. \$\endgroup\$ Apr 25, 2019 at 21:46
  • \$\begingroup\$ They bother with the pin because it lets you decide how you want to use it as a shield. For example, maybe the FET is used in a floating position where no FET terminals connect to GND. If the case is to be a shield, you need to connect the shield to GND because an unconnected shield can be worse than no shield. But if they omit the case pin and just connect the case to an arbitrary FET terminal, then you lose the ability to connect the case to a stable potential. \$\endgroup\$
    – DKNguyen
    Apr 25, 2019 at 21:57

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