Input capacitance of 4000 series and PICs

Nov 08, 2011 7 Replies

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Klaus Kragelund 14 years ago

I have an application in which I need to take into account the input capacitance of a given input.

I can feed the signal directly into the PIC we are using (PIC12 series). Microchip lists input capacitance as max 5pF

I may use a 4000 series IC, for which family specification lists

7.5pF.

But, anyone actually measured these or simelar inputs, to get an idea of what the real input capacitance is? (ok, I could bring out the LCR meter, but perhaps I get some golden input from this NG before I can do it tomorrow :-)

Thanks

Klaus

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Klaus Kragelund 14 years ago

Found the HCGuide, which details 74HCxxxx as typical 3.5pF and a derating to conservative number of 10pF. Guess the 4000 series has simelar values...

/Klaus

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Nico Coesel 14 years ago

Well, thats your answer. If you want to know whether you can count on it being exactly 5pf the answer is no. It may be much less. I would not let a design depend on the input capacitance of a chip being constant.

Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------

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Joerg 14 years ago

This family guide quotes a max input capacitance of 7.5pF for the HEF4000 series:

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Regards, Joerg http://www.analogconsultants.com/

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John S 14 years ago

Yabbut... I wonder if anybody has ever measured to verify. Might be interesting.

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Joerg 14 years ago

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I have, but can't recall because that was at least 20 years ago. It was much lower, I think well under 5pF. Had to do it because I was paralleling lots of inverters in a CD40106 to get more gusto out of it.

To some extent this will depend on the supply voltage.

Regards, Joerg http://www.analogconsultants.com/

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Tim Wescott 14 years ago

Below the stated maximum, yes. Constant - no, that's not a good idea.

www.wescottdesign.com

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upsidedown 14 years ago

According to the Motorola McMOS Handbook (1974) the MC14xxx series inverter input capacitance varies with input voltage due to feedback effects.

At VDD=10 V the capacitance is about 4 pF at Vin=0 or 10 V. However, there is a distinct peak between Vin=5 .. 6 V, peaking to 11 pF at 5.5 V.

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