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From: johne@hp-vcd.HP.COM (John Eaton)
Newsgroups: sci.electronics
Subject: Re: Crstal oscillator load capacitance
Message-ID: <2470006@hp-vcd.HP.COM>
Date: 2 Jan 91 16:46:34 GMT
References: <1990Dec21.193438.4172@mdivax1.uucp>
Organization: Hewlett Packard, Vancouver, WA
Lines: 25

<<<<
< For the circuit above, the load capacitance should be
<
<	Cload = C1 + C2 + 2 * pin capacitance
< 
< Now most design examples use C1 and C2 in the range of 20 to
< 33 pF, and pin capacitance is approx. 10 pF for most ICs, so
< Cload should be in the range of 60 to 100 pF. However, the
< standard Cload values for stock crystals (and the design
< examples I've seen) usually have Cload in the range of 20 to
< 40 pF.
----------
Cload is the capacitance as seen by the crystal.


   Cload = (C1+pinCap)*(C2+pinCap)/((C1+pinCap)+(C2+pinCap))


If you ignore the ground for a moment you will see that the
two caps are in a series (not parallel) configuration as 
seen by the crystal.


John Eaton
!hpvcfs1!johne