If you've ever taken a smoke alarm apart you might have come across this
method also. On the one I looked at one of the pins from the main IC was
bent up and connected directly to the output pin of the sense module. I
guess this is also a very high impedance circuit.
I've also seem app. notes recommending that this type of connection can be
used by soldering the critical connections in midair-just like one of my
rats nests:-) (but not readily acceptable to production :( ). The moral of
the story is that there can be significant leakage across a pcb - especially
after a few years of use in a dirty environment.
In the case below however, as there is a unity gain buffer following the cap
connection, guarding would appear to be a more manufacturing acceptable
solution to any leakage problem.
Regards
Richard

> -----Original Message-----
> From: Brian Aase [mailto:ulcsa@ESKIMO.COM]
> Sent: Friday, May 28, 1999 1:49 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: Sample & Hold Circuit (fwd)
>
>
> Actually, polypropylene and Teflon capacitors have leakage
> performance superior to Mylar.  The most effective circuit
> I have come across used a polypropylene cap mounted up off
> the PCB on Teflon standoffs.  The IC pins were bent upwards
> so as not to touch the PCB at all.  Then the whole business
> was factory-cleaned in a vapor de-greaser, and a plastic
> box covered the whole circuit.  Hold time approached two
> hours (more or less) with this arrangement.
>
> > Aside from the suggestions given consider the following...
> >
> > Use a Polyester capacitor. Remember, capacitors, although
> designed for
> > energy storage do exhibit leakage. The lowest leakage variety are
> > Polyester...
> >
> > PCB layout also contributes. Ensure that the board is clean
> and isolate
> > the capacitor connections as best as possible from GND and
> POWER tracks.
> > Also please consider the use of a guard ring around the Ch
> pin of the
> > LM398, TOP and BOTTOM of board. Read the application notes
> carefully for
> > the device. All that I have mentioned is contained within the LM398
> > datasheet...
>-snip-----
> > > > >
> > > > > >Dear Netters,
-------snip-------
> > Nino
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>