There are also some guidelines in
http://ww1.microchip.com/downloads/en/DeviceDoc/41215B.pdf

Tip #10

They say that the output will be limited to 1/2 of the output of the output
from the oscillator.

Thanks,

Gordon Williams

----- Original Message -----
From: "Jinx" <joecolquitt@clear.net.nz>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Thursday, December 07, 2006 9:56 PM
Subject: Re: [PIC] ADC voltage mismatch problem


>
>
> > What kind of capacitor and size would you consider suitable?
>
> I'm of the suck-it-and-see school of components so maybe
> I haven't got this quite right. I'm happy to be corrected so I
> may learn
>
> Your circuit, using 25mA at 3V3 => 3.3/0.025A = 132ohms
>
> Say you have a 100uF capacitor at 5.0V. The RC time constant
> is 132R * 0.0001F = 1.32ms, 5 times which is what you would
> allow to get the voltage up
>
> Charge (Q) is C*V = 0.0001 * 5.0 = 500 uCoulombs
>
> Assuming it takes a generous 50us to do the ADC, in 50us
> the capacitor will have lost
>
> 0.025A * 0.000050s = 1.25uC
>
> => charge is now 498.75uC or 99.75% or 4.9875V
>
> If the initial charge was to a little over 5.12V, the actual voltage
> would have dropped to 5.12V at the time of acquisition, making
> calcs a little easier and no fractions of bits to help results
>
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