If you've got a decent sized filter capacitor (e.g. 10n) between the input pin and ground I imagine that would constitute a 'very' low impedance to charge anything inside the Mega88 even if the capacitor was fed from a series 100k.
Dave Pallant
AVR-Chat@yahoogroups.com wrote on 13/09/2005 10:14:40:
> � � From the Mega88 datasheet:
>
> � � "The ADC is optimized for analog signals with an
output
> impedance of approximately 10 kOhms or less. If such a source is
> used, the sampling time will be negligible. If a source with higher
> impedance is used, the sampling time will depend on how long time
> the source needs to charge the S/H capacitor, with can vary widely.
> The user is recommended to only use low impedant sources with slowly
> varying signals, since this minimizes the required charge transfer
> to the S/H capacitor."
>
> � � You have to use a low impedance source.
> � �
>
> � � "Signal components higher than the Nyquist frequency
(fADC/2)
> should not be present for either kind of channels, to avoid
> distortion from unpredictable signal convolution. The user is
> advised to remove high frequency components with a low-pass filter
> before applying the signals as inputs to the ADC."
>
> � � Care must be taken here. Better use an OpAmp filter.
>
> � � Mark Jordan
>
> On 13 Sep 2005 at 9:44, david.pallant@prometheanworld wrote:
>
> > 100 Megohm on the mega88. I call that high as well!
> > Dave Pallant
> >
> >
> > AVR-Chat@yahoogroups.com wrote on 12/09/2005
19:15:56:
> >
> > > On 12 Sep 2005 at 14:04, kholt@sonic.net
wrote:
> > >
> > > >
> > > > Yes, I always put in an external
RC between any sensor and the
> > > > A-D input. It's a bargain in space
and cost, taking advantage
> > > > of the high input impedance of
the A-D (i.e.no opamp required).
> > > > Ken
> > >
> > > �Hummm, what do you call "high
input impedance"?
> > >
> > >
> > >
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