Lpc2000

Sorry if this is an obvious question.  But, I have looked at the User 
Manual, and all other documentation I have and can not find a 
specification for input impedance on the ADC input pins????

Does anybody know what this impedance is??

I am going to be feeding a signal with a 200k impedance to one of the 
ADC inputs and am trying to figure out if I need a buffer or not.

Thanks,
Dave Butler

At 04:48 PM 6/6/05 +0000, drb5599 wrote:
>Sorry if this is an obvious question.  But, I have looked at the User
>Manual, and all other documentation I have and can not find a
>specification for input impedance on the ADC input pins????
>
>Does anybody know what this impedance is??

Unfortunately I don't belive that's documented anywhere.


>I am going to be feeding a signal with a 200k impedance to one of the
>ADC inputs and am trying to figure out if I need a buffer or not.

Well, something.  An op-amp if that's the kind of buffer you are thinking 
of may not be needed.  Depending on your scan rate a capacitor or better, a 
small rc filter may be sufficient.  If you are only converting on this 
channel at 100 Hz or so a 1k /.01uF cap will provide some noise and Nyquist 
filtering and give you enough charge storage that you won't need the 
buffer.  You would need the filter anyway so....

It's likely that you will want the charge storage anyway so that you don't 
need an unreasonably fast op-amp.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,   be 
they legal, genetic, or physical.  If you don't believe me, try to chew a 
radio signal. "  -- Kelvin Throop, III
http://www.aeolusdevelopment.com/

I'm not sure, that You know if high source impedance will increase 
sample collection time.

drb5599 wrote:

> Sorry if this is an obvious question.  But, I have looked at the User 
> Manual, and all other documentation I have and can not find a 
> specification for input impedance on the ADC input pins????
> 
> Does anybody know what this impedance is??
> 
> I am going to be feeding a signal with a 200k impedance to one of the 
> ADC inputs and am trying to figure out if I need a buffer or not.
> 
> Thanks,
> Dave Butler
> 
> 
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
>  
> 
> 
> 
>

--- In lpc2000@yahoogroups.com, rseku <rseku@p...> wrote:
> I'm not sure, that You know if high source impedance will increase 
> sample collection time.

Yes, Good point.  I will not be sampling this pin very fast at all, 
so speed is not really the issue in this particular application.  I 
am really more concerned with making sure the impedances are not 
completely out of range.  I would like to assume it is in the 10k 
range????
I think I am going to use a buffer to start out with, I would really 
like this proto to work.  
Thanks for the input.
-Dave

> 
> drb5599 wrote:
> > Sorry if this is an obvious question.  But, I have looked at the 
User 
> > Manual, and all other documentation I have and can not find a 
> > specification for input impedance on the ADC input pins????
> > 
> > Does anybody know what this impedance is??
> > 
> > I am going to be feeding a signal with a 200k impedance to one 
of the 

> > ADC inputs and am trying to figure out if I need a buffer or not.
> > 
> > Thanks,
> > Dave Butler
> > 
> > 
> > 
> > 
> >  
> > Yahoo! Groups Links
> > 
> > 
> > 
> >  
> > 
> > 
> > 
> >