Lpc2000

At 09:41 PM 5/27/05 +0000, fl429 wrote:
>I did not see any specification in the Datasheet on the input
>impedance of the Analog inputs. When I switch A/D channels, this
>impedance will determine how long the sampling time should be before
>the conversion can start. When in Burst mode, there is no mention on
>the sampling time at all.

Since this is really a single A/D prefixed by a sample and hold and they 
don't mention any input buffering it is VERY unlikely that the inputs are 
high impedance.  The usual configuration would effectively simply be a 
sampling cap preceded by a couple of switches with a finite 
resistance.  Unfortunately, I don't think either of those values are 
documented.  Hint to Philips those are useful figures to have, even if only 
a range is provided.  In particular it would be really nice to know what 
the sample capacitance upper limit was.

>Can I assume that the 11 clocks already take care of this ?

Not w/o a little bit of care.  What I usually do is use an RC filter as the 
last, or only, element of my input Nyquist filter.  If the cap on that is 
next to the input and reasonably large it provides a large enough charge 
storage that it effectively nulls out the source impedance.  IE the voltage 
change from a 0.1uF cap charging or discharging the sample cap, which is 
usually only a few pF, is less than a single bit.  At that point the only 
element that limits the input sample time is the RC of the input switches 
and the sample cap.  The response time of the signal itself will be limited 
by the RC of the input filter.

One of the clearest outlines I've seen of the typical input interface is 
actually in an Intel app note: 
http://www.intel.com/design/mcs96/datashts/27211501.pdf  They don't, 
however, mention the need for Nyquist filtering to reduce sampling artifacts.

Robert

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