Lpc2000

At 09:01 PM 3/6/2006 +0000, Danish Ali wrote:
>The microchip switched-capacitor successive-approximation ADC
>takes a sample by charging up an internal capacitor (through an
>analog switch).
>When you tell it to convert, the analog switch is opened and then
>the conversion takes place on the voltage stored on that
>capacitor. (I do not know if the voltage on the capacitor remains
>unchanged or if it is left at a random voltage).

That's a description of a sample and hold circuit not a switched 
capacitor  SA convertor.  Most A/Ds work this way. The biggest exception 
would be those that require an external sample and hold circuit and 
possibly flash convertors. It would be possible to have a op-amp buffer in 
front of the sample and hold but I don't know of any micros off hand that 
do (OK there's an opportunity for someone :)

A switched capacitor convertor is probably the most common microcontroller 
convertor and I would be somewhat surprised if the LPC convertor was 
something different but that's masked by the sample and hold circuitry.

Note that the S/H cap also means that if you scan across channels too 
quickly you can introduce crosstalk between channels.

Robert


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