Lpc2000

Hi Richard,
Microchip do more than that - they give an explanation of why they
suggest that impedance.
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).

When the conversion is complete, the analog switch is closed again.
The impedance of your analog source (and the switch impedance)
determine the time it takes to charge the internal capacitor before
you dare start the next conversion.
This is known as the acquisition time.
It is up to YOU to provide the delay for the acquisition time. The ADC
has no hardware to assist you.
Also if you change the analog source, you have to wait the acquisition
time before initiating a conversion - or you will get a reading
somewhere between the old source voltage and the new source
voltage.

I have not seen anything that tells me the lpc family do A to D
conversion in a similar way. But figure 7 of the LPC2148 data sheet
gives values which can be interpreted in the "switched-capacitor"
way:
5pF is stray capacitance of the pin, the bond pad and the bond wire.
20k is the on-resistance of the analog switch
3pF is the capacitor you are trying to charge during acquisition.
So the acquisition time is the time it takes to charge that 3pF
through (20k + your source impedance) to the desired precision.
Leakage current would be dominated by the "digital" leakage
of +/- 3uA.

Does anyone know how the Philips 8051 family ADC behaved?
I suspect the LPC2000 ADC is based on that one.

Hope this helps,
Danish
--- In lpc2000@yahoogroups.com, "rtstofer" <rstofer@...> wrote:
>
> 
> Other manufacturers (notably Microchip) specify that the source
> impedance of analog signals must be below 10k ohms.  I certainly
> wouldn't be driving the converter with 100k.
> 
> Richard
>