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Thread: Real newbie question on ADC
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On 18 Aug 2002, at 0:26, Sean Alcorn (SYD) wrote:

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PIC has nothing to do with it - it is an EE question :-)

Used to work (servicing) expensive bench Digital Voltmeters (1uV resolution
on 10 volt range - work out the 'bits' equivalent for yourself).
They had a filter switch which basically just added an RC filter at the input to
get reduce some HF noise. BUT the integration time of the ADC was phase
locked to the mains frequency so that a reading was taken over one (or
multiple) cycles of the mains. This 'averaged' any mains ripple on the input to
near zero - assuming sinusoidal interference.
Basically if you have unknown frequency/waveform on your DC input then the
answer will be wrong regardless of filters.
If it is mains then you must look at the signal for e.g. 20mS (50Hz) or 40mS - I
am not sure if the PIC (or any) microprocessors have a facilty for setting
integration time...

regards,
George Smith

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