Q
The ADC noise tables on page 30 of the ADuC834 datasheet show that the
effective resolution
depends on the range and update rate selected. Why?
A
The ADuC834 features Sigma Delta ADCs. Sigma Delta ADCs use a method of
averaging (using a
sinc3 filter) to obtain the very high accuracy ADC conversion results. The more
averaging that is
done the more accurate the ADC conversion is going to be. With high update
rates the ADC gets to
perform less averaging in hardware on the chip. Hence the resolution will not
be as good. With
slower update rates more averaging is done on the chip and the resolution is
improved.
The resolution of the ADC also depends on the input voltage range. Noise added
to an input signal
with the ADC converting on the 2.56V range will affect the conversion result
much less (128 times
less) than the same amount of noise being added to a signal converting on the
20mV range. Hence
converting on a larger signal will produce an ADC result accurate to many more
bits.