Comments on Cutoff frequency of lowpass filter for AVcc
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Cutoff frequency of lowpass filter for AVcc
The ATmega16U2 datasheet suggests putting an external lowpass filter from Vcc (the digital supply) to AVcc (the analog supply).
However, the datasheet doesn't specify what type of lowpass filter (passive/active, RC/LC) should be used, neither does it specify the cutoff frequency fc. Here is my current schematic where I have used a an RC lowpass filter with fc = 159 Hz (schematic still in progress). Is this a good value?
What determines the filter type (passive/active), and what determines the cutoff frequency for the lowpass filter in this situation?
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Another option entirely is to add a voltage reference IC. There are two flavours of those: "series reference" and "shunt reference". Simplified, the former is a linear regulator of sorts and the latter is a zener diode of sorts. An article from AD here: Series or Shunt Voltage Reference?
As noted in the article there are a few subtle differences between the two making them more suitable in certain cases. But no matter which one you pick, you can go with one that's slightly below the ordinary VCC supply of the MCU. Most often the ADC pin is specified to not tolerate higher voltages than the main supply, but you can usually go far lower. Depending on the nature of the voltage you are measuring of course.
For example, for 5V supplied MCUs with 12 ADC, it is common to pick a voltage ref with 4.096V, since that would be an even divisor of 212 leaving you with a handy unit of mV in the software.
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