I am a newby to the world of electronics and IC chips, and am looking to make an ATMEGA328 based board to monitor 12v batteries. The board will be powered from the batteries being monitored, with a regulated power supply taking 12v input and outputting 5v to the board. The data sheet for the ATMEGA328 chip says that AVcc should be connected externally to Vcc via a low-pass filter. A bit of internetting tells me the low-pass filter will filter out noise above a specific frequency, and provides links to calculators that show the affect of different resistor / capacitor pairs.

My problem is that, I have no idea what frequencies I need to filter! All / any advice much appreciated.

Further info: voltage measurements will be via voltage divider circuits, amp hour consumption will be via sensors, all of which will be connected to analog pins for ADC conversion.


The filter is only needed if low noise is important to your application. For monitoring batteries, you probably don't need the accuracy level at which you would want the filter. For what it's worth, the Arduino Uno doesn't have such filter: it's AVCC is connected straight to VCC.

If you really want the filter, the exact characteristics are not critical. The datasheet suggests 10 µH + 100 nF, so this should be a safe choice.

Edit: To answer the question in the comment, yes, it is generally advised to use decoupling capacitors, usually something like 100 nF, even when powered from a voltage regulator.

Notice that both the VCC and the AVCC pins are very close to a ground pin. Best practice is to use one decoupling cap for each (A)VCC/GND pair, and to place that cap as close as possible to the pins.

  • many thanks for this, and apologies for not spotting this in the data sheet. If I choose not to insert the low pass filter, would there be any benefit in fitting a low value capacitor, to smooth the AVcc input, or should I expect the output from the regulated voltage supply to be okay? – Colin Jan 27 '18 at 19:22

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