I am laying out a pcb with an ATmega32U4, where I am using pins 26-30 as analog inputs. Pins 36-44 can also be used as analog inputs, but I'm using them for digital IO and JTAG. Both of these groups of pins have a nearby AVcc pin, which is used to power the analog part of the micro, and as a voltage reference for the ADC. The datasheet says that this pin should be separated from Vcc by a low pass filter, which makes sense, as that will reduce the ADC noise. But there are two pins, so should I:
- Use a low pass filter on each pin (adds parts where space is at a premium)
- Use a single low pass filter and connect the two pins together (awkward, as they are on opposite corners).
- Filter the one near the inputs I'm using and leave the other not connected.
- Filter the one near the inputs I'm using and connect the other to something else.
- Filter the one I'm using with a LC filter, and the one I'm not using with an L only. I've seen this on someone else's design, but I don't understand the logic behind it.
Now, it seems to me that if the two pins are connected internally, I would get good results from option 3. But although most of the datasheet refers to AVcc in the singular, there is a caption on figure 24-10 which suggests I need to filter both pins. Though maybe only if both sets are in use?