I'm content to have learned a little ...

I'm not done with you yet. You're gonna learn more and better, if you can get all the way to the end of this post.

a 0.1uF cap on the voltage divider for fc = 1 / 2(pi)(10,000)(0.1x10-6) = 159 Hz

That will probably work just fine. But, the appropriate calculation for the rolloff frequency is

f_{c} = 1/(2*pi*0.1x10^{-6}*1800) = 883 Hz

The effective impedance of the voltage sensing circuit is not 10k - it's the parallel combination of the 10k and 2.2k resistors, and that comes to about 1.8k, calculated to a reasonable precision. If you want a corner frequency at about 160Hz, you'll want to recalculate that capacitor.

One way to calculate the effective impedance is to consider the circuit with all the voltage sources replaced with short-circuits, and figure out what the resistance is looking into the circuit from its output. For this circuit, that's a 10k and a 2.2k in parallel, about 1800 ohms.

The other way to find the equivalent impedance of the voltage divider is to develop the Thévenin equivalent circuit. Here's how:

- Calculate the output voltage when the output is an open circuit - for this circuit, that's 12V*2.2k/(10k + 2.2k) = 2.16V.

- Calculate the current that the circuit would deliver to a short circuit at it's output - in this case, across the 2.2k resistor - for this circuit, that's just 12V/10k = 1.2mA.

- Divide the open-circuit voltage by the short-circuit current to get an effective impedance - in this case, 2.16V/1.2mA = 1803.28 ohms, to a ridiculous precision.

Thévenin's theorem says that a network of voltage sources, current sources, and impedances can be modeled as a single voltage source in series with a single impedance. The only difference between an actual network and its Thévenin equvalent is the amount of power that's consumed in the internal impedance. Otherwise, from the outside of the network, we can't tell the difference.

I bring all this up for two reasons: because it's extraordinarily useful and you don't seem to know it; and, because Thévenin's name is invoked only rarely in this forum - I find eleven references total, only five in English, and only a couple of those offering any real information. Without an understanding of Thevenin's theorem, it's hard to do much of anything other than guesswork in analog electronics. The Wikipedia article is here:

http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem ... output of the differential amp ... prefer to minimize voltage drop ... I imagine that the current between that op amp and the ADC is minimal ...

When you say that the ADC current is minimal, you minimalize its minimalness. It's really small. The ATmega328 datasheet says that the ADC is optimised for an input impedance of 10k or less. You've got about 9.9k to go. With a resistor 100 times as big, the capacitor could be 100 times as small, or, 0.1 uF. It'll be cheaper, take up less space, and be more reliable in the long run.