Highpass at low frequency
The cutoff frequency of a high pass RC circuit is given as ω = 1/(R*C) where ω is the angular frequency. This formula will yield the -3 dB point (20*Log10(VOUT/VIN)). The roll-off will be about 20 dB / decade.

When creating a high pass filter involving an A/D converter, the input impedance of the A/D can serve as the resistive portion of the filter. So for example, if you set your cut off frequency at 0.2 Hertz and you have a 10k ohm input impedance for your A/D converter, the required value for the series capacitor would be ~80 μF.