ADC input filter design for DC input signal
An anti aliasing filter should be always present. It would be better to sample at high sample rate, use a FIR filter to eliminate the environment noise 50/60Hz, thus the anti-alias filter can be a small RC with low TAU value. Sampling at 64sps would imply large RC filter and it is very close to 60Hz, so most probably you will pick lots of noise/garbage.
See this example:
Youtube , source PDF
You could use 1kHz sampling rate with a 300Hz cutt-off anti-alias RC filter.
$$f_{cuttoff}=\frac{1}{2\pi RC}$$
You said, your source has low impedance, so you don't need a buffer. Let we neglect the source impedance (=0) and we calculate as the entire resistance is the filter resistance R=1kOhm.
$$C=\frac{1}{2\pi Rf_{cuttoff}}=\frac{1}{2\pi\cdot 1k\Omega\cdot 300Hz}\approx 5.3\cdot 10^{-7}F\approx 0.56\mu F$$



simulate this circuit &ndash; Schematic created using CircuitLab
Above it is an anti-alias filter 300Hz cutt-off. Then you sample and filter with low pass FIR filter, for example 128 taps. You would get a precise measurement without environment noise.