RC Lowpass Filter between Amplifier and ADC input

If you are reading the ADC just once per second then you need to eliminate frequencies above 0.5 Hz to prevent aliasing. If you think your system will have noise at, say 10 Hz, then that noise will contaminate your readings. I recommend that you sample at a much higher rate, perhaps some multiple of the power mains frequency, and perform low-pass filtering in software. Even a simple moving-average filter would work and wouldn't take much processing.

ADC inputs typically are quite high impedance. I have often used 100K in series with an input with no dc loss. (and a capacitor to ground for filtering) If you are happy with the attenuation with that circuit then I would suggest scaling the resistors up and the capacitors down. I would not use electrolytic capacitors as they tend to have more leakage compared to other types. I would probably use a ceramic cap.

Edit:

I just looked at the data sheet for the part. Go take a look at page 3, Input Impedance. Loading certainly will not be an issue.

This 3-section RC should provide better rolloff at high frequencies. The random noise is dominated by that 3,000,000 ohm resistor with the 5Hz bandwidth, less than 1uV RMS.

^{simulate this circuit – Schematic created using CircuitLab}

Here is what Signal Chain Explorer (we used that to predict Gargoyles interferer levels) shows as the 3-pole rolloff. With 2 volts PP input, the ENOB is 19.7

Notice we are NOT including ANY ADC noise contributions.