Where to place a RC filter regarding decoupling capacitor?
Typically, the decoupling capacitor is the C part of the RC filter, and the non-ideal inductance and resistance of the PCB traces is the R part. Sometimes (and this is not terribly common), a very small resistance or inductance will be placed like this:



but if you have so much noise on your power rails that a 500 Hz filter is necessary, you probably have bigger problems. The datasheet lists the supply voltage rejection ratio as 75dB minimum, power supply noise should not affect your measurement very much.