DSP Cutoff vs Sampling frequency question on low pass filter
The basic formula for a simple filter relies on the ratio of time between samples and the time constant of the filter i.e. T\$_S\$/CR.

At 125MHz Ts is 0.000000008 and CR = 0.03 for a 5Hz cut-off.

Quite a few digital systems are going to produce errors unless you are working with decent floating point numbers. This is a simple digital IIR filter with a nod to the CR time of an analogue low pass RC filter: -



It's dead easy to implement in a spreadsheet and you can apply integer math (or whatever you are using) to see how it performs with the vast difference in sampling frequency and target cut-off frequency.