where 𝑋 1 (𝑛 -1) and 𝑌 1 (𝑛 -1) are the previous values of 𝑋 1 and 𝑌 1 and 𝛼 is a weighting coefficient that determines the cut-off frequency of the exponential filter. (𝑋 1 and 𝑌 1 are arbitrarily set to zero prior to the first sample). 𝛼 is set to a constant value of 7.85 × 10 -6 , which corresponds to a cut-off frequency 𝑓 𝑐 of 0.25 Hz and a time-constant 𝜏 of 0.63 s. The exponential filtering stage is repeated using the same 𝛼 value to provide better attenuation of non-DC interferences and hence a 'cleaner' solution: