filtering - How to decide to cuttoff frequecies of filter in case of using ADC( Flow: Analog-signal to ADC to bits to fir_filter to filtered_output) -


  1. fir filter has used removing noise. don't know frequencies of noise might adding analog feedback signal taking.
  2. my apparatus consists analog feedback signal using adc digitize value have apply fir filter remove noise, not sure noise noise added in analog signal environment or sort of noise comes there due adc ?
  3. i have code in vhdl.(this part easy can that).

my main problem in deciding frequencies.

thanks in advance ! tagging vhdl people working in vhdl might know filter.

let me start stating obvious: adc samples @ fixed rate , can not represent frequency higher nyquist frequency

step one: understand aliasing, , frequency higher nyquist alias signal noise. once understand need anti aliasing filter in hardware, in analog signal path before digitize it. depending on noise requirements of application may implement complicated 4 pole filter using op-amps; simplest use rc filter.

step two: setting filter cut off. don't set cutoff right @ nyquist frequency, make sure filter cutting before nyquist (1/2x... 1/10x, depends how clean , how noise present)

so you're kind of on sampling signal: filter cutting above signal, , sample rate high enough such nyquist frequency sufficiently higher. on sampling kind of data, captured intent of filtering further, , possibly decimating (keeping on in n samples , throwing rest out)

step three: use filter further remove noise between initial cut off of anti-aliasing filter , nyquist frequency. science on it's own really, let me start suggesting decimation filter: averaging 2 values. it's box-car filter of order 2, known sinc filter, , can re applied n times. after n times equivalent of fir using values of nth row in pascal's triangle (and divided sum).

again, filter choice science on it's own really. extreme decimation filters of sigma-delta adc. cs5376a datasheet explains they're doing; learn quite bit reading datasheet!


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