Comments on When do I need to put anti-aliasing filter in front of SAR ADC?
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When do I need to put anti-aliasing filter in front of SAR ADC?
Signal characteristics are:
- 0 to 3.3V span
- 0 to 100Hz range( there will be noise superposed on top with unknown characteristics)
- Output resistance Rout=4700 ohm
I want to sample that with a SAR ADC built in the MCU. ADC has following characteristics:
- internal sample and hold capacitance --> Csh = 7pF
- sampling switch resistance --> Rsh = 6kOhm
- sampling time range --> programmable from t_s = 0.1us to t_s = 16us
My idea was to sample the input signal at a much higher rate than needed and implement an RC filter in the firmware so I can modify it easily.
Is there a benefit and what would the benefit of putting an RC filter in front of the ADC be? Or I can implement it purely in FW without drawbacks?
Post
Anti-Aliasing filter depends on the following parameters;
- Signal BW
- Sampling Rate
- Sampling duration and thus BW of sample
- LPF attenuation of alias spectrum above 1. From your SNR spec or dynamic range desired for resolution
When sampling rates of 64x are used, often a 1st order LPF is all that is needed when 32x , a 2nd order filter, when 2x sampling rate, you need a brick wall filter that suffers from group delay distortion near bandedge. These are the benefits of over sampling (order vs SNR are tradeoff estimates only, not precise.)
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