Same here - 3.5kHz Sallen & Key would make a great anti-aliasing filter.Use an analog lowpass with a corner at 3.5 kHz. and the digital algorithm on the output of the LPF. That's what I would do.
Thank you ! I have been doing exactly same. Instead of using analog LPF with cutoff 3.5 KHz , I am using digital LPF with cutoff of 3.5 KHz (sampling rate of 1 MHz) and it is works well. However, the digital HPF is not working and i suspect it's because of Low cutoff (1.1Hz) and high sampling rate (1 MHz).Use an analog lowpass with a corner at 3.5 kHz. and the digital algorithm on the output of the LPF. That's what I would do.
Roll-off frequencies shall not deviate more than 0.2 dB, the number of poles required is 2 ( 40dB / decade).How critical is the accuracy of the roll-off frequencies, and what roll-off (number of poles) does the system require?
Since when did we measure frequency in dB?Roll-off frequencies shall not deviate more than 0.2 dB, the number of poles required is 2 ( 40dB / decade).
My bad. I meant the amplitude. I am trying to replace an existing analog filter with a digital filter. I would definitely use Sallen and Key filter if I were designing an analog Filter.Since when did we measure frequency in dB?
0.2dB in amplitude would be 2.5%. A Sallen and Key filter is as accurate as its resistors and capacitors, and you should be able to find 1% tolerance capacitors without too much trouble.
Look! In words of one syllable or less: you cannot replace an analog filter with a digital one. It just won't work. The problem is aliasing -- look it up. The analog front end cannot be eliminated, so don't even bother trying.My bad. I meant the amplitude. I am trying to replace an existing analog filter with a digital filter. I would definitely use Sallen and Key filter if I were designing an analog Filter.
True.The analog front end cannot be eliminated
No argument there. Still the thrust of the TS's post is the elimination of any analog contribution at all.True.
But with a 1MHz sample rate, the upper alias frequency would be 500kHz, so you should just need a simple 1-pole, low-pass RC filter at somewhere above 3.5kHz in front of the A/D.
Moving average (running average) is a low pass filter right?If you think the sample frequency is the problem with the low frequency HP filter, can't you just average the A/D samples (say a running average of 10-100 samples) to give a lower sample frequency to work with for the digital filter?
Right.Moving average (running average) is a low pass filter right?
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