Member "gf" told me not make another thread so I post this question here.
I rather meant that all this stuff would better fit into your "Audio ADC" thread which already did exist before. These questions are not related to the topic "Most accurate signal generator".
There is another topic that I've been thinking for over a month. Can you share any software that can demonstrate the power of digital filter especially oversampling that can apply lowpass or bandpass filter with brick wall frequency response. My BMA-200 that uses the AMP01 has 2 order Butterworth filter with -12dB/octave response while my gtec USBamp has no amplifier but only ADC that directly maps the microvolt signal at +- 250mV. I'd like to know if it's oversampling (ability to do moving averages) capability can make me see signal that I can't with the BMA (between 1 to 2400Hz), and worth spending $3000 just to get the software to run it. Here is the spec of the USBamp. Pls share sofware to demonstrate the power of oversampling via digital filter. How much can it beat 2 order Butterworth filter with its almost brick wall response?"
Remember that a 2nd order Butterworth filter with less vertical low-pass edge only adds about 7.5% noise compared to the digital brick-wall filter.
You answer the question yourself. The equivalent noise bandwidth of a 2nd order Butterworth is about 1.08 x cut-off frequency, so the difference is not large.
If your Butterworth filter is not digital, but resides in the analog frontend, then additional noise may be introduced in the signal path between the filter and the ADC (including ADC noise). Then an additional digital filter may help if you don't need to full 0...sample_rate/2 bandwidth. [ But don't overreact before you have evidence that this is really relevant in your setup -- it could also be negligible. ]
So does digital filter oversampling only remove the noise above the cut-off...
Any filter removes (or better say attenuates) frequencies outside its passband. A filter cannot distinguish useful signal and noise, but only frequencies. Consequently, without attenuating components of the useful signal as well, a filter can only eliminate/reduce noise in a frequency band which is not covered by the useful signal.
https://www.gtec.at/product/gusbamp-research/
"Each of the 16 analog to digital converters operates at 2.4576 MHz. Oversampling 64 times yields the internal sampling rate of 38,400 Hz (per channel and for all channels!). In addition, a powerful floating point Digital Signal Processor performs oversampling and real-time filtering of the biosignal data (between 0 Hz – 2,400 Hz). Therefore, a typical sampling frequency of 256 Hz yields an oversampling rate of 9,600. This results in a very high signal to noise ratio, which is especially critical when recording evoked potentials (EP) in the EEG or identifying small amplitude changes in high-resolution ECG recordings. You are measuring far below the noise-range of conventional amplifiers."
Any delta-sigma ADC is based on oversampling and subsequent decimation to the output sample rate (which implies digital filtering), and that includes all today's audio ADCs. That's not special, but that's simply how a delta-sigma ADC works. And the mentioned "typical sampling frequency of 256 Hz" is not suitable for you, as it would limit the bandwidth to 128Hz (theoretically, and in practice even less, say 100...120Hz), but you want 1000Hz bandwidth for the useful signal. The low sample rate is not relevantt anyway for the noise consideration, but only filtering matters, and filtering can be done at a higher sample rate, too [although it requires more processing resources then].