Hi,
I'm looking at PCM1808 and the internal antialiasing filter.
According to the datasheet the cutoff frequency for the LPF is 1.3MHz and the delta-sigma modulator is 64 times FS.
Does that mean that an external LPF is needed if FS is less than approx. 41kHz?
I will be using it with FS at 16kHz
Best regards
Gert Jensen
Hi Gert
According to this they would not want filter to go below 1MHz -3dB cutoff, so I take back implying that the low sampling rate would require better antialiasing, adding an external LPF would just be required if the current 1.3MHz is not satisfactory for your application overall independent of the sampling rate.
So leaving without LPF would be fine in terms of performance unless you especially want to maximize antialiasing though the reduction to 1Mhz.
Best Regards,
Carson
Yes sorry Gert you are right, my math was wrong.
I cannot say that the 1.3MHz is based off Fs x 44.1kHz, if anything it is based off it being about (2^16) multiple of audible band of 20Hz. Which I would guess would be correlated
The antialiasing filter is set at what it is because that give optimal THD+N when paired with the other ant-noise systems of the device. Adding a LPF to your device is not really a matter of what sampling rate is chosen but more so the incoming signal you will be sampling and if a significant amount of noise and harmonics will be present below that 1.3MHz range.
But of course if you are using a low sampling rate I imagine you signal of interest will be below 8kHz, so adding a filter of 64*8kHz could benefit overall performance but how much would actually change performance is really dependent on application specifics if it would be worth adding in that analog circuit and wouldn't say it is a requirement based on sampling rate chosen, hence why we say this below:
Best Regards,
Carson
