Re: [Xylo-SDR] A/D anti-aliasing filters

[Lyle Johnson <kk7p@wavecable.com>]

> Specifically: the TI / Burr Brown PCM1804 has a built in "linear phase 
> antialias digital filter" that is fairly sharp and very deep. Does it 
> require any additional filtering when connected to the capacitors of a 
> quadrature sampling detector, like on Flex Radio? It almost seems like 
> it will do more harm than good. Does anyone have practical experience on 
> this?

The anti-aliasing filter in the 1804 is an integral part of the overall 
ADC design.  Remember that this is a sigma-delta converter, and the 
filter characteristics are part of how TI achieves such excellent 
performance for this part.

The QSD is by nature a filter as well.  The 1804 filter won't get in 
your way as long as you are operating in its intended bandwidth.  But I 
think you'll find the cutoff of the 1804 is much sharper than the QSD.

To get the rated performance from the 1804, you must interpose a 
differential amplifier as outlined in the TI data sheet using very low 
noise amplifiers.  Not doing so will likely degrade the noise 
performance of the part.

73,

Lyle KK7P