Hi all,

just wondering, is it usually necessary to have a digital fitler? I was told that in general the on chip analog filters we design are not good enough for most applications. Therefore, people usually design an analog filter just good enough for ADC, and then use a digital filter since a digital filter is able to better approximate a brick wall response...

Is this true?

thanks,
Aaron

Aaron,

That isn't true at all.  There are plenty of applications for which analog filters are not only "good enough" but essential.  Take for example and RF receiver.  There is usually an LNA, mixer, analog filter, then ADC.  The question is, how do you spec the required dynamic range of the ADC?  Assuming there is enough gain before the ADC, the ADC is specified by looking at the ratio of the peak expected blocker to the desired.  In some cases the blockers can be huge.  Without a good analog filter, the ADC may require enormous dynamic range and take a lot of power.  If you put a good analog filter in front of the ADC, you may be able to knock the blockers down my many dB, and relax the requirements of the ADC.  This is much more than simple anti-alias filtering.  There are also applications where analog filters are the only way to go due to speed requirements, such as in 10G Ethernet and high performance read channels.  There are important uses for both analog and digital filters in integrated systems.

aaron_do wrote on Jun 28^th, 2007, 6:32pm:


no, not always necessary, but again depends on your system (signal environment, modulation, demodulation technique etc. etc.).
For example doing demodulation in an FSK (like bluetooth) system using a discriminator, the only channel filtering you'll get will be in the analog domain.
 

Hello Aaron,

delta-sigma modulators are a popular ADC architecture for RF receivers. Using a delta-sigma ADC
greatly relaxes the anti-aliasing requirements and the subsequent digital decimation filter removes
shaped quantization noise and residual interferers at the same time.  You only need to make sure that the blocker's power
level are low enough to be within the dynamic range of the ADC.
In case of a nyquist converter you probably will need to reject interferers below the ADC noise level to
avoid anti-aliasing.

Regards