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[dsp] Analog front end design

Scott Miller scott at opentrac.org
Wed Apr 20 17:31:34 UTC 2005

Got some questions on analog front end design and CODEC selection.  My
project is a dual-port TNC - I'm mostly concerned with 1200 baud support at
the moment, but I do plan to support 9600 baud.  Not sure exactly what
flavor yet.

I was looking at a nice dual-channel 16-bit CODEC from ADI (AD73322L), but
the problem I ran into with that chip is that it has a limited selection of
sample rates, and they're not baud rate multiples.  Maybe the clock could be
changed, but I don't know enough about sigma-delta ADC design to know if
that's really a good idea.  Another possibility is to use the MCU's on-board
ADC, but it would require the addition of a buffer and it'll only do 10 bits
of resolution.  I'm not sure that'll give enough dynamic range, especially
if I need to do much filtering in software.

Another CODEC from TI/Burr-Brown might be an option - the PCM3501 is
specifically designed for modem applications and has a more suitable
selection of sample rates.  It's got a much lower maximum rate than the ADI
part at only 26 kHz, but maybe that'd be enough.  It's also only a
single-channel device, so I'd need two of them.  It'd probably be a more
expensive solution than the ADI CODEC, and would take more board space.

While the on-board ADC option would be cheapest (and I think it'd be
interesting to do a 1200 baud only version without a codec) if the dynamic
range is sufficient, it'd also complicate the anti-aliasing.  The ADI and BB
chips have decimation filters and would save me some trouble there.

I might also keep the 1200 and 9600 inputs separate, each with their own
filters, and use an analog switch to select the appropriate input.  Or just
use a separate ADC input for each and select in software - I've got enough
inputs available.

Suggestions?  Would either of these chips be suitable?  Is there another
that'd be a better option?  Is a 10-bit ADC sufficient?



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