Re: 8 channel E*G with AVR and bluetooth

Grant

I hope you realize that you need to use at least a 4-layer PCB for such a
design.  A high density IC such as the ADS1299 needs at least a solid
ground plane and a dedicated +3V3 plane or a mixed power only layer would
also be a good idea.

You cannot run the micro and a sensitive device such as the ADS1299 from
the same regulator without at least using some inductors and low ESR caps
to separate the noisy micro (switching noise) from the V-analog supplies of
the ADS.  Otherwise use two regulators. One for digital and one for analog
supply lines.

I personally would rather use a 16-bit or even an entry level 32-bit
processor.  I am sure the ATMega168 will be sufficient but the cost
difference these days is so small, it approaches nil.  I use Microchip and
don't know the Atmel products.  I do know that Atmel makes good stuff that
is arguably better than similar PICs.  But coming from the PIC world, 8-bit
processors are usually limited to the number of UARTS, SPI modules etc.  I
would think that this is also true in the ATmega world.  With 16/32 bit
processors you should be able to upgrade to bigger RAM/ROM devices in the
same footprint should you decide to do more fancy software at a later
stage.   But as I've said, that's just a personal view.

Study the ADS1299 datasheet and follow all recommendations to the T,
especially power supply recommendations.  This is REALLY important.  Use
10nF in parallel with 100nF decoupling caps on critical supply lines.  You
really can't spend enough time on power supply decoupling, cleaning and low
impedance ground paths when using sensitive analog, or mixed signal devices
such as the ADS129x.  Also keep any tracks or components that carry digital
signals as far away from the analog stuff.  Don't even consider using
polarized caps anymore.  Use high density ceramics instead.  They are
cheaper than tantalums and have orders better ESR.  They are also smaller
in footprint.

Regards
Stefan

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