AVR-Chat

----Original Message----

From: AVR-Chat@yahoogroups.com
[mailto:AVR-Chat@yahoogroups.com] On Behalf Of leon Heller
Sent: 27 October 2008 14:17 To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Inductor at AVCC

> ----- Original Message -----
> From: "Enki" <enkitec@gmail.com>
> To: <AVR-Chat@yahoogroups.com>
> Sent: Monday, October 27, 2008 1:40 PM
> Subject: [AVR-Chat] Inductor at AVCC
> 
> 
> > 
> > The ATMEGA48 datasheet shows a 10uH inductor connected
> > to AVCC pin for filtering. 
> > 
> > Why 10uH?  Could I use 220uH?
> 
> They probably use 10 uH because it works better than
> anything else. 
> 

Or that's the value they had lying on the bench at the time and it
worked ok.

-- 
Tim Mitchell

Generally speaking, more inductance is better.

In reality, more inductance means also more resistance, and more
interwinding capacitance.
So, in a given system, over a range of similar type inductors, one
particular inductor value will perform best.

If you have a bulk supply available at higher voltage, you might use a
separate linear regulator to supply this pin.
Their PSRR figures vary, but some pretty high values are available.

I'm using the separate regulator approach in an extreme low light
video camera.  I use two ISL-9000 chips, which have PSRRs at about
90dB.
They are fed by a switcher which outputs just enough voltage to keep
the ISL-9000 in regulation (keeping my efficiency high), and runs in
the frequency range where the ISL-9000 has maximum PSRR. The switcher
is also very well filtered.   In the camera chip, there are two
digital supplies, and two analog supplies, so I use one ISL-9000 for
digital, and one for analog.  The camera chip has NO  PSRR on the
sensitive analog supply pins, so 1mV of noise there translates to 1
bit of noise in the picture.  The ISL-9000 is the best regulator I've
found for PSRR, it beats anything that Linear is producing, and AFAIK
anything from National, TI, etc.

Engineering IS the art of compromise.  Cost, size, weight,
complexity.. Figure your priorities, and evaluate different
approaches.


As far as the inductor again, I have also used a 1 ohm resistor, and a
high quality cap.