AVR Freaks

If the chip use has a USART bring that out to a small header even if not used.

• jumper on the power rail in various places so as to be able to measure the current drawn in major sections
• jumper to isolate drive signal to fets or transistors so that drive can be applied manually or via external signals
• grounding resistors on base or gate (N-Fet) so that those devices do not switch on randomly

Pads for an inductor on AVcc.

1. Where both analog & digital circuitry exists on the same PCB, place ferrites or small inductors between digital & analog circuitry that share Vcc, Vee and analog & digital common ground returns, to reduce digital noise levels on the analog side of things.

2. Where both analog & digital circuitry exists on the same PCB and where elevated analog & digital return currents are present, tie all common analog & digital ground returns carrying elevated current flow to a single common entry point on the PCB, minimizing cross coupling of ground noise into the analog side of things.  This would include running separate ground returns from on-board power supplies - run a separate "isolated" ground return from each voltage regulation system to a common point and the place of power entry.

3. The supply lines between the raw DC power source and the PCB should be ran with the largest gage and the shortest length as practical.

4. Where both analog & digital circuitry exists on the same PCB, it is critically important that each digital IC have an appropriate "Despiking" capacitor across the Vcc & GND and Vee & GND pin pair and placed as close as possible to the IC, itself.

5. When analog & digital circuitry both share the same PCB, it is highly suggested that, where possible, the analog circuitry and the digital circuitry be keep in separate sections of the PCB.  This facilitates reduced interference to analog circuitry due to radiated digital noise.  Isolation of the analog & digital circuitry also helps with PCB power buss isolation and management, as well.

6. Where analog or digital signals breach the boundaries of the others sphere of influence, I.E. an analog signal breaching the digital sections and sphere of influence, that analog signal should be guarded on both side along it's path of travel, helping guard against magnetic radiation from the digital circuitry.  The same goes for a digital signal entering the analog section.

7.  Where greater immunity of external signal radiation is critical, micro-strip (also known as strip-line) methods should be used to further manage analog-side noise immunity and increased digital-side waveform quality, better transmission line impedance control and reduction of radiated emissions.

8. When looking at digital signals capable of sub-nano second rise & fall times, use a scope probe whose GND lead is as short as possible.  This helps reduce inductive ringing, post transition, providing a more accurate representation of the digital signals you are attempting to observe.  It also aids in timing measurement, as well.

In my experience over more than four decades of bench work, lack of attention to good PCB power buss structure has been a very common design oversight.  It seems engineers often take great pride and joy in the details of the exotic, and the fun stuff.  I've often heard engineers say "Oh, I can get a power supply anywhere!", never realizing that good on-board PCB power buss structure concepts are even more important then the exotic or the fun stuff.  In fact, I've seen a few really great designs fail because of poor PCB power buss structuring and the PCBs had to be totally re-laid out, from scratch.

Not only will these guidelines help in establishing accurate, stable analog ADC readings, these practices will help ensure that all on-board analog circuitry exhibits the most noise free operation possible. 

For ADCs, such as what is within the 8-bit family of AVRs, the layout isn't terribly critical, unless you are measuring say, micro-volt levels and then amplifying those levels to be compatible with the AVR ADC operating levels.

Where it really gets stinky is when you are using external ADCs that are say, a 12, 14 or 16 bits.  Where the minimum design requirements of the AVR ADC would be acceptable for the AVR ADC, using a 12, 14 or 16 bit ADC under the same PCB layout philosophy would be a waste of the higher bit ADCs.  The reason is  because the least significant 2, 4 or 6 bit of the 12, 14 or 16 bit ADCs would probably get lost in the noise.

The fact is, using a 12 bit ADC, you could probably get away with just designing up a double sided PCB, with careful layout.  But with 14 and 16 bit ADCs, a multi layer PCB would be a must, where the ADC analog input signals and other analog signals could be better protected and routed.  This is also where the analog and digital GND planes would most definitely require some sort impedance between the analog and digital GNDs, such as a Ferrite or small inductor.

The key here is impedance and not just simply resistance.  Impedance, while related to resistance, differs from resistance in that, a typical resistor passes both DC and AC current nearly equally well.  But when using a Ferrite or inductor,  these passive components readably pass DC current undisturbed but they react to AC current by way of inductive reactance: XL = 2*Pi*F*L. where, 2*pi = 6.28, F is the frequency of the noise and L in the inductance of the ferrite or inductor. 

To determine the actual impedance (Z), the equation is: Z = sqr(R^2 + L^2).  Where if viewed as a right triangle, the adjacent side (or base) = R, the opposite side = L and the hypotenuse = Z.  Or, as those goofy mathematicians call it, Pythagorean Theorem.

http://www.electronics-tutorials.ws/inductor/ac-inductors.html

http://en.wikipedia.org/wiki/Pythagorean_theorem

Ideally, at DC levels, the impedance will be very near zero Ohms.  But at those pesky frequencies that might cause problems in a mixed analog/digital power bus structure, you want the impedance between GND systems to be as high as possible, but still practical.

The Screen shot that you have attached would be an improvement to running just a free trace and it will help to isolate channel to channel coupling, as well as providing some shielding from nearby radiated energy.