Computer Display?

[kellyseye]

Parallel a 0.1uF capacitor across the 47uF. Whilst the 10k pot to derive the 3V supply is very crude you show no decoupling of the 5V supply itself or the attached ICs - all of which are required.

The data lines to the PIC should be via twisted pair and, if necessary, decoupled to ground themselves.

[Charlie]

Parallel a 0.1uF capacitor across the 47uF. Whilst the 10k pot to derive the 3V supply is very crude you show no decoupling of the 5V supply itself or the attached ICs - all of which are required.

The data lines to the PIC should be via twisted pair and, if necessary, decoupled to ground themselves.

Good advice on decoupling 5V, but not good advice about adding 0.1 uF across the 47 uF. In some instances, for example using a large electrolytic capacitor with high ESR/ESL this could help with high frequency noise, however tantalum devices are likely as good as the ceramic in this case, so this will not do anything useful and will drive you crazy trying to solder it on. Worse, in some cases, you could introduce instability with a second pole in the filter, so this approach should be used with caution. And adding caps to the data lines will reduce the range and degrade the signal - very bad idea. Also twisted pair will be more harmful than helpful in this instance, since the signal is unbalanced.

Honestly, the problem is most likely the poor 3V supply - fix that before investing any time in other approaches.

[Ramius]

Hi All!
First thank all of you for all the great information, ideas, and opinions which are highly valued! I did find 3.0 volt regulators (MCP1700-3002E/TO) I would like to ask the group thier opinions on the best approach to decoupling. There are two methods possible using proto type boards (the boards with all the holes). First is to take the power and ground leads and run them parallel at the top of the board and then add connections to the I.C.'s from these "rails". The catch is that pin 1 is + power and pin 7 is ground. Thus the connection to ground would be long. The second approach is to take the ground and make a "C" shape buss with heavy ground wire so that the grounds would be shorter. What would be the advantages /disadvantages to either approach and is one approach better than the other when it comes to I.C. power and power supply decoupling? Thanks, Ed

[HenrikOlsson]

Hi Ed,
In your case I'd keep the GND connection as short as possible, the connection to the positive rail then becomes whatever length is required. Then place the decoupling capacitor right across the supply pins of the device or as close to the supply pins as you possible can.

/Henrik.

[Ramius]

Hi Henrik!
Thanks! One idea was to use 18ga buss wire for ground while 22ga for power. If I understand correctly then the negatives leads of the capacitors would be short and the positives would be long? Wouldn't this be allowing any spikes or signals to be on the positive supply rail? Thanks, Ed

[HenrikOlsson]

Hi Ed,
Not that I think it matters in this application but if you're going to use different gauge wire use the heavier gauge wire for the ground bus and the thinner for the power bus. The connection from the IC to the GND will be kept short, the connection from the IC to the power rail perhaps a little longer. The capacitor should be placed across the power supply pins of the IC and its purpose is to act as a local "power reservoir" for the IC as well filtering noise on the supply rail, bypassing it "down" to GND.

It's very important but really not that complicated for the general stuff we're tinkering with.

Here's a link to an article describing and showing how and why and here's a more technical app note from Analog Devices.

/Henrik.

[Ramius]

Thanks Henrik!
I basically understand what the intent is and from all the examples show the caps are mounted with equal lead lenghts which is where I became confused. (http://www.vagrearg.org/content/decoupling breadboard setup figure 4) Of course the ground will be connected directly to the ground pin of the I.C. It just seemed that with pulse widths indicating a frequency of 2.4 mhz that having parallel capacitor wires (caps on the 3.0 volt side and caps on the 5.0 volt side spaced about an inch apart and at a higher than ground potentials (3v and 5v) you would be inductively coupling the signals thus the need for a better understanding. Best, Ed