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Avoid noises in mixed signal design

2017-12-22 10:33  
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Today most of all embedded systems consist of two part circuitry a€“ digital and analog. Digital part is usually controller, its timing circuit and other input output devices. Frequently there is an analog part on same board like ADC, OP amplifiers, sensors and other analog circuitry. Such designs are called mixed-signal designs. Where digital and analog part meats a€“ the grounding problems starts. Fact is that each conductor has its own impedance, so any current flowing result in voltage drops. Ground wires and planes isn’t exceptions. Digital and analog grounds can generate significant electromagnetic radiation that adds noises to signals we need. So the over all system quality drops because o poor design.

mixed_signal_grounds.JPG

 

In a good design analog ground palane and digital ground plane should be separated. With multilayer PCB this can be done very easy. Another issue is that digital signal traces shouldn’t cross analog ground and analog signal wires shouldn’t cross digital ground plane area. Well of course and try to avoid aligning digital and analog wires as they can catch each other radiated noise.

How to deal with these problems. Well first thing is to recognize problematic areas correctly. Then you will be able to implement right design. Correct implementation is all about correct part placing and correct wiring.

If there are more than one analog parts on the board, then each of them should be separated so that even different analog parts don’t overlap.

Different_analog_signal_grounds.JPG

 

According to this example it is important to say, that each good IC manufacturer is trying to separate analog pins from digital pins in a package. For instance lets take Atmega8. See how nicely analog part is separated from digital part:

mixed_signal_IC.JPG

 

So that easier could be digital and analog part separated on PCB.

One more actual problems I want mention here is ground loops. Ground loops exists when electrical devices are connected to ground in more than one way. Simply speaking if devices are connected to more ground pints than on, then ground loop exists. Ground loops can generate significant noise, especially in audio and video equipment. Following example represents wrong ground implementation as there is obvious ground loop:

Ground_loop_example.JPG

 

In example above the ground has one loop, and the bigger loop(the longer wire), more noise it will pick up. So the circulating current will generate more noise radiation. There only one suggestion is to usecommon ground pointor so called tree like grounding:

Ground_example.JPG

Usually ground loops are post-factum problem a€“ when you realize that system is generating too much noise. Then the only solution can be to make a cut in a ground loop path hoping that this helps a bit.

So by concluding we can generate some general rules, how correct grounding should be done:

First rule is – Try toseparateanalog and digital ground planes. This can be easily done in two and more layer PCB’s. With one layer PCB there are much more limitations to do this, but again-you can follow the idea and try to separate analog part from digital.

Second rule a€“ Try not to crossanalog ground with digitalsignal wires and avoid crossingdigital ground with analogsignal traces. Ideally speaking analog part of PCB should be totally isolated from digital part speaking in terms of placing.

Third rule a€“ if its possible try to keepground planesassolidas possible. If board is double or more layered, then one side of board should be ground polygon. Remember thicker wire is less impedance it have it means less radiation because of voltage drops.

Fourth rule a€“ try not to alignanalog traces along digitalthat they could catch each other generated noise radiation.

Fifth rule a€“ avoidground loops. Use Common ground connection ortree like groundingstructure.


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