How to prevent noise from digital circuitry?

[chicken_feet]

p/s:sorry for posting again in this forum, i thought here would be more suitable.thanks.
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Hi all,

I always design and draw circuitry that involves microcontroller and audio circuitry. Most often I encounter digital noise that goes over the audio circuitry, especially if they are on the same PCB. I have tried many methods either to reduce or isolate the noise, but I would like to know if there are any other precaution that i do not know of.

Usually, my microcontrollers are running with an external crystal in Megahertz with I2C or RS485 serial communication. The problem with audio is that any signal that are pick up will be amplified so many times over, especially when the audio circuitry deals with channels selection at the source level (low level).

There have been few successes and many failures at reducing these pickup noise especially when these devices shares the same ground. They have to share the same ground, because using AC (transformer with different ground isolation) is not an option and I have to use the standard 24Vdc input power. There have been several instances where noise from the ground of a different device are pickup on another.

I welcome any sort of comments, experiences, or different way of doing things.

Thanks in advance.


Best regards,
chicken_feet

 

[Btrend]

it seems that you face a board level noise, right?
do you use multiple power or multi source for different ICs in your board ?
how many layer of ur PCB ? do u have dedicated GND layer and VCC layer ?

 

[chicken_feet]

it seems that you face a board level noise, right?
do you use multiple power or multi source for different ICs in your board ?
how many layer of ur PCB ? do u have dedicated GND layer and VCC layer ?

Hi,

Most of the digital IC's are powered by a 7805 while those at the analog section are powered using a 7818 or another seperate regulator. I only have two layers and usually, the top layer is fill with polygon of ground signal. For the analog portion, I do not put in the ground polygon.

 

[IanP]

General advice for mixed signals PCBs is to lower impedance on all analog lines. In some cases this can be achived by using low-impedance 600Ω-to-600Ω audio transformers (limited bandwidth), or linear optical isolators (example: HCPL4562 or similar) .. both ways provide total isolation of DGND and AGND ..

 

[freeinthewind]

1.u can separate the ground plane to two parts, connecting them to a point.
2.u can use the whole ground plane and make it as far as possible from analog circuits to digital.

 

[wholx]

check out this doc. some ideas for multipoint ground system.
a parallel ground connection may be helpful to your design.

 

[chicken_feet]

1.u can separate the ground plane to two parts, connecting them to a point.
2.u can use the whole ground plane and make it as far as possible from analog circuits to digital.

Thanks everyone for your input and suggestions. Both the above idea are already incorporated in all my pcb's. Anyway, I will read up the pdf file. Thanks all.

 

[Kral]

Try to connect the analog, digital grounds together at one point only, and make sure that this point is physically close to the audio ground (common) input pin of the board connector. Also, using differential input devices for the low level audio circuits will minimize the effects of common mode noise generated by the digital circuitry. Power supply bypassing capacitors on both digital and analog ICs is a must. If you don't use a ground plane, use a checkerboard pattern for the digital power and ground (vertical Vdd/Vcc traces and horizontal ground traces on separate planes, or vice-versa). This minimizes the loop area in which the current from the bypass caps must flow. Inductance is proportional to loop area.