This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PGA2311 , digital noise in analog section(output)?

Other Parts Discussed in Thread: PGA2311, LM337, LM317

Hello,


I made preamp circuit using PGA2311 (atmega8 MCU and MAX7219 led display driver)

BUT, when I connect input to pga2311 i get loud noise, obviosly from digital plane. And I can here PWM used for MAX7219 brightness in my speakers. When I disconect input, I stll can hear that noise but much, much, much weaker, almost not audible, but still present. and when I disconect pga2311 output from my power amp, then, nothing can be heard.

I think I separeted digital and analog ground plane. Here is my pbc output:

Digital power supply is done with small mobile phone charger connected to connector named POWER-DIGITAL, and analog power supply for pga2311 are done using power toroidal transformer which is also used for power amplifier, but in pga2311 case., it is regulated, first via transistors to lower voltage from 54V to 15V and then via LM317/LM337 to lower to +/-5V.

I am not sure why I get so much noise in analog input/output of pga2311. I can here how PWM pulse/pause changes(on display driver IC) like changing frequencies

Thank you.

  • Hi, Oliver,

    Sorry for the delay. I have asked my colleague to work with you on this issue.

    -d2
  • Hello Oliver,

    Unless I'm interpreting your PCB design incorrectly, it looks like you don't have any ground planes on your layout. With no ground copper in between your traces, it's easy for signals on adjacent traces and layers to capacitively couple to one another. Having no ground plane also forces signal and power return currents to take long paths back to their sources, traveling through other nodes which also increases noise pickup. Finally, your analog and digital ground are not in fact separate but are directly shorted together.

    I recommend following the PCB guidelines given in Figure 6 of the PGA2311 data sheet, as copied below. This should resolve your noise pickup issues.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog - Op Amps

  • Yes, I saw that guidlines in datesheet, but I did not understand it correctly. Should I connecto both digital and analog ground together ? I dont think PGA could work without connection both grounds together in one point as this picture you posted shows? This picture shows both grounds are connected in one point as I did in my schematic.

    Now, I am not sure what is ground plane exactly? Is it separete PCB layer or is it just a copper areas connected to ground all over every traces? like in this picture for example:

    thank you

  • Should I make something like this(this is only one side copper gnd plane but I suppose I should make on both side, because my pcb is double layer):

  • Hi Oliver,

    A ground plane is just an area of copper on your PCB which is connected to GND potential. On some multi-layer boards, designers will dedicate entire layers of copper to use as GND and power planes. However, on a 2-layer PCB like yours I recommend placing ground planes on both top and bottom layers.

    Engineers have different opinions on whether digital/analog ground plane separation is required. Based on the training I've received from experts on grounding and PCB layout, such as Henry Ott, separate planes are not required. What you instead should do is use a single plane, and lay out your components so that the digital area is on one side and the analog area is on the other side. Route your traces so that currents from analog and digital signals remain on separate sides of the board, and they will not interact.

    The PGA2311 pinout makes it easy to do this, since the digital pins are on the left and the analog pins are on the right. I recommend using the layout that you have, but place ground planes on top and bottom. The image below from one of my upcoming TI Precision Designs gives an example of the use of the ground copper planes. All the areas of solid red and blue are ground copper. Traces on other nets are kept separate, but any component pins or vias connected to GND connect directly to the copper area.

    Post an image of your new PCB layout and I will gladly review it for you.

    Best regards,

    Ian Williams

  • Hello Ian,

    This is how I made ground plane:

    aand as it can be seen from screenshot, analog gnd of pga2311 is connected to digital gnd via one track on bottom layer. So, is this enough for separating digital from analog returning currents?

    TThank you.

  • Hello Oliver,

    The top layer looks good. However, are the planes on the bottom connected to GND? It looks like the copper has not flowed over the traces, which remain separate. Are they assigned to separate nets?

    Best regards,

    Ian Williams

  • O, yes Ian, you are right, bottom copper is not connected to gnd, I forgat to assighn net to copper. because I was doing it in a hurry to post here.

    and here is a final design:

    Now if it is good enough I would start to make this board to see if it will collect noises. What is your experience with poured copper connected to gnd? are they less noises?

    Thank you.

  • Hello Oliver,

    The new layout looks good.

    In my experience, having ground copper planes is a huge help to reducing noise. As I said in my first post, it prevents noise from coupling into your input and output signals (since the noise instead couples to GND), and provides short and direct paths for your return currents (preventing them from traveling near sensitive nodes).

    Best regards,

    Ian Williams

  • Ian,

    I made pcb as you and pdf document suggested and do many other things to suppress/remove noises.
    1. Reorganize IC MAXx7219 pwm led display driver. I place it on separeted pcb connected with flat cable with this(main) pcb (and noise still present, but with much smaller amplitude).
    2. I made pcb with poured copper connected to GND (now, noise is much less).
    3. I separete input audio signal gnd and digital power supply gnd (now, noise completly dissapered)
  • Hello Oliver,

    I'm glad to hear that your noise issues disappeared! Let me know if you need any further assistance with the circuit design or layout.

    Best regards,

    Ian Williams