Asking about AD1937 noise

Hello Song,

This is difficult for me to determine the source of the noise without more information. The information you sent is helpful. I did look at it and I do not see a problem right now. So I will need to know what other tests you have done. What is the source of the audio signal to the DAC? Do you know that it is a clean quiet signal or does it come from the ADC? So you will need to isolate and know that the signal going to the DAC is quiet.

Assuming that is all quiet and good. It does not sound like there is a clocking issue to the DAC so I will not ask for those details and register settings at this time. More likely this will end up being a power supply, grounding and layout issue. It almost sounds to me like it is a switching power supply or other clock getting into the audio. How power and ground is implemented can make a huge difference. So if you are still having this issue then I will need to know the details of your voltage regulators, what it is supplying? Where is it on the board? How grounding is done? I would also like to see your circuit board layout. Feel free to contact me via private message if you do not want to post all this detail on a public forum.

Thanks,

Dave T

Hello, Dave.

Thank you.

I just apply power to the board without any programming and I can hear noise from headphone.

I guessed that by default DAC is off and expected not to hear any noise from DAC OUT.

Because I didn't send any I2S signal at all.

But my current board has noise.

The power voltage is 3.02V, the power is coming from power supply. Can it cause the noise?

Thank you.

Songhee.

Hello Songhee,

To answer your question about if the power supply can cause noise, the answer is yes. A switching type of power supply with a fairly low switching frequency can be heard or it can beat with other signals and create audible artifacts. You did not mention the type of regulator. I also noticed that you are not using the internal regulator.  I suppose you do not have 5V on your board so you cannot use the internal regulator? It is a quiet regulator but if you only have 3.3V then it is not an option.

So I have looked more closely at your schematic and I have noticed a few things.

1) You are listing 3.3V as the power supply voltage but you say it is measuring 3.02V. Where did you take this measurement?

2) You are using two grounds, GND and AGND_ADI. Since all the ground pins on the Codec are internally connected you do not need to provide different grounds. It is best to use a ground plane and connect the parts to this plane with traces as short as possible. All of our evaluation boards are done in this fashion and it is how we get maximum performance from the part.

3) Power planes, I recommend you use a power plane as well but this gets a little trickier because you do want to have separate planes for the digital and analog power. The power planes need to have several decoupling caps spread around the edges of the plane and inside of the plane if it is large.

4) Power supply decoupling. I see you are using a ferrite bead to isolate the digital power from the analog power. This is good but there is not enough decoupling on the analog side. You need a 100nf cap for each analog power pin. The cap needs to be on the same side of the PCB as the part and as close to the part as possible. Then the via connections to the power and ground planes need to be away from the part. So the traces should go from the via to the cap then to the AD1937. If the via is between the cap and the AD1937 pins then that can cause noise pickup due to the inductance of the via. This seems to be a small difference but it is significant when it comes to reducing high frequency noise.

You also need one large cap on the Analog power side of the ferrite bead. A 22uf would be nice but a 10uf should be good enough. This does not need to be very close to the part but not too far.

You also need to have a 100nf cap next to each digital power pin. You have it labeled VCC_01 and VCC_02. This will help reduce any noise before it has a chance to get very far on your board. I see you have a 4.7uf cap on the 3.3V. This is probably fine but I like to see at least 10uf. The digital current draw stays fairly constant so you do not need a huge reservoir.

5) The CM (Common Mode) pin, pin 52, you have decoupled with a 100nf and a 10uf. The 10uf is a minimum. We find a slight performance benefit when a 47uf cap is used. So if you have room and budget for it then increase this size. Also, these need to be close to the pin and connected to the ground plane in the manner I mentioned in comment #4.

6)In the amplifier schematic I see you have an AUD_VREF. Where is this coming from and how is it developed? What is the decoupling? How far is it from the op-amps? This is all very important. You can use the CM pin for this reference but you would need to have an op-amp to buffer the reference. I can provide more detail if you like.

6) You are using the DAC outputs single ended. It is more difficult to reduce noise pickup in a single ended circuit. If it is possible I would use the differential filter circuit shown in figure 33 of the datasheet. I have not simulated your output buffer to see what the filter corners are but it looks to me that there is not enough filtering of the outputs. So you will pick up a lot of HF noise especially when running single ended. You have almost no filtering of the output. You can also look at the evaluation board design. That output filter is a little different from the datasheet but it does work well. download the user guide UG-040 and look at the schematics. http://www.analog.com/static/imported-files/user_guides/UG-040.pdf

I think this is more than enough for now. I know there is a lot in this post but if you follow these recommendations you should have good results but there are still other layout issues that could cause problems. Without my seeing your layout it is difficult for me to comment on them.

Thanks for reaching out for help. We are glad to provide it.

Sincerely,

Dave T.

Thank you so much for this advice.

I will update this information.

Thanks again.

Songhee.