freeDSP main thread

[SubSoniks]

Originally Posted by Baldin

Sure, but the question was also if any of you experience the same, and how to overcome this using the internal DACs and output opamps.
Of course one could end up with so many additional components that just using a DAC which has already taken care of this is a better option.
But would like to hear some experiences. Thanks.

There is a 1.5V DC offset on the output of the ada1701, a blocking capacitor is used but you still have a change in voltage on the output as it charges and discharges causing it to pop on the outputs. You could deal with it by adding a mute circuit to connect / disconnect the output before the adau1701 powers on/off.

[CyberPit]

Originally Posted by SubSoniks

There is a 1.5V DC offset on the output of the ada1701, a blocking capacitor is used but you still have a change in voltage on the output as it charges and discharges causing it to pop on the outputs. You could deal with it by adding a mute circuit to connect / disconnect the output before the adau1701 powers on/off.

Hello Baldin & SubSonicks-San,
Here is an example adding power on/off mute on ADAU1701.
DIY DSP PROCESSOR PROJECT PAGE
(Solid-state mute requires negative voltage when releasing mute.)

[Baldin]

Very interesting CyberPit

Thinking if it can be done even more simple ... sensing Vcc with a Zenner (here 3 diodes), and using this to mute the output at start up and close down

Have not tried in reality yet ... but it seems to work in sim ...

Any thoughts

[CyberPit]

Baldin-San, I also like to design as simple as possible.

There are few things to remind the constraints for real-world simulation.

- DAC output waveform voltage may exceeds 0.6Vpeak from the GND level. We should prevent form the waveform peak clipping.

- Need a negative voltage power supply for perfect releasing of mute-Tr driving function.

- Need a reserved positive power supply after power-down. (To Keep muting after the power down)

[dymoneum]

Hey everybody! I have the exactly same problem with freeDSP SMD B:
Output 2/3 are lousy noisy! Maybe it is a layout mistake? In my opinion, the whole board is not designed really professional...
I will try a linear regulator instead of the switching regulator. Any more ideas?

[dymoneum]

supplement: will at FIRST not change the voltage regular. Instead I will look around the negative supply rail for driving the mute circuit (charge pump)

[dymoneum]

For all who have noise problems with freeDSP SMD:


The layout is a big FAIL!
seems like someone did his first try in layouting...


Dear future PCB designer:
First of all make a proper grounding concept. A circuit like this should have a groundplane. DON'T cut the groundplane through and through with any kinds of signals!
If your layout program produces small copper islands, please leave them out instead of connecting useless 10 square millimetre islands with one via.
Don't mix analog and digital signals close together. At least, take care for some ground shielding between them! Keep analog signals away from everything - especially digital signals! And yes, a charge pump running on few hundreds kHz is noisy too! If you decide to use a charge pump, don't place it close to the output filters. And again: use a lot of GND. And vias are not so expensive anymore, use more of them, they will be your friends



To solve the noise problem:
cut the signal lanes from DSP outputs 2/3 close to the DSP. Take a piece of thin shielded wire and connect the outputs straight to the output filters (the two 2k Resistors).

I will do more modifications, mainly fixing some GND rubbish.

[dymoneum]

Further huge improvements in noise made by replacing the switching regulator (1,2MHz!!?!!) with LM317. I still can`t believe how someone can design such a bad thing and how to sell it commercially