5
\$\begingroup\$

In an audio system, I am reading samples from an sd card and sending those back to a 16 bits DAC for audio playback.

My prototype works but is too noisy. I discovered than using 2 separate Vcc/3.3V (provided thus by 2 different regulators) is helping quite a bit. So the MCU and Micro sd card are powered by regulator 1, and the DAC by regulator 2. I also separated physically on the board the analog from the digital circuitry.

The last thing I want to do, is to use 2 ground AGND and DGND.

The question is : If I can afford it "spacewise", is it a clever idea to use a top ground pour as the DGND and the bottom ground pour as AGND ? They would rejoin only at the power entry connector.

\$\endgroup\$
  • \$\begingroup\$ What is the part number of the DAC are you using? \$\endgroup\$ – Wossname Oct 13 '17 at 6:50
  • \$\begingroup\$ The PT8211, a very cheap 16 bits audio DAC. \$\endgroup\$ – Nicolas SoundForce Oct 13 '17 at 8:53
7
\$\begingroup\$

you'll have a lot of capacitive coupling between the two planes - you're better off putting all the analog in one clean place, with a single ground plane, connected to the rest of the world at just one point (so digital ground currents don't decide to take a short cut across it), and a carefully decoupled single power supply point

\$\endgroup\$
  • 1
    \$\begingroup\$ Yep. Two ground planes = a capacitor, which is very effective at coupling high frequency noise and spikes. \$\endgroup\$ – pipe Oct 13 '17 at 9:49
  • \$\begingroup\$ That's what I was wondering. Great information. \$\endgroup\$ – Nicolas SoundForce Oct 13 '17 at 10:00
  • \$\begingroup\$ @pipe. Is that necessarily true for audio? You only care about noise in the audible range up to 20 kHz, so the pf of capacitance between the planes probably wouldn't be a concern. Or am I ignoring some aliasing effect where that noise could wrap back to the audible range? \$\endgroup\$ – jalalipop Oct 13 '17 at 15:09
  • \$\begingroup\$ The audio outputs have sallen-key low pass filters at 19khz indeed. \$\endgroup\$ – Nicolas SoundForce Oct 13 '17 at 16:05
  • 1
    \$\begingroup\$ Put all the analog in one clean place, yes. But don't split the ground. \$\endgroup\$ – mkeith Oct 13 '17 at 16:26
2
\$\begingroup\$

It sounds like your system is all digital except the DAC output. Most of the time, I advise against using separate grounds, especially if passing radiated emissions testing is a concern. Personally, I would not have two different ground planes. It would be OK to have two ground planes and connect them together with lots of GND vias.

But it is a very good idea to study the placement of components and the routing of traces to make sure digital signals and digital return currents on the GND plane are kept away from sensitive analog signals. In your case, you need to focus on the voltage reference for the DAC and the DAC output itself. Those are the two places noise would be most likely to cause problems. Review any documents that provide guidelines for filtering DAC reference voltage inputs.

Generally, make sure the digital systems are well designed, with bypass caps and so-on.

Just for the sake of filling out the answer, the most sensitive signals are microphone signals. Traces must be routed with extreme care. Line level signals can also be pretty sensitive, especially if it is line-in and you are feeding the signal to any type of amplifier with high gain and/or high input impedance.

Headphone outputs are usually not very sensitive. Speaker outputs are fairly immune to low level noise. If you have problems with headphone or speaker outputs, the problem was probably created at the input side.

\$\endgroup\$
  • \$\begingroup\$ Unfortunately it is a very low-cost DAC without voltage reference input. Radiation testing is not an issue as this is small serie project. \$\endgroup\$ – Nicolas SoundForce Oct 13 '17 at 8:55
  • \$\begingroup\$ If the DAC doesn't have a reference input, then the DAC VCC is the reference input. Filter the VCC that goes to the DAC, but don't split the planes. \$\endgroup\$ – mkeith Oct 13 '17 at 16:24
  • \$\begingroup\$ Will do, I had placed a 100nf as well as a 10uF just next to the DAC. Considering that I am using 2 voltage regulators (from 12V input voltage to 3.3V), one for the digital section and one for the analog section, is it worth it to use a maybe a pi filter at the DAC VCC ? or another filter ? \$\endgroup\$ – Nicolas SoundForce Oct 13 '17 at 17:57
  • \$\begingroup\$ A series element makes the filter better. You can always use a zero Ohm resistor. I don't know if you need pi, or just RC. (Or LC). Sometimes LC filters ring, so an RC filter is OK if the DC current is not too low. \$\endgroup\$ – mkeith Oct 13 '17 at 19:09
1
\$\begingroup\$

You don't want 0.5 volt spikes with 2 nanosecond risetimes, repeating every 100 nanoSeconds, getting into your analog signal chain. The opamps cannot filter out that energy. The opamps have no control over their output pin, up at 1/(2n+2n) = 250MHz.

Regarding VDD filtering for DAC that uses VDD for VREF: if 1uH and 1uF LC filtering, then use 1_ohm resistor series dampening.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.