I am using this DAC in a PA design. The firmware folks say that during initialization, the output goes to 1.65V. (This is our mid level voltage which drives preamps. 3.3 VCC) This voltage output is causing a pop to be heard through the speakers. It would be beneficial if the DAC would come up with 0V out of it at init rather than 1.65V. I found it baffling that this was not possible. Before I get into the data sheets too much, is this a possible outcome for this part? If not, do you have any suggestions for me on how I might mitigate this issue?
(I should mention that the 'fix' I have implemented is to turn off amp, init DAC and turn on amp. This improves the pop output to where the pop is much lower, but having the DAC output 0V at init should eliminate it completely.)
The DAC has to be biased up to a common mode voltage. That is just the way it works. with no signal in it will sit at 1.65V and as the input signal goes positive or negative the output will swing above 1.65V or below 1.65V. The range in which it has to work between is AVDD and Ground (but it will not swing all the way to AVDD or GND). So the part can never swing below ground so therefore it has to sit at 1.65V with no input. Now the output is differential so when one pin goes more positive the other will go more negative so the difference will be double the voltage swing. If you are DC coupling then you will have an issue. You will have to AC couple the outputs. Let me look at your schematic so I have an idea of what you are doing so I can give more directed advice.
Also, if you could tell me exactly the part number you are using? Is it the 1966 or the 1966A?
I am using the ADAU1966A. Here are snippits of the schematic.
Hmmm... Well, I don't see an issue with what you are doing. Your note says there is a cap on the amp input so it is cap coupled eventually. If that cap is really large (220uf or so) it may be taking extra time to charge but changing that to 4.7uf will not eliminate the pop just reduce it. Then I also do not know the impedance you are driving to know how much of a hit you would take on the low end response.
So if you do have a 75W amp you probably have a lot of gain in the system making the turn on surge very loud. I do not know of any register settings that would help, and they would be too late anyway since you have to power up the change them! So I think you can look into two approaches:
1) reduce the cap size to minimum for your performance goals, Then lower the gain of the amp so that full scale output of the DAC will be right at clipping of the amp. If the DAC clips before the amp does then you have too much gain. Then see how bad the turn-on thump is. If it is still too much then go to approach 2.
2) Devise a turn-on mute circuit that will mute the audio until the DAC has had a chance to power up. Is there a shutdown/Mute pin on the amplifier that you could use to delay the turn on?
I will think about this some more and see if I can come up with any other ideas.
We have implemented number one and have tried to turn off the amp as you suggest in number two. The amp turning on again causes a thump that our customer does not like. (Much better than the pop, BTW) I was just trying to solve the actual problem with the DAC. I do have a question, can I init the DAC then attenuate the output quickly? Can you tell me if this might help? Looks like I am between the rock and the hard place again. Sigh.
Are you using the DAC in Standalone mode? In that mode it will power up with the DACs on. However, if you are not in the Standalone mode, and programming it with a system controller, then when the DAC powers up it is muted. In fact, it powers up in a software powerdown state. The Master Powerup bit is in register 0x00 bit 0 and it powers up as a zero which is Master Power Down.
If it is the thump of the common mode voltage coming up that is the problem. (I think it is) then the volume down will do nothing because it is in digital before the actual DAC analog output stage. However, you can try to shut off the common mode voltage reference and see if that helps. You will not be able to run the DAC that way but you can then turn it on later and perhaps that would help. The register bit for it is located in the PLL_CLK_CTRL1 register, bit 1.
It sounds like the amp is also an issue.