adau1446 create dc offset

Hello Richard,

The DC source can be made to appear on the output. There is nothing special about its output. I whipped up this project:

When I switch from the audio input to the DC I see full scale DC output from the DAC. The only difficult thing about this interface is that it wants to be in 5.23 format where full scale is 1 and negative full scale is -1. Half scale is 0.5 so getting very small offsets is a bit tricky. What helps is to use a readback cell set to hex to see what the data word looks like.

What makes it a bit easier is to use a slider. They are a log function but it does seem to be a little easier to operate. Just set the DC source to 1 and divide it down to what you need.

For this next project I inserted it into the audio signal and I could see the signal riding up and down on a DC level as I adjusted it. So this is certainly possible.

Now, where you may be running into trouble is if you are sending the DC into blocks that have a HPF. It will be important to make sure that does not happen.

Then there is the DAC. Check that there is no HPF enabled on the DAC. Some DACs have this feature.

Then the last thing to check is to make sure that the DAC outputs are not cap coupled. I had to look at it before the caps of the evaluation board. This means that you must be direct coupled from the DAC outputs. Almost all DACs output sit at the common mode voltage around AVDD/2 so this makes interfacing quite difficult. You will not be able to dial out that DC offset because it is required for the DACs to function. So if you are wanting to use the DSP to remove a small offset off of an Op-Amp it will not be possible without cap coupling somewhere. I suppose you could use an extra DAC output as a reference for an op-amp stage but there will be a lot of HF noise on the DAC outputs that would need to be filtered.

So in the end here I think I need to understand more exactly what the DC offset is and where it is coming from. Feel free to share your schematic of the output stage.

Thanks,

Dave T

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