How to feed digital data from SigmaDSP core to a PC

The ADAU1781's serial outputs are in I2S format by default. This is a standard interface for board-level connections between audio ICs, but is not typically seen as an interface on a PC. In order to get the audio data into your PC you will either have to connect via analog, as you've described, or use something like an SPDIF transmitter IC to convert your data from I2S to SPDIF.

If your AVDD is 3.3 V, then the full-scale output voltage on the analog outputs is 1.0 V. This is a typical full-scale level and is likely what the line input level of your USB interface is. You may want to check on what the gain setting of your input signal is.

If you go ahead with the plan to use analog outputs, a good way to get the levels right is to create a test project in SigmaStudio that has a sine tone source connected to the analog outputs. When active, this source will output a full-scale sine tone that you can use as a test signal to determine the proper gain staging at your audio interface's analog input.

Thanks for the response guys. I've figured out the issue with getting the analog values; it had to do with my audio interface.

Now that I can get the signal, I hear some sort of electronic distortion on the output, I've uploaded a short wav file so you can hear it.

I can't tell what its source is. I used a volume control block to mute the signal path but it made no difference. I also tried adjusting the various gains of the codec chip from the Digital Setup tab in Sigma Studio but it is always there. And it can't be from my audio interface because the same noise is present on another interface.

The noise you're hearing is likely power supply noise coupled over the USB connection to the evaluation board. This noise is sometimes related to the power supply connected to a wall outlet; if you have a laptop, you may want to try running it on battery power (not plugged in) to see if that reduces the noise. Another option is to directly supply +5V to the evaluation board, rather than using the USB power supply. You can do this by removing jumper J1 on the ADAU1781 evaluation board and directly supplying power on header J15.

JeradL you are correct that the source of the noise is the USB cable. The only way I could get rid of the noise was by powering the board from a DC battery and completely disconnecting the USB cable so that control communication is lost.

By the way is this noise issue common in customer designs utlizing ADAU1781 and other SigmaDSP codecs? or is it mostly specifc to powering from USB cable on the evaluation board? it is just that the main reason we are excited about the digital MEMS microphone is that we have a fairly noisy circuit and we thougt the built in amplifier and ADC would give us the best results given those conditions, but if the codec itself is so susceptible then that eliminates the digital mic's benefits.

and thanks again for your prompt responses, they have been immensely helpful.

Like Jerad mentioned, the source of the noise is really your AC power supply (i.e. the wall outlet). A high quality Power Supply Unit (PSU) in the computer can help to remove some of the noise. You could take the same setup (computer, power cable, USB cable, and evaluation board) and plug it into a "clean" power supply (like one found in an electronics lab) and this noise would be reduced significantly. So, you see, this noise is really not related to the USB cable or the evaluation hardware, it's mostly the noise coming from your wall outlet.

In your end application, using a DC power source with good noise rejection characteristics can help remove this noise. In systems that are battery-operated (cell phones, digital cameras, etc.) the noise of the power supply drops significantly. Other methods of removing noise from the system include power supply decoupling capacitors and careful PCB design.

A USB islolator should help. AD produce the ADUM4160 for this purpose:

http://www.analog.com/en/interface/digital-isolators/ADuM4160/products/product.html?ref=ASC-PR-118

http://www.analog.com/library/analogdialogue/archives/43-06/USB_isolator.pdf

You can avoid construction if you wish. AD make an evaluation board for this product also. There are other board designs available also, e.g.

http://www.circuitsathome.com/products-page/usb-interfaces/

which is also available with peripheral-side power supply.

There are readymade USB isolator products available - using e.g. fibre connections - but they tend to be very expensive.

best regards

Gerry

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EZ Admin