ADAU1761 signal leak in mute?

I would like to understand a little more about your system setup. Please answer the questions below so I can have a more full understanding of the issue.

Does your hardware resemble Figure 27 in the datasheet? Are the microphones connected to LINN and RINN? Is the CM signal connected to LINP and RINP?

Do you have any audio sources connected to LAUX and RAUX, or are the microphones the only audio inputs to the ADAU1761?

Do you have any I2S or TDM inputs on the serial port?

Could you please share your project file, or just the register settings? A screenshot will work as well. (You can also share the register settings file. In the Hardware Configuration tab, right-click the ADAU176x cell and select Register Settings...Save)

Thank you for your answer.

At this time, I'm just using the ADAU1761 as a pass-through mixer.

I'm connecting a desktop computer mic to input 1 of the EVB with a jack, the mic bias 1 jumper is closed, and I attached an amplified speaker set to the "stereo output".

I attach my sigmastudio setup.

The problem with the differential setup is with our board, I'll ask the hardware engineers to give me a schematic.

Thank you.

Hi,

I attach a schematic which depicts our current configuration.

Thank you in advance.

Matteo

Matteo,

What kind of microphone are you using? Is it single-ended? I'm a little confused if you're having the problem in single-ended mode, differential mode, or both.

Hi Brett,

I see the problem in our board, which is using differential mode, as you can see from the schematics.

I tried it also on the ADAU1761 Evaluation Board, which I controlled through SigmaStudio, and I found the problem is there both in single-ended and in differential mode. As I said in my original question, I found a workaround for single-ended mode, but not for differential mode.

Regards,

Matteo

Are you using an electret microphone for these tests? If so, how is it connected to both your board and the ADAU1761 evaluation board? When you say that you're using differential mode, is the microphone connected as shown in Figure 33 of the ADAU1761 datasheet? It might be helpful to see a schematic of how the microphone is connected into your system.

Regarding the tests with the evaluation board, I used a normal electret desktop computer microphone in single-ended mode.

I'll send you the rest of the schematic with the connection to the microphone for our board, which instead is using a differential microphone connected to the right differential input.

Here is an enlarged schematic which depicts also the microphone connection for our board. In this board we're having signal leak in differential mode. Disconnecting the microphone from the ADAU by removing the two 0R resistors in the input path is effective, so it seems that the ADAU is leaking.

Thank you

Hi,

Looking at your schematic, the P connection (A-IN2-P) needs to be taken care of differently for single ended and differential hookup. In the single ended case the P input should be grounded. For the differential case you need to place an equal value resistor from the P pin to ground as for the N pin to MICBIAS (in this case 2.2K). At the moment when you connect differential it seems like there is only the 47K resistor pulling the P pin to ground?

Best Rgds,

Alan

Hi,

in our schematic, the MICBIAS pin in Not Connected (the 0R is Non Soldered), while the microphone is phantom powered by an external source, since we need to give it more power (and a somewhat long cable). There are two 47k resistors to ground, just to weakly tie the signal to something.

Is the connection you mention required anyway, i.e. the 2.2k resistors to micbias and gnd? Do you suspect that as the source of our problem?

I still wonder why the problem is there also in the single-ended mode implemented in the ADAU Evaluation Board.

Thank you.

Hi,

Which 0R resistor in the schematic are you refering to? I see a 2K2 resistor connecting to MICBIAS. You don't need the MICBIAS with your external biasing however i am somewhat confused by how your external biasing of the mic works. It appears that you are biasing both the P and N connections with a 6K8 resistor to +12V? I've attached a diagram of how we usually connect differential operation with biasing.

Best Rgds,

Alan

Hi,

you're right about the resistor, it's a 2.2k. So, the micbias in our schematic is disconnected and the power for the microphone capsule is from the phantom source. It's powered that way so that the power supply DC is on both the differential signals and can't introduce noise.

Do you see the problem in the evaluation board as I described it in my original post? Could you please check?

I also attached the microphone to input 1 in the EVB in differential mode, as it is shown in the attached pdf (of course I removed the micbias jumper), and the problem doesn't go away even when I use the trick of keeping the input stage unmuted, as described in the same original post.

Thank you

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