Dragonboard Mic/Audio Input


#1

My question is two fold:

  1. Why don’t I need an AC coupling capacitor between the output of my analog mic and the MIC3_P pin on the board? The audio routing application note from qualcomm says I can just wire the output of my mic directly in, but I feel like there needs to be a capacitor to block DC, and so it seems weird that the application note says I don’t need one.

  2. Can I use one of the mic inputs with a single ended mono audio jack? I don’t see why I wouldn’t be able to, but I can’t find any examples or testimonials saying it has been done. It seems weird to me that this board would be incapable of taking in line audio, so I just want to make sure.

Cheers.


#2

Hi @mbstrange

The input circuitry on the Microphone ADC has been optimized to receive a signal that has a DC offset (about 0.9V), hence you do not need a DC blocking capacitor. Taking the blocking capacitor out of the circuit reduces both cost and distortion.

If you want to use the microphone inputs for a line-in signal you will of course have to turn down the gain, and provide a DC blocking capacitor. You will need to DC Bias the signal, normally a Line input has an impedance of about 47k Ohms, you can achieve the required DC Bias and input impedance by using two 100k Ohm resistors between GND and the 1.8V Microphone bias supply to terminate the ADC side of the DC blocking capacitor.

In general you probably want response down to roughly 20Hz with a input impedance of 47k Ohms, hence you need about 0.22u Farad capacitor. A Polypropylene capacitor is best, but ceramic will work.

Full disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other posting may not reflect the opinions of my employer.


#3

@ljking I’m using a MEMS microphone that takes 3.3V, so I think I would still need a DC block - also the codec diagram seems to suggest that the inputs are internally biased - why would I need to bias it myself?

This diagram also suggest that the bias is at 1.8V, not .9


#4

Hi @mbstrange

Hmmm… most MEMS microphones (but not all) output Digital, not Analog and need connections to GPIO0 and 1 (which also requires Device tree changes). Based on the fact that your microphone wants 3.3V, I would suspect it is a digital MEMs microphone.

The diagram you have attached doesn’t show the full circuit, you haven’t considered the contribution of the microphone in the circuit. Although you are correct the bias supply voltage is 1.8V, this is not the voltage that will be on the pin during operation.

A typical analog microphone has a DC resistance of about 2k Ohms. The internal bias resistors shown in the diagram are “pull up” resistors with values of about 2k Ohms. When you have the 2k Ohm resistor pulling up to the 1.8V Bias supply, and the 2k Ohm microphone pulling down to GND, you end up with a effective offset voltage of 0.9V at the input.

When you are connecting an external line input, you don’t want to use the internal 2k pull-up resistor, since this will do two things you don’t want. 1) it will give you and input impedance of 2k Ohms which is non-standard, and 2) will leave with a 1.8V DC offset, which is incorrect, you want a 0.9V offset at the inputs. Hence two 100k resistors and a capacitor…

You will need to ensure that the internal bias supplies are disconnected when you are using the inputs as line inputs.

FYI - the diagram you attached contains the note “Capless Input only” under each ADC, this is the (hidden) answer to you original question about AC coupling capacitors.

Full disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other posting may not reflect the opinions of my employer.


#5

Hi @ljking I have this mic, which is an analog MEMS mic. https://www.digikey.com/product-detail/en/knowles/SPU0410HR5H-PB/423-1138-1-ND/2420982

How do I disconnect the internal bias supplies?


#6

Hi @mbstrange

The Knowles mic you are using is exactly the same mic that is used on the Audio MEZZanine board. This mic runs on any bias supply between 1.5V and 3.6V so you can use the internal 1.8V bias supply. No need for a 3.3V supply, or for any external components other than the mic.

Here is a small fragment from the Audio Mezzanine board schematic:

This shows the appropriate connections for the Knowles mic. The diodes D9 and D7 are for ESD protection and are not necessary to get the circuit running. The 100nF capacitor C18 reduces the supply noise and makes the mic a little quieter but is also not critical to getting the mic running.

As far as turning the bias circuits on/off, this is buried in the amixer or tinymix commands described in the Audio Application note. You can find some more discussion in this Thread. Perhaps @srini can shed some light on turning internal bias and external bias supplies on/off, for use as a line-input.

Full disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other posting may not reflect the opinions of my employer.