How to explain this Audio Headset Circuit

[kian0079]

Hi all,

I am looking at a reference design for an audio headset. The schematic is attached. May I know what is the purpose of the capacitors C78 - C81?  Are they necessary and do they need to be placed closer to the headset jack or closer to the microprocessor? Also, are the ferrite beads and transient suppressors necessary?

Thanks in advance!

[Andy Chee]

These components need to be positioned physically close to the socket.  These components form a low pass RF filter, and stop RF from interfering with microprocessor function.

You need to keep in mind that the fleshy human body can act as an antenna and static charge generator, therefore, any sensitive inputs that a human can touch with their finger, should be protected from these effects....if you want a robust and reliable design that is.

[kian0079]

Following up on my question. I am also quite puzzled by how the headset detect works. The CDC nets(connections) on the schematic goes to an audio codec (as part of an android hardware development kit). I do not have the datasheet of the audio codec. But when I am trying to find out is how can I get the headset to be permanently detected regardless of whether a headset is plugged into the stereo jack or not.

Looking at the schematics, when the headset is not plugged into the stereo jack, pin 5 is connected to pin 2 on the stereo jack. This essentially creates a 'short circuit' between the left speaker output and the headset detect pin. Conversely, when the headset is plugged into the stereo jack, pin 5 is disconnected from pin 2 of the stereo jack. Does that mean that if I leave the CDC_HS_DET net floating, the audio codec will think that a headset has been detected?

[Andy Chee]

Does that mean that if I leave the CDC_HS_DET net floating, the audio codec will think that a headset has been detected?

Most probably, yes!  The easy way to test this is by inserting a new plug with no cable, or a scrap plug with the cable cut off.

Strictly speaking, the net probably isn't floating and is internally pulled up within the chip, or a resistor elsewhere on the schematic.

[RoGeorge]

In case you want to do that trick only to start the radio without a headset plugged, the radio can be made to start, but it won't work.  The headset antenna is really needed for FM reception.

[bson]

They form LC filters with the ferrite beads, right?