Sound Card In and Outputs

[sled]

Hey there,

I'm currently working on a development board for digital signal processing and I stumbled upon a (maybe trivial) problem.

My goal is to use an external USB sound card for signal in and output for a metal detector. What I'm designing basically is an interface board that plugs between the USB sound card and the metal detector's coil - nothing new.

However, when I had a closer look on the output of the sound card I found out that the audio jack's sleeve is a virtual ground potential at ~1.67V (half the supply voltage) so the output signal swings from 0V to 3.3V, centered at 1.67V.

I took the sound card apart and figured out that the output is DC coupled (no capacitor).

The problem I now face is that I don't know how to create a common ground between my amplifier PCB and the sound card - what shall I do with the jack's sleeve which is at 1.65V?

I've attached a very simple schematic of the sound card and the datasheet of the sound card's chipset.

[Silver Dollar]

Many digital devices do the same thing. The processor runs on 3v or 5v so the ADC does too.
You just have to level shift your signal before delivering it to the ADC.

Here's a single ended input for a PI detector;

[sled]

Many digital devices do the same thing. The processor runs on 3v or 5v so the ADC does too.
You just have to level shift your signal before delivering it to the ADC.

Here's a single ended input for a PI detector;

[ATTACH]29036[/ATTACH]

Yep, 1.65V is the center voltage or virtual ground for the output which swings between 0 and 3.3V referenced to the sound card's absolute ground potential.

Now I have my amplifier board that has its own power supply, and usually you connect the grounds of the sound card with the ground on your amplifier board through the sleeve of the audio jack. But I'm not quite sure if I can do that because the ground of the audio jack is not the real ground on the sound card - it's a virtual ground at 1.65V.

[Silver Dollar]

Well as long as your power supplies are separate you can use the sleeve of the audio for ground reference for the amp.
If they share a power supply then you need to isolate them somehow. A cap inline with the audio will loose the ground
reference and swing positive and negative relative to the input line.

Ground is just a reference. If you look at a lot of the PI schematics you will sometimes see the positive battery terminal
used as ground. It only matters when you are using a scope or needing a chip to have positive or negative voltage. There
can be only one reference in a given circuit so if you have 2 devices with a common power supply the ground on that supply
is common. If you then output a signal centered on 1.35V then the amp (which is on the same supply) needs to rereference
the ground (by a cap in series or level shifted through an op amp).

[sled]

yep it seems like they just saved the money/space for the big output capacitor on the sound card and just biased the sleeve of the audio jack at the center voltage

The pain is that both circuits (sound card and amplifier) are connected via main's earth through the shield on the USB connector, so I can not connect the 1.65V to ground because it would create a short circuit through earth. I think a simple solution is to just solder in a jumper wire so that the sleeve is the "real" ground and then put the output capacitor on my amplifier board

I've attached a simple sketch of the problem.