Soundcard input bias question

Hi all,

I am building a soundcard input buffer for use with simple PC oscilloscope (<20kHz) software while I save up for a real scope.

The circuit in the first attachment minus the output divider to out_gnd is what I was originally working on. I can't for the life of me remember where I found the schematic. It works but the output is of course centred at +4.5V. I don't know if that's going to kill a soundcard input, but I'm given to understand they want a maximum of 1V peak to peak. Is that true?

So I added the output divider, which at least works in the sim. It costs 2mA but I can live with that.

Anyway, I've since decided to build this one instead, but I would still like to know: is it reasonable to solve the bias problem with the output divider I've added?


Thanks,

Torben

I've no idea what you're asking, or why?.

Soundcards have AC coupled inputs, so a DC bias is meaningless.

I've no idea how much it would take to damage a sound card, but I would expect it would be a LOT more than 1V - but in any case, don't feed it more than that!.

What I am trying to ask is this: even if the DC bias wouldn't hurt the input of the soundcard, would the divider be a reasonable solution? I'm thinking now that just a series cap would be easier.

[Edit: argh. That is not what I'm trying to ask. What I meant was: forget the soundcard. Assume the device we're outputting to has no AC coupling, or that I don't know whether it does. Would the divider be a suitable solution, or would the series cap be sufficient?)

Thanks for the tips,

Torben

Torben said:

What I am trying to ask is this: even if the DC bias wouldn't hurt the input of the soundcard, would the divider be a reasonable solution? I'm thinking now that just a series cap would be easier.

[Edit: argh. That is not what I'm trying to ask. What I meant was: forget the soundcard. Assume the device we're outputting to has no AC coupling, or that I don't know whether it does. Would the divider be a suitable solution, or would the series cap be sufficient?)

Click to expand...

Both are completely different solutions to completely different problems - the sound card is AC coupled, so your divider would do absolutely nothing - just a coupling cap is all you require (and you may not even need that - but I would include one).

Nigel Goodwin said:

Both are completely different solutions to completely different problems - the sound card is AC coupled, so your divider would do absolutely nothing - just a coupling cap is all you require (and you may not even need that - but I would include one).

Click to expand...

OK, very good to know. But again--what if I wasn't feeding a soundcard, but something not AC-coupled?

My line of thinking was this: if the circuit I'm feeding may not be AC coupled, would the output divider provide a useful ground reference for it, or would a coupling cap be a better answer? I'm assuming that my buffer and the device I'm feeding have separate grounds.

If not, then what would an example be of a problem the divider would solve?


Torben

It all depends EXACTLY what you're trying to do, and EXACTLY what you're trying to it with.

Assuming you were using a PIC analogue input, that accepts input voltages from 0V to 5V (assuming standard minimum settings). If you're wanting to use that as a scope type input, you would want an input range of +2.5V to -2.5V. To do this you could bias the PIC input at 2.5V - however, you would normally do this in the opamp feeding the PIC, not with a potential divider direct to the pin - which would effectively reduce the input impedance of the A2D.

Thank you! I think that answers my question.


Torben

One more heads up, my soundcard inverts the AC signal. Better test before you build it.