PC Soundcard maximum in/out ratings

[izua]

soundcard maximum voltage

Hi guys!
I've received a CMI 8738 (C-media) soundcard from a friend, and I'm planning on using the AUX or CD inputs as probes for a scope, and one of the extra outputs, such as Modem or PC-Speaker for a signal generator.

However, I'm not sure where to look for specifications on impedances, maximum current, maximum voltage on ins/outs, etc.

I've made some time ago an EKG device with opamps, and the output was way above 3V. Burnt my mic port on the motherboard, so this time i'd like to be on the safe side.

Thanks,
izua

 

[echo47]

sound card maximum input

Most sound card buyers don't need such specs, so the card manufacturer doesn't bother publishing them. I wouldn't be surprised if some card manufacturers don't know or don't care exactly what the specs are. They may have designed their card by simply copying an example design from the sound chip manufacturer's data sheet or application note. You may have better luck reading that info, and measuring the card's performance yourself.

A typical line-level audio signal is around 0.5 volts RMS. The microphone input is much lower - several millivolts. A good sound card will have headroom for higher sound peaks, but different cards will clip (or be damaged) at different levels.

Beware, most sound cards block DC. That will hurt oscilloscope performance.

 

[izua]

soundcard max input

uuuh, i've asked on c-media forums, and no one seems to know.
has anyone done something like this before?

what sort of protection can i build, the most rudimentary one?

I was thinking of putting two diodes between the probes, in inverse parallel.

 

[echo47]

pc sound card voltage output

You can download the CMI8738 datasheet from the manufacturer, but it does not tell the absolute maximum rating of the analog pins.
https://www.cmedia.com.tw/?q=en/datasheets

The 8738 runs on +5V and GND, so I'm guessing it would be bad (possible damage) to apply an analog input voltage greater than those limits.

The datasheet says the analog input and output voltages are 1.1V RMS typical. I'm guessing that means the chip's maximum output is around 1.1V RMS, and its maximum input before ADC clipping is around 1.1V RMS.

Your particular sound card may have additional components that amplify or attenuate the signals between the 8738 chip and the audio I/O connectors. Maybe you can examine the card and trace the circuitry. It probably has a DC blocking capacitor, so if you apply a signal to the card with peaks +2.5V and -2.5V, it probably becomes +5V and 0V at the chip's analog input pin. That's just a guess.

Two back-to-back parallel diodes may be fine for the microphone input, but they would probably limit the voltage too much for the line input. For that input, maybe use two back-to-back series zener diodes. Don't forget to add a current-limiting series resistor if your signal source is powerful enough to damage the diodes.

 

[Endolith]

pc line input max voltage spec

Beware, most sound cards block DC. That will hurt oscilloscope performance.

The C-Media USB audio chips I've seen (CM108, CM119, CM6300) actually don't block DC, which is why I'm going to use them for the same purpose. A simple, convenient way to get DC or low-freq measurements into a computer.

You can figure out the maximum and minimum voltage by just testing it. Start recording, apply 0 V through a blocking cap and see if it's at 0 in your recording software, then increase the amplitude of the sine wave until it starts to clip. You probably want the gain at minimum for best signal-to-noise ratio.

Usually the maximum input voltage is supply+0.5 V, so definitely don't go over that, but it probably has internal diodes to protect it against short overvoltages.

What sound card is this, specifically?