sound card voltages

 

I was just wondering what the typical voltages input to a sound card
are.  I want to try to view some signals on my sound card.

Thanks.

 

On 30 Jan 2007 23:35:13 -0800, bob@coolgroups.com wrote:


If you use the Line In, you can typically apply around 1 Vrms
(just under 3 V peak-to-peak).  The mixer input slider
controls will affect this to a certain extent.  If you reduce
the slider setting, you may be able to handle somewhat
higher voltages.  The sound card chipset can probably
handle 15 V, but better safe than sorry.  

The Mic input is typically more sensitive, but usually
has a fairly crappy preamp that rolls off frequencies
above 8 or 9 kHz.

You can use my (shameless plug) Daqarta for Windows
software for viewing signals as waveforms, spectra,
or color spectrograms.  It has an Auto-Calibration
feature to determine the relative sensitivity of the
mixer's internal volume controls, so you can adjust
sensitivity in dB.  Also has instructions for performing
a simple absolute calibration using only a cheap DMM
(no sensitive AC volts range) and a battery.

(Note:  Version 3.03 should be out in a few more days.)

Best regards,




Bob Masta
 
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
           www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!

 

Thanks for the information, Bob.

BTW, do you know if a sound card can accurately measure a DC voltage?

Also, do you know if you can usually use a sound card to measure
 the power level of a high frequency (i.e. 30 MHz) sine wave?

Thank you.

On Jan 31, 5:15 am, NoS...@daqarta.com (Bob Masta) wrote:

 

bob@coolgroups.com wrote:

   No, they are AC coupled.  In fact, the microphone input has a DC bias
for electret microphones.



   Anything over 20 KHz is pushing your luck with most sound cards, 30
MHz is 1500 times that.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

On 31 Jan 2007 17:50:44 -0800, bob@coolgroups.com wrote:


To add to Michael's post, you can measure DC by
converting it to AC which the card can accept.  The
simplest way to do this would be to chop the DC signal
at a kHz or so.  This can be as simple as 2 elements of
CD4016 (etc) that connect the input to the signal or ground
when driven by a simple CMOS oscillator.

The rectangular wave will easily be accepted by the
sound card.  Then you just measure the peak-to-peak
voltage to get the original DC value.  

If you have a  spectrum analyzer mode in your software, you
can read the peak height of the fundamental as a single
measurement instead of 2 separate waveform measurements
for peak-to-peak..  This isn't as accurate for a couple of reasons:
The peak height will be subject to "leakage" of its energy into
sidelobes unless the sample rate just happens to be a multiple
of the chopper clock (which effect can be partially cured by
applying a window function), and also there is the fact that
the fundamental amplitude of a square wave is not the same as the
peak-to-peak amplitude (it's actually higher!).  But if you
have a lot of measurements to make, both of these effects can
be calibrated for.

Daqarta simplifies the true peak-to-peak measurement by
providing triggered operation (so you get a stable trace,
which isn't essential for the above spectrum method),
and by allowing you to place separate cursors at two places
on the waveform.  Besides readouts of the two waveform
values, there is a 3rd readout that shows their difference,
which is the desired peak-to-peak value.   In general, the
chopper frequency won't need to be particularly stable
for this (unlike the spectral method) as long as the drift
is not so great that the cursors "fall off" their waveform
phases.  A simple RC oscillator is way better than
good enough.

In principle, you could use the above approach with your
30 MHz  signal by first rectifying it to DC and then applying it to
the chopper.

Best regards,


Bob Masta
 
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
           www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!