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Sound Card DC Measurements
Standard Windows sound cards do not respond to DC or frequencies lower than a few Hz. (See Why Sound Cards Block DC Signals below for explanation.)
There are two general approaches to get true DC response from a standard sound card: Modify the card, or add an external circuit to convert the DC into an AC signal which can be easily measured by an unmodified card.
Board layouts are in the Documents - Daqarta - Circuits folder. Besides the individual circuit discussions under these topics, see Daqarta Printed Circuits for a general overview of construction options, including Printed Circuit Construction and professional board fabrication.
An alternative approach to measuring continuous DC or very low frequency signals is by means of an external frequency-to-voltage circuit and the Fcal option of the Frequency Counter. While this only handles one channel, and produces a numeric display instead of a normal waveform trace like the above circuits, it has the advantage that it easily supports non-linear sensors (such as thermocouples) by means of calibration tables. If you need a time-series of values instead of just a numeric display, you can use Data Logging to send the readings to a text file.
Why Sound Cards Block DC Signals:
Standard Windows sound cards have DC blocking (also known as AC coupling) capacitors on each input line. There are two reasons for this:
First, DC and frequencies below about 20 Hz are almost always unwanted in audio signals. They are not generally audible except under extreme conditions, such as with very large speakers driven by very powerful amplifiers, where low frequencies are felt more as vibrations by the body than as sound by the ear.
However, very low frequencies can cause annoying Doppler distortion when the speaker cone advances and recedes while reproducing ordinary audible tones. An audible tone is shifted upward in pitch as the cone moves toward you, and lower in pitch as it moves away.
Further, DC in an input signal (often caused by leakage from a "phantom" microphone power supply) can reduce the available input range of the sound card ADC. For example, if the normal full-scale input signal is +/-2.5 volts, but there is 1 volt of DC added to this, then that input is actually seeing a range from +3.5 to -1.5. But the ADC can't handle anything over +2.5, and (assuming the desired signal is symmetrical) you would need to reduce the level to +/-1.5 to prevent clipping distortion from the ADC.
The second reason sound cards are AC-coupled is that this allows a single supply voltage for the ADC, such as +5 V. The input may swing +/-2.5 volts, but the input capacitor shifts this to 0 to +5 volts. When no signal is present, the ADC sees +2.5 V.
Note that some "audiophile" sound cards may claim that they are DC coupled, but they do not really pass DC or low frequencies like a true DC-responding device. The claim relates to the fact that the card does not have a capacitor in the input signal path, which could in principle cause subtle distortions of the sound.
But since it is still important to keep DC and low frequencies out of the audio path, these cards use a "servo" method. They do have a capacitor, but it is not directly in the signal path. Instead, it is used to obtain the average (DC) value of the output from an internal preamp stage, which is then subtracted from the input to give a net average value of zero. In effect, the overall circuit behaves just like a normal capacitor-coupled input, in that it still blocks DC and very low frequencies. (You could not use it, for instance, to directly measure temperature, atmospheric pressure, or stress in a bridge girder.)
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