Data AcQuisition And Real-Time Analysis
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The following is from the Daqarta Help system:



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Polarity Determination - Full-Scale Ranges

Controls: Calibration Menu >> Full-Scale Range

Some sound cards invert signal polarity on input or output channels, or both. Daqarta must know the correct polarity in order to generate and display waveforms properly.

If you know that a channel is inverted, you just enter the Full-Scale Range value with a minus sign. Daqarta will retain that and apply the needed correction whenever that channel is used.

The best way to determine polarity, without lab-type equipment, is using a 1.5 volt battery. You can use a standard male-male stereo audio cable plugged into Line In on your sound card. Connect a mini-gator clip lead to the shaft of the free plug, and hold it against the negative terminal of the battery.

Briefly touch the tip of the plug to the positive battery terminal, and note which way the Daqarta trace deflects. Sound card inputs (and outputs) include capacitors that block sustained DC signals, so the signal jumps to the battery voltage and rapidly decays; you just need to note which direction it first jumps.

If the trace goes up then the input polarity is correct. Otherwise, enter minus signs for Line In (both Stereo and Mono fields) of the Full-Scale Range.

Once you have entered the proper sign for the input channel, remove the clip lead and plug the cable into Line Out (or Spkr or Headphone Out). Use the Generator to create a waveform; the default 440 Hz sine is fine, but just about anything will do. Note if the input and output traces have the same polarity; if not, enter a minus sign ahead of the Wave Out Full-Scale Range value.

The above assumes you have already done an Auto-Calibration with the Duplex Delay option. That will insure that the input and output traces are properly aligned, so that you can tell if the polarity is the same. Without proper alignment, a delayed wave may look like an inverted one.

If you haven't done Auto-Calibration, you can use a Ramp or Pulse waveform to remove any ambiguity. A ramp that with a long Rise duration and steep fall will appear to have a slow fall and fast rise if inverted. Similarly, a narrow pulse with a long dwell time will be obvious if inverted.

Note that there is a peculiarity about the way the Range polarity interacts with the Generator output and the trace display. When you specify an inverted Wave Out value, both the generated wave and the display of that wave are inverted. The idea is that the display shows the actual output; if there is an inverting amplifier somewhwere after the digital-to-analog converter (DAC or D/A), then the generated digital signal must be pre-inverted to make it come out right after the inverter. The display inverts the generated signal to match.

However, if the Generator is already runing when you change the Range sign, it is not restarted with the inverted polarity. There is typically a second or two of prior waveform data already "in the pipeline" on the way to the DAC, which has to work its way out before the inverted data is seen. But since the display begins inverting as soon as you change the sign, you'll see a second or two of inverted prior data before the display and the data are both using the same sign.

See also Full-Scale Range Dialog, Output Voltage Method, DaquinOscope Method, Input Voltage Method, Calibration Menu


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