Controls: Calibration Menu >> Full-Scale Range

This approach to Full-Scale Range calibration involves measuring the Generator output with an external voltmeter and setting the Output Range, then applying the now-calibrated signal to the input to get the Input Range.

To measure the output voltage accurately you need a separate voltmeter that has a sensitive AC Volts range, ideally one that can read 3 decimal places (1.999), or at least 2 places (19.99) AC volts full-scale. Connect it to the output and set the Generator to produce a sine wave at maximum volume and 100% Level. The frequency should be in the range of 50 to 1000 Hertz for best accuracy on most meters. The reading from the meter will be an RMS value, which you must convert to amplitude by multiplying by 1.4142 (the square root of 2). Enter that value for Wave Out in the Range dialog.

If the sound card output distorts at the maximum setting (see below), you might want to run at a slightly softer volume. This is possible as long as you have done a relative calibration (either manually or with Auto-Calibrate). Just use spectrum mode (with RMS and Y-log off) to read the peak value that Daqarta reports for the output, which assumes whatever Range value is currently present (1.000 Volt default).

You can use the Spectrum Cursor Track option to automatically position the solid cursor at the peak, and use the Spectrum Cursor Peak option to correct for leakage energy that might otherwise appear as spectral "skirts". (If you wish, you can avoid this leakage completely by setting the Generator frequency to fall exactly on a spectral line using the Step Lines entry mode, but this is probably overkill here.)

The true Wave Out range will then be the current Wave Out range (default 1.000) times the true amplitude divided by the amplitude reported by Daqarta.

RangeOut * Vmeter / Vcursor

You can avoid the need to convert the RMS meter reading to amplitude first, if you set Daqarta to read RMS as well. Just remember to toggle that off for the Input readings that will follow.

Now Daqarta has become a calibrated signal generator which you can use as a reference signal source to calibrate the inputs. Adjust the output volume and/or Stream Level until you get a large (nearly full-scale) input, without over-driving it into distortion. One easy way to do this is to monitor the spectrum instead of the waveform; when you drive the input too hard, you will see a sudden onset or rise of spectral peaks at harmonic frequencies. This is much more sensitive than judging sine-wave distortion by eye.

It is unfortunately all too common for sound card inputs to distort well before the input signal reaches full-scale... sometimes as little as half-scale.

Write down the cursor value that Daqarta reports. Since Daqarta is assuming whatever Range value is currently set (typically 1.000 Volt full-scale), it is unlikely to be the true input voltage... yet.

Now read the signal level that produces that input. To do that, toggle off all the display channels except for the output channel, so that the solid cursor reports the (calibrated) Generator output voltage. To find the true full-scale input range, multiply the current Range by the output voltage divided by the apparent input voltage.

RangeIn * Vout / Vin

For example, suppose the true output is 250 mV where the apparent input is 0.600 Volt with the default full-scale range of 1.000 Volt. Then the correct full-scale value will be:

1.000 * 0.250 / 0.600 = 0.4167

Enter the computed value as the Range value for that Input line.

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