AC measurements
PrintThe measurement of the analog characteristics of AC signals (signals whose amplitude changes as a function of time), including voltage (RMS, average, peak, or peak to peak), and waveform parameters.
Tips on AC measurements
High-frequency measurements often require an oscilloscope probe whose frequency response can be adjusted to compensate for cable length. Usually, you can calibrate the probe by connecting it to a known, high-quality square wave. Then, adjust the probe to give as clean a square wave as possible without ringing or corner rounding.
High-voltage measurements often require voltage dividers (attenuators) that can be obtained in probes. Be extremely careful when measuring circuits above 42 V since they can present a significant shock hazard. Carefully follow any safety warnings on the DAQ card or instrument, and make sure that you don't touch any "hot" or floating grounds.
Know how much of a load you can place on the unit being measured. If, for example, the output impedance of your UUT (Unit Under Test) is 100 kohm and you are connecting an instrument with an input impedance of 1 Mohm, you may get up to 10% error in your measurement.
The ratio of the output impedance of your device to the input impedance of your instrument should be at least three times lower than your expected accuracy. For example, if you expect an accuracy of better than 1%, the output impedance should be less than 0.3% of the input impedance of your instrument or DAQ card.
Buffer or conditioning amplifiers may be necessary between the unit you are testing and your measuring system if you have a long distance, have too low a load impedance, or are subject to noise. The more sensitive to noise and loading that your UUT is, the closer to it you should connect the conditioning amplifiers.
An example of basic RMS measurements is given in this interactive Demo using the LabVIEW Player.
Approaches:
Approach 1
1. Verify that your system is free from noise and hum. Although you can sometimes reduce noise and hum by averaging, the measurement time will always be longer than when measuring in a "clean" system.
2. If you know the waveform of the signal that you are measuring, you can often improve the accuracy (and speed) significantly. For example, if you know that you are measuring a sine wave, you should choose measuring tools that make it possible to trigger and average the value over an integral number of periods of the signal. National Instruments DAQ products have this capability.
3. Use digital signal processing techniques to achieve the highest accuracy possible. For example, some products provide sophisticated signal processing that gives state-of-the-art RMS measurements with much shorter measurement times than conventional instruments.
Approach 2
1. If you are using a conventional voltmeter, make sure to analyze the entire measurement chain, including how long it takes to acquire, transfer, and process data in the controlling computer.
2. If you are measuring the RMS voltage and/or frequency and phase of a sine wave, you should use a digitizing product (such as an NI-DAQ product) and a set of advanced algorithms to give maximum speed.
3. Multi-channel measurements: Although multiplexed solutions are the least expensive, they may take more time than simultaneously sampled measurements. For example, if you need to measure 16 channels of 100 Hz signals, a 100 kHz MIO card (multiplexed) will be able to provided 6,000 samples/s per channel. This is more than adequate to accurately measure a 100 Hz signal, and therefore, the multiplexed approach is fast as well as the most economical.
If, however, you need to measure four channels at 500 kHz, there is no multiplexed solution fast enough to accurately characterize each channel in real time. Therefore, you should choose a simultaneous sampling solution, such as the PCI-6110E, which samples four channels simultaneously at 5 Msamples/s.
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