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Understanding AC Power Source Measurements, Part 4: Analog or Digital?

June 23, 2016


I-IX_SII_Main_Image The California Instruments iX Series are AC sources that use a digital measurement system.


Programmable AC power sources are primarily used to provide a low distortion, precisely controlled sinusoidal voltage to a unit under test, but some AC sources, such as the California Instruments I-iX Series II, perform measurements as well. In this series of blog posts, we'll discuss various aspects of making measurements with your AC power source:

  • Part 1 describes the benefits of using sources for measurement and how to make voltage and current measurements.
  • Part 2 describes how to make frequency and power measurements.
  • Part 3 describes how to make power factor and crest factor measurements.
  • In Part 4, we'll talk about how AC power sources perform measurements.

Analog or Digital?

Measurement systems can be implemented in one of two ways, analog or digital. Analog systems use one or more RMS converters to determine the RMS level of the voltage and current signals. For true power measurement, an analog multiplier is used which multiplies voltage and current and feeds the output into an RMS converter.

The California Instruments iX Series uses a digital measurement system.More often than not, analog measurement systems use a single RMS converter in combination with a multiplexer. The multiplexer routes the required signal to the RMS converter. The RMS converter output is digitized using a high resolution, but slow, sample rate A/D converter. No high-speed A/D conversion is required with this approach. This significantly reduces cost and required board space.

AC power sources with analog measurement systems generally cost less and provide very accurate readings.

A digital measurement system, on the other hand, digitizes voltage and current signals in realtime using high-speed analog-to-digital converters (ADCs). To make true power measurements, each channel (voltage and current) must either have its own ADC or a sample and hold circuit to ensure voltage and time measurements are made at the same time. If not, calculations for power and power factor will be inaccurate.

The instrument stores the measurements in memory and processes them to produce true RMS, power, power factor and crest factor readings. Digital measurement systems use a digital signal processor (DSP) in order to perform the required calculations fast enough.

The California Instruments iX Series are AC sources that use a digital measurement system. To ensure that the measurements are highly accurate, the measurement system use a pair of 18-bit sigma-delta analog-to-digital converters to digitize voltage and current values.

While digital measurement systems are generally more expensive than analog systems, they have capabilities not found in analog systems that justify the higher cost. Since the time domain information for the voltage and current is available, the DSP can apply digital filtering and FFT algorithms to provide harmonic analysis of both voltage and current. In addition, the time domain waveforms can be displayed on the front panel of the AC Source or on a PC, providing the operator with more details on the voltage and current than would be possible with an analog measurement system.

For more information, dcontact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.