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Calibrate power supplies, boost signal quality

04 Jan 2016  | Chris Grachanen

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Power supply users occasionally inquire about what they need to get power supplies calibrated. The typical justification given for calibrating power supplies is output accuracy. In other words, how well does a power supply's output setting and displayed output values (feedback) compare to its actual voltage and current?

Users frequently challenge this justification, especially those who actively monitor the outputs of their supplies using a voltmeter and current shunts. Actively monitoring a power supply's output with a meter would seem to render the need for calibration mute, but this isn't the case when you consider the integrity of the power supply's output.

Power supply signal integrity is basically concerned with the correctness of generated outputs without significant alteration by noise, harmonic distortion, transients, and so on. Significant is used in this definition because all power supplies exhibit some degree of alteration from outside influences such as changes in line voltage, but are designed so that these influences don't degrade performance. For DC power supplies, signal integrity is primarily concerned with Periodic and Random Deviation (PARD) (figure 1) defined as:

The term PARD is an acronym for "Periodic and Random deviation" and replaces the former term ripple and noise. PARD is the residual ac component that is superimposed on the dc output voltage or current of a power supply. It is measured over a specified bandwidth, with all influence and control quantities maintained constant. ... Attempting to measure PARD with an instrument that has insufficient bandwidth may conceal high frequency spikes that could be detrimental to a load.


Figure 1: DC output of power supply and superimposed PARD component.


For AC power supplies, signal integrity is mainly concerned with Total Harmonic Distortion (THD) (figure 2) defined as: Harmonic Distortion is described as the interference in an AC power signal created by frequency multiples of the sine wave. Total Harmonic Distortion is used as a measure of the amount of Harmonic distortion in the system.

Figure 2: An oscilloscope can show the differences between a clean sine wave sine wave (top) and one with distortion, something you can't see with a meter.


It's easy to ascertain from the figures how excessive PARD for DC power supplies and excessive THD for AC power supplies can compromise power supply signal integrity, which may adversely affect the operation of the circuits/devices they power. It may also be apparent from the above waveforms the challenge to quantify PARD and THD using just a monitoring voltmeter and current shunts.

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