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 Lecture for Measurement

Posted 2020/2/26

DC-power-supply (AC-DC power supply)

Noise due to digital part circuit of power supply

As introduced in the previous article, “Circuit method and features of DC power supply,” linear regulator type power supplies have excellent noise performance, but humming noise and noise due to disturbance may be superimposed.

In addition to that issue, noise caused by the digital circuit may also be superimposed, so this article will describe noise sources that are often overlooked.

It is well known that noise such as clocks affects the analog part of a power supply unless the digital part and the analog part such as a regulator circuit in the case of a power-supply device are isolated.

So what else we need to be aware of?

Figure 1: Isolation between analog and digital circuit

1. Noise emitted from the display

Noise from the digital section that is often overlooked is the noise emitted from the display.

As for countermeasures against noise from the digital circuit, although clock sources and the CPU tend to be attention, but the display also emits noise.

Figure 2: Noise emitted from the display

As well as the noise emitted from the front of the display noise is also emitted to the inside of the power supply.

If this noise from the display affects the analog circuit, the low noise power supply will not able to maintain its original performance.

The graph below shows the noise characteristics of a general linear power supply.

Figure 3: Noise characteristics of linear power supplies

Several spectra exist in the megahertz range, and represent noise emitted from the display.

No matter how the digital and analog circuits are isolated on the board, the noise emitted from the display may jump the space and affect the analog circuit.

When display was using the seven-segment LED, their noise affects only few. Due to the general use of LCDs that emit large noise, it is now necessary to pay attention to the noise from the display.

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Bias-voltage and control-voltage source for sensors and devices.

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