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Methods of preventing noise for power adapter

Date:2018-08-16 Hits:1350
Methods of preventing noise for power adapter

Methods of preventing noise for power adapter

Noise on the power adapter: If it is a linear power adapter, the first low frequency of 50Hz is a serious source of interference.


Since the primary incoming AC is inherently impure and is a sinusoidal wave of waves, it is easy to cause electromagnetic interference to the adjacent circuit, that is, electromagnetic noise. In the case of a switching power supply adapter, the noise is more serious, the switching power supply adapter operates in a high frequency state, and there is a dirty harmonic voltage in the output portion, which can generate a large noise to the entire circuit.

     Prevention method: Reasonable grounding, transmission of analog signals by differential structure, put decoupling capacitors at the output of the power adapter of the circuit, electromagnetic shielding technology, separation of analog digital ground, bottom line of signal lines, ground isolation, etc. In fact, I said that these are just the tip of the iceberg in terms of noise removal. Even those who have worked for 30 years will not fully master all of these technologies, because understanding the need for such things requires a strong technical foundation and considerable experience, but what I told you is basically enough. The noise floor is caused by the circuit itself, due to the impureness of the power adapter, the phase margin and gain margin of the circuit, etc., and the circuit itself and the device. This part needs to be improved during circuit design. Other noises are due to factors such as unreasonable circuit layout and wiring, electromagnetic compatibility, inter-wire interference, and the like.


The elimination of analog circuit noise is more dependent on experience than on scientific basis. The situation that designers often encounter is that after the analog hardware part of the circuit is designed, it is found that the noise in the circuit is too large, and the design and wiring have to be re-designed. This "try it out" design approach will eventually succeed after several twists and turns. However, a better way to avoid noise problems is to follow some basic design guidelines and make use of noise-related fundamentals when making decisions early in the design process.


The role of the preamplifier in the audio system is critical. This article first explains how an engineer should properly select components when designing a preamplifier for a home audio system or PDA. Subsequently, a detailed analysis of the source of noise provides guidelines for designing low noise preamplifiers. Finally, taking the preamplifier of the PDA microphone as an example, the design steps and related precautions are listed.

A preamplifier is a circuit or electronic device placed between a source and an amplifier stage, such as an audio preamp placed between a disc player and an advanced audio system power amplifier. The preamplifier is designed to receive weak voltage signals from the source. The received signal is first amplified with a small gain, sometimes even adjusted or corrected before being transmitted to the power amplifier stage, such as audio front. The amplifier can first equalize the signal and tone control. Whether it's a home audio system or a PDA design preamplifier, you have to face a very headache, which components should be used properly?


Component selection principle

        Because of the small size and performance of op amp ICs, many of these preamplifiers use this type of op amp chip. When designing a preamplifier circuit for a sound system, we must clearly know how to select the appropriate specifications for the op amp. System designers often face the following issues during power adapter design.

1. Is it necessary to use a high precision operational amplifier?

The input signal level amplitude may exceed the operational amplifier's error tolerance, which is not acceptable for op amps. If the input signal or common-mode voltage is too weak, the designer should use a high-precision op amp with a very low compensation voltage (Vos) and a high common-mode rejection ratio (CMRR). Whether or not a high-precision operational amplifier is used depends on how many times the amplification gain is required by the system design. The larger the gain, the more accurate the operational amplifier is needed.

2. What kind of power supply voltage does the operational amplifier need?

This problem depends on the dynamic voltage range of the input signal, the overall supply voltage of the system, and the output requirements. However, different power supply rejection ratios (PSRRs) of different power adapters can affect the accuracy of the op amp, which is battery-powered. The system is most affected. In addition, the power consumption is also directly related to the quiescent current and supply voltage of the internal circuit.


3. Does the output voltage need to be full swing?

Low supply voltage designs typically require a rail-to-rail output to take advantage of the entire dynamic voltage range to increase the output signal swing. As for the issue of rail-to-rail input, the configuration of the op amp circuit has its own solution. Since preamplifiers are typically configured with inverting or non-inverting amplifiers, the input does not need to be full swing because the voltage common mode (Vcm) is always less than the output range or equal to zero (with very few exceptions, such as a floating grounded single, Supply voltage operational amplifier).


4. Is the issue of gain bandwidth more worrying?

Yes, especially for audio preamps, this is a very worrying issue. Since human hearing can only perceive sounds in the frequency range of about 20 Hz to 20 kHz, some engineers ignore or despise this "narrower" bandwidth when designing an audio system. In fact, important technical parameters that reflect the performance of audio devices such as low total harmonic distortion (THD), fast slew rate, and low noise are all necessary for high gain bandwidth amplifiers.

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