Suppress circuit noise

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/ 2019-05-24 / News

For the noise nominal in electronic circuits, it can be broadly considered that it is a general term for all signals except the destination signal. At first, the electronic signals that caused the noise emitted by audio equipment such as radios were called noise. However, the consequences of some non-purpose electronic signals on electronic circuits are not all related to sound. Therefore, people gradually expanded the concept of noise. For example, those electronic signals that cause white lines on the screen are also referred to as noise. It may be said that all signals except the intended signal in the circuit, regardless of whether it affects the circuit, can be called noise. For example, ripple or self-oscillation in the power supply voltage can adversely affect the circuit, causing the audible device to emit hum or cause the circuit to malfunction, but sometimes it may not cause the above consequences. For such ripple or oscillation, it should be called a kind of noise of the circuit. There is also a radio wave signal of a certain frequency, which is a normal destination signal for a receiver that needs to receive such a signal, and a non-target signal, that is, noise, for another receiver. The term interference is often used in electronics, sometimes confused with the concept of noise. In fact, there are differences. Noise is an electrical signal,Interference refers to an effect that is an adverse reaction to the circuit due to noise. There is noise in the circuit, but there is not necessarily interference. In digital circuits. It is often ostensible to observe that mixing small spikes on a normal pulse signal is undesirable, but a noise. However, due to the circuit characteristics, these small spikes are not disturbed by the logic of the digital circuit, so it can be considered that there is no interference.

 

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When a noise voltage is large enough to disturb the circuit, the noise voltage is called the interference voltage. Or the maximum noise voltage applied by a device while it is still operating normally is called the interference immunity or immunity of the circuit or device. In general, noise is difficult to eliminate, but it can be tried to reduce noise. Intensity or increase the immunity of the circuit so that noise does not cause interference.

 

How noise is generated in electronic circuits? How to suppress

 

This is mainly due to the digital circuitry and power components in the circuit. In digital circuits, high frequency digital levels are common and these levels can be generated. Two kinds of noise: 1, electromagnetic radiation,Just like the antenna of a TV, it emits electromagnetic waves to interfere with the circuit next to it, which is the noise you are talking about. 2. Coupling noise means that there is a certain coupling between the digital circuit and the circuit next to it. The noise can directly affect other circuits directly on the electrical device, and this noise is more powerful.

The noise present on the power supply: If it is a linear power supply, the low frequency 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. If it is a switching power supply, the noise is more serious, the switching power supply 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, decoupling capacitors at the output of the circuit, electromagnetic shielding technology, separation of analog digital ground, bottom line of signal lines, ground isolation and many more. In fact, what I said is only the tip of the iceberg in terms of noise removal. Even those who have played for 30 years will not fully master all of these technologies.Because understanding the possession of such things requires a strong technical foundation and considerable experience, but I have told you that these are generally sufficient.

The noise floor is caused by the circuit itself. Due to the impureness of the power supply, the phase margin and gain margin of the circuit are not suitable, etc., the circuit itself and the device. This part needs to be improved during circuit design.

Other noise is due to factors such as unreasonable circuit layout and wiring, electromagnetic compatibility, interference between wires, and so on.

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.

 

Design method for low noise preamplifier circuit

 

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 the 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 perform 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 Principles

 

Because of the small size and excellent performance of op amp ICs, many preamplifiers are currently available. These types of operational amplifier chips are used. When designing a preamplifier circuit for a sound system, we must clearly know how to select the appropriate specifications for the op amp. System design engineers often face the following issues during the design process.

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 very 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 op amp 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 (PSRR) of different power supplies can affect the accuracy of the operational amplifier. Sex, where battery-powered systems are the 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 common mode voltage (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.

 

Learn more about noise

 

Before designing a low noise preamplifier Engineers must carefully examine the noise from the amplifier. In general, the noise of the op amp comes mainly from four aspects:

1, thermal noise (Johnson): due to irregular fluctuations in the electron energy of the current in the electrical conductor The resulting thermal noise with broadband characteristics has a direct relationship between the square root of the voltage rms value and the bandwidth, electrical conductor resistance, and absolute temperature. For resistors and transistors (such as bipolar and field effect transistors), such noise effects cannot be ignored because their resistance values are not zero.

2, flicker noise (low frequency): noise due to the continuous generation or integration of carriers on the crystal surface. In the low frequency range, such flicker appears in the form of low frequency noise, which becomes "white noise" once it enters the high frequency range. The flicker noise is mostly concentrated in the low frequency range, causing interference to resistors and semiconductors, and the bipolar chip is more disturbed than the field effect transistor.

3. Shooting noise (Schottky): Schottky noise is generated by current carriers with particle characteristics in the semiconductor, and the root mean square value of the current and the average bias current of the chip and There is a direct relationship to bandwidth. This noise has the characteristics of broadband.

4, popcornfrequency (popcornfrequency): If the surface of the semiconductor is contaminated, it will produce such noise, the impact of which is as long as several milliseconds to a few seconds, the cause of noise is still unknown, under normal circumstances, There is no certain pattern. The use of cleaner processes in the production of semiconductors can help reduce such noise.


In addition, since the input stages of different operational amplifiers have different structures,Therefore, the difference in transistor structure makes the amount of noise of different amplifiers very different. Below are two specific examples.

 

Bipolar input op amp noise: The noise voltage is mainly caused by the thermal noise of the resistor and the high-frequency shot noise of the input base current. The low-frequency noise level The size depends on the low frequency noise generated by the base current of the input transistor flowing into the resistor; the noise current is mainly generated by the shot noise of the input base current and the low frequency noise of the resistor.

 

The noise of the CMOS input op amp: the noise voltage is mainly caused by the thermal noise of the channel resistance of the high frequency region and the low frequency noise of the low frequency region, and the corner frequency of the CMOS amplifier ( The cornerfrequency is higher than the bipolar amplifier, and the broadband noise is much higher than that of the bipolar amplifier; the noise current is mainly generated by the input gate leakage noise, and the noise current of the CMOS amplifier is much lower than that of the bipolar amplifier, but the temperature rises. 10 (C, its noise current will increase by about 40%.

Keywords: Circuit noise
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