Analog Electronics - Considerations for Low Noise System Design

For the nominal noise in electronic circuits, it can be generally considered that it is a general term for all signals other than the target signal. Initially, people called those electronic signals that caused noise from audio equipment such as radios as noise. However, the consequences of some non-target electronic signals on electronic circuits are not all related to sound, so people gradually expanded the concept of noise. For example, those electronic signals that cause white spots on the screen are also called noise. It can be said that all signals in the circuit except the target signal, regardless of whether it affects the circuit, can be called noise. For example, ripple or self-excited oscillation in the power supply voltage can have an adverse effect on the circuit, causing the audio device to emit AC sound 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 in the circuit. There is also a radio wave signal of a certain frequency. For a receiver that needs to receive this signal, it is a normal target signal, but for another receiver it is a non-target signal, that is, noise. The term interference is often used in electronics, and sometimes it is confused with the concept of noise. In fact, there is a difference. Noise is an electronic signal, while interference refers to a certain effect, which is an adverse reaction to the circuit caused by noise. The existence of noise in the circuit does not necessarily mean interference. In digital circuits, it is often possible to use an oscilloscope to observe that some small spike pulses mixed with normal pulse signals are not expected, but a kind of noise. However, due to the characteristics of the circuit, these small spike pulses will not affect the logic of the digital circuit and cause confusion, so it can be considered that there is no interference.

When a noise voltage is large enough to interfere with a circuit, it is called interference voltage. The maximum noise voltage applied to a circuit or a device while it can still maintain normal operation is called the anti-interference tolerance or immunity of the circuit or device. Generally speaking, noise is difficult to eliminate, but we can try to reduce the intensity of the noise or improve the circuit's immunity so that the noise does not cause interference.

The first trick to low noise system design is to apply as much gain as possible in the first stage. The figure below shows two examples of an amplifier front end with a gain of 10. It can be seen that applying all the gain to the first stage is much better than distributing the gain between two stages. Please note that sometimes the requirement for the best bandwidth performance may conflict with the requirement for the best noise performance. For bandwidth, we want each gain stage to have similar gain, while for noise, we want the first stage to have all the gain.

The second tip is to pay attention to the source impedance. There are two reasons for this: first, the larger the source impedance, the more noise there will be in the system; second, the amplifier must be well matched to the source impedance, and if the source impedance is high, the current noise characteristic may be more important than the voltage noise characteristic.

The third tip is to pay attention to the feedback resistor. If you choose an ultra-low noise op amp but use a large feedback resistor, it is impossible to achieve a low noise circuit. In the same phase (Figure 7) or inverting configuration, pay attention to the feedback resistor, which is equivalent to the noise source referred to the output. The other resistors are equivalent to the voltage source at the input, or more accurately, the voltage source at the input of the inverting configuration. As mentioned earlier, when designing a low noise system, the first stage application has a high gain, in which case Rg noise dominates.