“Controlling noise in analog and mixed-signal circuits is important, but not straightforward. Signal integrity depends on controlling noise in the signal chain. If not controlled, the noise can interfere with the system and even affect the operation. This article will first introduce various noise sources and types, look at how noise enters the signal chain through various components, then describe how to use PCB layout techniques to help control noise, and finally review some techniques for controlling noise.
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Controlling noise in analog and mixed-signal circuits is important, but not straightforward. Signal integrity depends on controlling noise in the signal chain. If not controlled, the noise can interfere with the system and even affect the operation. This article will first introduce various noise sources and types, look at how noise enters the signal chain through various components, then describe how to use PCB layout techniques to help control noise, and finally review some techniques for controlling noise.
Noise can arise from a variety of sources and manifest in a range of frequencies, bandwidths, and spectral distributions (Figure 1). Noise comes in the form of common-mode or differential-mode energy. It can originate within the system or from an external source. Regardless of its specific characteristics or source, noise is an undesirable form of energy.
Figure 1: Noise is caused by a variety of sources and can manifest in a variety of shapes, sizes, and frequencies. (Image: Altium)
Source and type of noise
There are a range of noise sources in analog and mixed-signal systems. Some common noise types include white noise, pink noise, Johnson noise, quantization noise, and popcorn noise.
White noise has a flat spectrum and is uniformly distributed throughout the frequency domain. Reducing the bandwidth removes white noise from the analog signal. Reducing the bandwidth of the amplifier by a factor N reduces the RMS white noise by the square root of N.
Pink noise, also known as flicker or 1/f noise, has a frequency dependence that is inversely proportional to frequency. The pink noise energy at each frequency level drops off at a rate of about 1 to 3 dB per octave. This is in contrast to white noise, which has the same energy at all frequency levels. Therefore, pink noise is strongest at low frequencies, but at high frequencies it decays, making white noise the dominant noise source. Designing to eliminate pink noise at the PCB level presents certain challenges.
Johnson noise, also known as thermal noise, is the random excitation of electron motion as a function of temperature. Johnson noise is unavoidable and can only be completely eliminated at absolute zero.
Quantization noise is the result of errors introduced by quantization in analog-to-digital converters (ADCs). It is nonlinear and signal dependent. This is caused by the error between the ADC’s analog input voltage and the output digitized value.
Popcorn noise or burst noise, it is low frequency, it is caused by device defects, it is completely random and therefore unpredictable.
Various sources of noise are inherent in Electronic components and combine into the noise figure of the input and output. Noise analysis determines the noise level, the noise floor, below which any signal will be indistinguishable. The noise floor of an interconnect or component is defined by the input noise from all sources, the bandwidth of the component or circuit element, and the noise figure of the interconnect or component. Noise can enter the signal chain through various components such as:
• ADC, which generates thermal and quantization noise.
• Amplifiers producing broadband and 1/f noise
• Voltage reference, producing wideband and 1/f noise
• Jittered clocks.
• Power supplies, especially switch-mode converters, generate various types of periodic and random noise.
• Printed circuit board (PCB) layout that couples noise from external sources into the system.
• Sensors that transmit various external noises into sensitive systems (Figure 2).
Figure 2: Example of a noise source in a sensor node. (Image: Texas Instruments)
Current spikes that appear on some signals can create noise in the PCB. In analog circuits, these spikes are usually caused by changes in load current, while in digital circuits, current spikes are caused by transistor switching. Incorrect grounding or floating grounds can also cause noise. Grounding is especially important. In PCB systems with a maximum signal frequency of 1MHz or low frequencies, a simple single ground point is usually sufficient. If higher frequencies are involved, a star or multi-point ground architecture is usually required. Some designs combine single-point grounding for low-frequency modules and multi-point grounding schemes for high-frequency modules.
Figure 3: Multi-point grounding is an effective tool for reducing noise. (Image: Photoelectronics)
Component placement is also important when minimizing PCB noise sources and levels. Some common suggestions include:
• Placing power components close together on the same layer reduces inductance between vias.
• High frequency components to keep trace lengths as short as possible.
• Place decoupling capacitors as close to the power pins as possible to reduce current spikes from signal switching and minimize ground bounce. A rule of thumb for selecting multilayer ceramic capacitors (MLCCs) for decoupling is to use 0.1 µF MLCCs at 15 MHz and 0.01 µF MLCCs for higher frequencies.
Given the many types and sources of noise, it is important to actively reduce and control noise in analog and mixed-signal designs. Common suggestions include:
• Use higher order filter circuits when needed to control noise outside the desired bandwidth. Filtering can be used to control analog noise sources and to control the rise/fall times of digital signals.
• The specified rise time cannot be faster than a device such as a comparator that reduces high frequency harmonics.
• When sampling with the ADC, the noise can be distributed over a wider bandwidth and the overall noise can be reduced by using a higher sampling rate and anti-aliasing filters.
• Amplifier noise can be reduced by specifying a device that does not exceed the necessary bandwidth, and filtering can be added to reduce the effective bandwidth if necessary.
Summarize
Noise is an undesired but inevitable product of analog and mixed-signal circuit design. Noise comes from a variety of sources and has a variety of energy characteristics or formats. It can enter the signal chain through a variety of mechanisms. Noise analysis determines the expected noise level in a given design, called the noise floor, below which any signal will be indistinguishable. Designers can use a range of tools to control the impact of noise on system performance.
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