Table of contents
Explanation of technical terms
5. Calculate the noise of the signal chain
overview
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This article describes the steps involved in performing a theoretical analysis of the noise performance of a high-speed, wide-bandwidth signal chain. Although a specific signal chain is chosen, the steps presented apply to all types of signal chains.
It is mainly divided into five steps: give assumptions, draw a simplified schematic diagram of the signal chain, calculate the equivalent noise bandwidth of each signal chain module, calculate the noise contribution of each module at the output of the signal chain, and finally add all the noise. The analysis shows how all noise contributions can be described using simple mathematical calculations. Knowing each block's contribution to the total noise allows the designer to make appropriate modifications to the design (eg, component selection) to optimize its noise performance.
This article focuses on the last step
overall architecture process
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When designing a measurement signal chain, it is important to use noise analysis to determine whether the signal chain solution has low enough noise so that very small signals of interest can be easily extracted. A detailed noise analysis can save time and money in the production process. This article will outline the main steps required to perform a signal chain noise analysis. We will use the Power Optimized Current and Voltage Measurement Signal Chain from Analog Devices Precision Wide Bandwidth Technology page as an example.
Explanation of technical terms
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ADC:
Analog-to-digital converter, or A/D converter, or ADC for short, usually refers to an electronic component that converts an analog signal into a digital signal. A common analog-to-digital converter converts an input voltage signal into an output digital signal. Since the digital signal itself has no practical significance, it only represents a relative size. Therefore, any analog-to-digital converter needs a reference analog quantity as a conversion standard, and the more common reference standard is the largest convertible signal size. The output digital quantity represents the magnitude of the input signal relative to the reference signal
noise:
With respect to the undesired offset of the measured and supplied quantities, this offset occurs more or less randomly, usually with a wide frequency spectrum. Note: Noise is part of the period and/or random offset (PARD), measured under specified conditions.
technical details
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5. Calculate the noise of the signal chain
To add up all noise contributions, use the sum-of-squares method:
noise spectral density
Considering the ADC sampling frequency, the noise spectral density (NSD) can be calculated.
Important Notes
The NSDs of different devices can only be added directly if measured over the same bandwidth.
The choice of signal filter resistor values depends on the noise requirements of the application, the power dissipation of the signal chain, and the bandwidth of interest.
Active filter configuration
The choice to use active or passive filters in the signal chain will depend on the application. The active filters used in the analysis feature low power consumption and low noise. However, its distortion performance across the frequency range is not very good and may not be suitable for some applications.
When active filters are used, noise is generated by the amplifier that is part of the active filter. Passive filter circuits do not use filter amplifiers, so this is not needed.
This is specific to the amplifier driver used.
Here, we can use a single-ended equivalent circuit with all the noise appearing at the positive input of the op amp. All other calculations remain as above.
summary
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It will be roughly divided into five steps in total, and this section mainly talks about the first and last step. Designers will be able to analyze and calculate the noise performance of the selected signal chain. Analysis will provide useful insights into how different components in the signal chain affect noise performance and how to minimize noise (for example, by varying resistor sizes, changing components, or minimizing the equivalent noise bandwidth). This allows the designer to create a proposal that ensures the signal chain can extract the very small signal of interest, helping to save time and money. The description will continue later.
These two conclusions have also been drawn, and these two conclusions are also very important, some of which are emphasized here.
The NSDs of different devices can only be added directly if measured over the same bandwidth.
The choice of signal filter resistor values depends on the noise requirements of the application, the power dissipation of the signal chain, and the bandwidth of interest.