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          Signal Chain Solutions for Medium Bandwidth Applications


Figure 1. Signal chain processing has proved invaluable to condition-based monitoring of critical assets in factories, processing plants, and manufacturing facilities.

A simple signal chain receives analog data from a sensor and conditions it so that the digital output is precise, accurate, and reliable. Conditioning is accomplished through a series (or chain) of signal conditioning hardware such as ADCs (Analog to Digital Converter), filters, amplifiers. But, while this explanation may seem simple, designing a signal chain for a real-world application such as condition-based monitoring can prove formidable.
Signal Chain Categories

One of the easiest ways to begin a discussion of signal chains is to look at how they are categorized based on bandwidth: narrow, medium, and wide.  
Signals of interest for narrow bandwidth signal chains are in the range of DC to approximately 10 kHz. Among the applications for these precision signal chains are DMM (Digital Multimeters), SMU (Source Measurement Units), weight scales, battery testing and inspection, chromatography, seismic survey equipment, energy metering, and semiconductor manufacturing.  
Signals in the range of DC to 1 MHz are considered wide bandwidth. These signal changes are used as solutions for DC linearity, THD (total harmonic distortion), SNR (measurement noise), settling time performance, and closed-loop or measurement latency. In addition, they are often found in EV (Electrical Vehicle) technology and medical equipment.  
There are also medium bandwidth signal chains used with signals in the range of DC to 500 kHz. Typical applications include CBM (Condition-based Monitoring), power quality, multimodal DAQ (Data Acquisition), and others shown in Figure 2. 
Figure 2. Typical application areas for medium bandwidth signal chains. 

This type of signal chain is recommended when time-domain and frequency-domain analysis are needed, hence their common use in vibration monitoring. And working with these types of signal chains poses several design challenges.
Design Considerations for Medium Bandwidth Signal Chains

When implementing any type of signal chain, precision, accuracy, repeatability, and reliability are always crucial. In addition, aliasing, quantization noise, and switched capacitor input are always considerations for general-purpose signal chain design.  
Latency performance, linearity, and noise are essential for data that requires time-domain analysis (such as sonar, CbM, motor control, and position monitoring). However, for frequency domain analysis (which includes audio, power quality measurement, and dynamic signal analyzers), bandwidth, noise, and total harmonic distortion receive the most emphasis when configuring the signal chain.  
Time-critical data measurements, such as motor control, rotation/position monitoring, or analog I/O, require low-latency solutions. In addition, when phase accuracy across multiple channels is essential, as would be the case for DAQ instruments and power quality measurement, noise and dynamic range with total harmonic distortion must be addressed.  
And for most practical applications, these requirements are combined. For example, a CbM system may use data from the same sensors for both time and frequency domain analysis, and there may be a need for phase accuracy across two or more channels. In addition, motor controls often need time-critical data suitable for time-domain analysis. Engineers often find it challenging to meet these multiple design requirements while still achieving precision, accuracy, repeatability, and reliability.  
Signal Chain Implementation for a Rotary Encoder

For example, consider implementing a medium bandwidth signal chain for an encoder used to monitor the rotational position of a motor shaft. The system must provide low latency performance with minimal noise and excellent linearity and be low cost while requiring minimal board area. 

Figure 3. Simple block diagram of a rotary encoder signal chain. 

Figure 3 shows the basic design of this type of signal chain, including a precision reference, reference buffer, anti-aliasing filters, and ADC drivers. However, finding the right components to achieve accuracy, precision, and reliability while still meeting design constraints can be challenging -- and one of the solutions to these challenges is to take advantage of a medium bandwidth signal chain platform.  
Figure 4 shows the ADI Medium Bandwidth Signal Chain platform components that fulfill the basic design specifications and fall within cost and board space constraints. This approach uses ADI Precision technology such as the AD8656 low noise precision dual amplifier, ADA4945 low noise fully differential ADC driver, the AD4681 dual simultaneous sampling SAR ADC, and the LTC6702 low power dual comparator.  

Figure 4. ADI Medium Bandwidth low cost reduced board area solution for a simple rotary shaft encoder.

These solutions utilize 14-bit performance for cost-sensitive applications and a reduced data rate to achieve a lower cost signal chain. In addition, the use of integrated functions aid in both minimizing the number of hardware components required and the board space required. And, referring back to Figure 3, the Medium Bandwidth signal chain solution also uses an integrated LDO, reference, and reference buffer to reduce the size of the solution further.
ADI Signal Chain Platforms

ADI has several platforms for developing signal chains. These platforms are engineered specifically for precision signal chains and provide outstanding performance. In addition, ADI has worked to simplify the customer design journey and provide unrivaled support and tools for design that can significantly reduce customer time to market.  
The Medium Bandwidth platform provides best-in-class accuracy, stability, and resolution for signals up to 500 kHz. It also supports trade-offs of accuracy, bandwidth, or stability to meet cost, density, or power consumption requirements. In addition, its measurement and drive solutions provide accurate insights into the dynamics of complicated electromechanical systems to support enhanced condition monitoring, measurement, and control. 

Developing medium bandwidth signal chain systems must balance conflicting performance measures and fall within cost or power consumption requirements. Engineering such a solution can be complicated and lead to unexpected delays and extensive testing. But, using a signal chain platform -- such as ADI Medium Bandwidth -- can significantly reduce the complexity and time required to achieve a highly accurate, precise signal chain system.