Portable Precision Analog Signal Chains

Taking Precision Analog Signal Chains on the Road

Image Source: Vitaliy/stock.adobe.com

By JJ DeLisle for Mouser Electronics

Published July 19, 2024

Sensor technologies are the pivotal input components of the Internet of Things (IoT), diverse automation systems, and the tools we use to measure and analyze the performance of various technologies. While many sensors today incorporate the analog-to-digital conversion process, traditional analog signal chains are important to precision measurement applications, particularly in test and measurement scenarios.

Test and measurement instruments (Figure 1) typically combine high-performance sensors, analog processing, digital-to-analog conversion, digital processing, digital storage, and digital communication technologies. For low-precision test instruments or general communication and sensing technology, analog signal processing circuitry is often minimal, sufficient to deliver adequate-fidelity signals to the analog-to-digital converters (ADCs) where most signal processing occurs.

Figure 1: An ultrasonic test used to detect imperfections in a thick steel plate. (Source: sakarin14/stock.adobe.com)

However, relying solely on digital signal processing is insufficient for test and measurement instruments. The incoming fidelity of the analog signals dictates an instrument's maximum achievable precision and error-correction capability. This is why analog signal processing is still alive and well in the age of digital electronics. Despite the prominence of digital processing, there is no substitute for quality analog signal chain design.

Designing portable instrumentation introduces additional challenges. The market demands compact, feature-rich, precise, and user-friendly test and measurement instruments that can operate over a broad performance range—all while incorporating the latest wired and wireless communication technologies and maintaining low power consumption. For portable test and measurement instruments, the analog processing must be done using ultra-low-power (ULP), high-precision designs that are as compact as possible.

This article examines the trade-offs of implementing a low-power precision signal chain for portable measurement.

Defining Analog Signal Chain, Precision Signal Chain, and ULP

The analog signal chain refers to the circuit pathway traveled by an analog signal, extending from sensors like thermocouples, strain gauges, and pressure transducers to the final analog processing stage prior to digitization or further processing such as collection, display, or storage. It commonly encompasses other components like amplifiers, filters, and associated electronics preceding digitization at the ADCs.

A precision analog signal chain is designed with performance parameters that introduce minimal uncertainty to the system while conditioning the signals to desired levels and preparing for capture at an optimal level. A low-power analog signal chain is designed for prolonged low-power use on limited supply power, such as from a portable battery.

Low-power systems are usually deployed in environments where size and weight matter and where it may be difficult to provide external power access or change batteries. Hence, many low-power systems are designed to operate using microamps of current at low voltages for extended periods. In extreme cases, a ULP system may be designed to operate for months or years on a single coin cell battery. These ULP systems are generally deployed in environments where access is difficult but may provide vital safety, security, or monitoring functions for critical systems or building maintenance.

Low-power analog systems typical of portable test and measurement systems are generally run from relatively efficient portable battery power packs or low-voltage DC sources (such as USB power supplies, battery banks, 12V automotive power systems) or renewable energy supplies (such as solar, wind, and electromagnetic energy harvesting). These systems can also receive power from the inertial energy of mobile systems, such as when a user is walking or moving. Additionally, crank-based rotational-to-electrical energy generators can be used for equipment designed for extremely remote environments where other renewable sources aren't viable. Examples include scientific excursions in cave systems, mines, underwater environments, remote mountainous regions, tundra conditions, and other extreme environments. Future trips to the moon or other celestial bodies may also necessitate ULP/LP analog signal chains with various types of low-power supplies designed for even more extreme environments.

Critical Performance Parameters

The following is a list and discussion of the most common critical electrical performance parameters for precision analog signal chains:

• Noise floor, added noise, and dynamic range
• Maximum/minimum voltage, current, and power
• Frequency range
• Linearity
• Uncertainty
• Differential or single-ended
• Common-mode rejection
• Power supply rejection
• Additional sensor/transducer input characteristics

The noise floor, added noise, and dynamic range of an analog signal chain determine the minimum and maximum voltage, current, or power of the signals that the signal chain can carry while introducing only specified uncertainty. If a signal falls below the noise floor, including the inherent noise from elements within the signal chain, it may not be captured accurately. Analog and digital filtering methods can be used to enhance the ability of a system to extract signals from noise, but these require foreknowledge of specific signal characteristics, add complexity, and introduce additional power usage. Signals above a certain voltage, current, or power range may exceed the handling capability of the signal chain and result in distortion, overload, or even stress-induced hysteresis in some analog circuit components and devices. The linearity of a signal chain plays into these limitations as well, where worse linearity results in lower dynamic range and constraints on the maximum frequency and resolution of a signal.

Every signal chain has a minimum and maximum frequency of signals it can effectively carry. Typically, the noise floor, signal level, frequency range, and linearity of an analog signal chain are the bounding parameters on the capabilities of a system, where every analog component, device, and routing also contributes to the overall uncertainty of the captured signals. Filtering and signal-processing methods, such as windowing and averaging, can reduce uncertainty. Ultimately, the uncertainty can limit the lower level, or signal strength, of the acceptable signal range and the reliability of the signals captured.

Depending on the type of transducer or preferred signal chain, the transducer and the preferred signal chain configuration may be differential or single-ended. Differential signal chains tend to present greater noise immunity and benefit from better common-mode rejection and power supply rejection. However, some transducer types are purely single-ended, and a single-ended signal chain tends to be more energy efficient and have lower complexity. A differential signal chain requires all differential components, devices, and routing, roughly double the complexity of a single-ended signal chain. Additionally, only differential amplifiers and filter designs can be used. There are circuit methods to convert single-ended to differential and vice versa, which may be desirable in high-noise environments or when a single-ended signal chain is more viable.

Some transducers present unique characteristics that must be accounted for with additional circuitry. For instance, transducers with extremely low current and voltage might need very high-gain instrumentation amplifiers or specialized current-mode amplifiers to preserve the signal. In other cases, it may take a long time before a transducer reaches a stable state suitable for making a reading (i.e., the response time of the transducer). In this case, engineers can include additional analog circuitry that can accumulate the signal energy without loss or introduction of errors over an extended period.

Trade-offs of Portability

To make an analog signal chain portable, the electronics and circuitry are typically constrained in size, weight, and power. Analog electronics also need to be resistant to shock, vibration, g-forces, and pressure and protected from environmental dangers such as corrosive gasses and fluids, humidity, moisture, debris, physical contaminants, and temperatures outside the specified range.

Large analog transducers and circuit components that lack power efficiency, robustness, or environmental resilience are often unsuitable for portable tests and measurement instrumentation. Moreover, portable instruments (Figure 2) are often employed where bench-top or laboratory instruments are not feasible. They may also be used in critical industrial, governmental/defense, medical, research, or security applications. Therefore, these portable test and measurement instruments carry significant liability, prompting customer demand for reliability and accuracy assurance.

Figure 2: A soil test is used to assess the pH of the soil in an agricultural application. (Source: wellphoto/stock.adobe.com)

Size and power constraints limit the design and complexity of analog filters, typically composed of inductors, capacitors, resistors, switches, and transistors arranged in specific configurations. Constraining filter size and complexity also results in reduced filter performance, which may impact the overall performance of an analog signal chain. Similarly, limiting analog amplifiers' size and power supply results in restrictions to the gain, linearity, frequency range, and topologies available. Curtailing amplifier capabilities can significantly reduce an analog signal chain's overall dynamic range and maximum/minimum voltage, current, and power capabilities. Also, high-gain amplifiers with low added noise figures are essential for extracting extremely low-power transducer signals; hence, reduced amplifier capability also limits the types of transducers that can be used. Limiting the power and size of analog signal chain components also means that more precise circuit topologies and components/devices may be excluded from an analog signal chain, which may result in higher degrees of uncertainty in the signal chain from using less precise circuit elements and designs.

Engineers can use calibration standards and protocols to enhance the performance of a given transducer and analog signal chain. However, calibration methods require additional components, steps, and circuitry built into a test and measurement device. Therefore, it may be more practical to include an internal or otherwise more compact calibration system for a portable instrument instead of external calibration electronics. However, these systems are not at the same performance level as laboratory-grade calibration systems and standards. The calibration circuitry may also be simplified due to limited space, power, complexity, and cost design constraints.

Hence, additional design efforts may be needed to overcome these limitations with analog components, devices, and circuit techniques. It may also be necessary to use technologies that are more power efficient for the digital electronics of the system; so if part of the analog signal chain is integrated into an IC, then that portion of the analog signal chain may be in a smaller process node that isn't optimized for analog performance. Having much more compact circuitry may also present challenges when the analog circuitry is placed close to noise generators or crosstalk from other parts of the analog signal chain or digital circuits. If not properly accounted for, noise and crosstalk from different parts of the circuit can reduce the dynamic range of the analog signal chain or enhance uncertainty. Portable electronics are also often more difficult to shield, so there tends to be a higher degree of interference and noise from outside sources.

Conclusion

The performance of portable test and measurement equipment typically does not match that of bench-top or laboratory instrumentation due to the need to balance the trade-offs described in this article. However, with innovative design and extensive experience, engineers can circumvent many of these design challenges and trade-offs associated with making analog signal chains portable. Improvements in ULP electronics, advanced materials, and circuit topologies have made it possible to create compact, efficient, and high-precision analog signal chains. By integrating cutting-edge technologies and robust design practices, developers can achieve remarkable accuracy and reliability in portable instruments, meeting the growing market demand for smaller, more capable, and energy-efficient test and measurement solutions. While there is always a cost, advancements in this field continue to push the boundaries, enabling high-performance portable instrumentation for a wide range of critical applications.