Design Tips: Talking about cost and performance optimization of efficient signal chains (Part 1)

Mouser Electronics

From industrial process control and measurement to high-speed communications and imaging, efficient signal acquisition is the foundation of various applications. With such a wide range of application categories, it is critical to match the appropriate application components and create a signal chain to meet performance requirements at the lowest possible cost. However, with the expected rapid development of embedded sensor systems (providing acquisition signals for the Internet of Things), balancing cost and performance becomes even more important. With the number of IoT devices expected to reach tens of billions, the resources saved at each level of the signal chain can be aggregated to save amazing resources.

For the designer, building an efficient signal chain means balancing the specifications of each individual component to achieve the target performance level at each stage of the signal chain. While some applications require the highest possible device specifications (see Figure 1), designers may often use more cost-effective components to create a complete signal chain after achieving the required level of performance and functionality.

Figure 1: High-performance analog components, including analog-to-digital converters and multiplexers, enable CERN’s LHC (Large Array Collider) to measure magnetic field regions with the highest possible performance.

Ideally, the most basic form of a signal acquisition circuit contains only one component: an analog-to-digital converter (ADC), which converts the analog signal input from a sensor or other input source into a digital signal. However, for any practical application, real-world signals are not so simple, and further signal conditioning is required, including signal amplification and filtering (see Figure 2). For applications using active sensors and additional components such as digital-to-analog converters (DACs), accurate reference voltages and amplifiers are required at the front end of the system to provide the required excitation current or voltage to the sensor.

Figure 2: Prior to data conversion, a typical analog signal chain requires conditioning to compensate for small-signal inputs, signal compensation, and other signal characteristics unique to each application.

Signal Conditioning

Typically, sensors and transducers generate small amplitude signals. Without amplification, these signals can only meet the requirements of a portion of the full dynamic range of the ADC, most likely resulting in loss of details due to the limited resolution of the ADC and the influence of converter quantization errors.

Therefore, designers typically need an analog front-end (AFE) amplifier to increase the amplitude of the input signal to meet the full dynamic range requirements of the ADC. Equally important, the input amplifier ensures that sensors and transducers remain properly loaded while also buffering the front end from the effects of load transients that can appear at the input of some types of ADCs when the signal is sampled.

Engineers can find amplifiers that span a wide range of functions and performance. Although it is often desirable to find an amplifier with the highest possible performance, engineers can significantly reduce design costs by rigorously comparing the amplifier's specifications to the characteristics of the input signal and the required output resolution. For example, when the signal changes slowly and remains well above noise, using an instrumentation amplifier (IA) with the fastest slew rate and lowest noise may simply increase costs. Similarly, an amplifier with the best linearity specifications may simply outperform the ADC, providing results that are accurate enough to have quantization errors but meet the performance requirements of the signal chain overall.

Based on signal characteristics and application requirements, engineers face more stringent requirements. They can choose from a variety of full-featured amplifiers, such as high-precision IA, low-noise amplifiers (LNA), and programmable gain amplifiers (PGA). However, the performance characteristics provided by traditional amplifiers are suitable for most applications. For example, rail-to-rail input and output (RRIO) low-noise amplifiers such as Analog Devices' AD850x, Maxim's MAX963x, and Texas Instruments' OPA320 series can maximize dynamic range and minimize noise in a wide range of signal acquisition applications, and are a cost-effective option.

Although traditional single-ended input amplifiers are adequate for many applications, many signal acquisition applications require differential inputs, and good common-mode rejection is key. For example, applications using bridge sensors or designs that operate in very noisy environments require higher common-mode rejection characteristics for the amplifier's differential inputs. In fact, some differential amplifiers such as Analog Devices' AD8476 and Texas Instruments' THS4531 are specifically designed to address differential signal conditioning requirements and include features designed to simplify interfacing to ADCs. Speaking of ADC interfacing, the integrated laser-trimmed resistors found inside Analog Devices' AD8476 can help reduce component count and cost in signal chain designs (see Figure 3).

Figure 3: Differential amplifiers such as the ADI AD8476 have integrated laser-trimmed resistors that provide the ability to adjust the output to the ADC interface requirements, helping to simplify signal chain design for differential input requirements.

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