Electronic instrumentation and measurement designs push the boundaries of performance, power, and integrated features to meet the design challenges presented by the different industries in the marketplace.
Instrumentation allows engineers to see and understand what is going on within different electronic or mechanical processes or systems. These devices obtain, analyze, and present data, permitting engineers to monitor and control machinery and make all necessary corrections. During testing or prototyping, instruments enable circuit improvements so that new and better designs can be created.
As highly capable instruments with extensive functionality and usability are needed to meet the design challenges presented by various applications, we’ll look at the characteristics these instruments require to meet customer expectations and then examine important properties of one key design component—the instrumentation amplifier.
Different Instruments, Different Requirements
Once the operating conditions and performance specifications of an instrument have been determined, the functional components of the device must be identified in blocks. Selection of suitable sensors, signal processing circuits, digital and passive components, and power supplies must be made. Meeting these needs requires a complete bill of materials (BOM) of high-performance components such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). The former is used to convert analog signals such as the output from a temperature transducer, a radio receiver, or a video camera into digital signals for processing. Conversely, DACs are used to convert digital signals back to analog signals.
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Many applications require substantial signal conditioning to reduce noise, increase dynamic range, and compensate for sensor non-linearity before an instrument can measure the signal effectively and accurately. For example, signal conditioning in analog front ends of medical instruments presents the design challenge of detecting small signals in the presence of large differential DC potentials. Consider electrocardiography (ECG), the acquisition and recording of electrical activity of the heart (Figure 1). Each of the cell membranes that form the outer covering of the heart cell has an associated charge that is de-polarized during every heartbeat. These impulses appear as tiny electrical signals on the skin, which can be detected, translated into a waveform, and amplified by the ECG.
Figure 1: Block diagram of an ECG device (Source: Texas Instruments)
A key element of the process, the instrumentation amplifier (or in-amp) is a specialty amplifier type. It is generally the first component at the measurement front-end, so its performance is critical to the entire instrument’s performance.
In general, signals obtained from sensors have very small amplitudes and must be amplified before processing and display can be accomplished. Simply put, the in-amp’s function is to extract the small signal of interest (a differential signal riding on top of a large common-mode signal) from, say, pressure or temperature transducers and other signal sources in noisy environments and amplify the difference between two input signal voltages.
In-amps are widely used in medical equipment such as ECG and electroencephalogram (EEG) monitors, blood-pressure monitors, and defibrillators. Other examples of instrumentation applications where in-amps might be used include audio (as microphone preamps, for example), high-frequency signal amplification in cable radio-frequency (RF) systems, and high-speed signal conditioning for video imaging. In-amps can also be used for motor monitoring (to monitor and control motor speed, torque, etc.) by measuring the voltages, currents, and phase relationships of a three-phase AC motor.
The main difference between an instrumentation amplifier and an operational amplifier is that an op-amp is an open-loop device. In contrast, an in-amp comes with a preset internal feedback resistor network that is isolated from its input terminals. As with op-amps, output impedance is very low.
Selecting an in-amp that is a good fit for a particular circuit design requires a clear understanding of its characteristics and how they are portrayed on product datasheets. A few of the properties that define a high-quality in-amp include:
Let’s now consider a couple of real-world examples found in the Newest Products section of Mouser.com. Analog Devices’ AD8428 in-amp is designed to accurately measure tiny, high-speed signals. It claims to delivers industry-leading gain-accuracy (0.2 percent), noise, and bandwidth. ADI AD8428 (Figure 2) features a fixed gain of 2,000 and is one of the industry’s fastest in-amps. The high CMRR (130dB minimum) of the AD8428 prevents unwanted signals from corrupting the acquisition data. The part’s pinout is designed to avoid parasitic capacitance mismatches that can degrade CMRR at high frequencies. This ADI device is said to be well-suited for use in sensor interface, medical instrumentation, and patient-monitoring applications.
Figure 2: Functional block for AD8428 (Source: Analog Devices)
Texas Instruments INA826 Precision Instrumentation Amplifiers offer extremely low power consumption and operate over a very wide single or dual-supply range. The common-mode rejection ratio exceeds 84dB across the full input common-mode range from the negative supply up to 1V of the positive supply. Using a rail-to-rail output, the TI INA826 is well-suited for low-voltage operation from a 2.7V single supply as well as dual supplies up to ±18 V. TI INA826 Instrumentation Amplifiers are designed for use in various applications, including industrial process controls, circuit breakers, battery tests, and ECG amplifiers.
Conclusion
To meet the design challenges presented by the different industries in the marketplace, electronic instrumentation and measurement designs push the boundaries of performance, power, and integrated features. We’ve presented the characteristics modern instruments require to meet customer expectations and examined the important properties of the instrumentation amplifier. Mouser Electronics has a large portfolio of in-amps and other parts and design resources available to help the engineer tasked with acquiring, monitoring, controlling, or measuring different signals, processes, and protocols.
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