プレシジョン ラボ シリーズ：絶縁型アンプと変調器

[MUSIC PLAYING]

Hello, and welcome to the Texas Instruments Precision Lab series on isolation. Precision Labs is a comprehensive online curriculum for analog engineers. More videos can be found by going to ti.com/precisionlabs. In this video, we will discuss what isolated amplifiers are and when to use them. This video will cover the basic aspects of isolated amplifiers. First, we will talk about what an isolated amplifier is.

Next, we will discuss when, where, and why isolated amplifiers are used. Third, we will cover common applications for isolated amplifiers. And forth, we will cover the key electrical and isolation specifications for isolated amplifiers. Let's start by defining what an isolated amplifier is. An isolated amplifier is a special function amplifier with an output separated from the input circuitry by an isolation barrier, which is highly resistant to electromagnetic interference.

This interference, also known as EMI, is caused by external sources, and when ignored can degrade the performance of the signal chain or cause errors in the data. While there are several different methods for achieving isolation, Texas Instruments employs a capacitive isolation barrier. The purpose of the isolated amplifier is to provide protection against electrical shock hazards in high-voltage applications or isolate parts of circuits or different circuits from each other by breaking the ground loops.

So what is the difference between an isolated and a non-isolated amplifier? Let's take a closer look at the functional block diagram of an isolated amplifier. Just like traditional non-isolated amplifiers, isolated amplifiers have an analog signal on the input and an analog signal on the output. The input stage of an isolated amplifier consists of either a fully differential or single-ended input that drives a delta-sigma modulator. The gain of the amplifier is fixed and set by internal precision resistors.

The delta-sigma modulator uses the internal reference voltage in clock generator to convert the analog input signal to a digital bit stream that is then transferred across the isolation barrier. The bitstream is processed by an analog filter on the low side of the isolation barrier and presented as a differential analog voltage at the output. Texas Instruments capacitive isolation technology offers two different types of isolation barriers, basic and reinforced.

Basic isolation is implemented with a single-capacitive barrier, while reinforced isolation uses two-capacitive isolation barriers in series. For more information on types of isolation, please check our basic versus reinforced isolation Precision Labs video. Isolated amplifiers are widely used in many industrial and automotive applications. But why are they needed? One common reason for using an isolated amplifier is to break ground loops.

Electrical installations can cause large ground potential differences between two remote nodes, Ground 1 and Ground 2. A direct connection between these two grounds closes the ground loop. Equipment like electric motors can inject large noise currents into the ground, thus creating a ground loop current that can appear in the signal path. Isolated amplifiers remove the signal path noise from the ground loop and protects the system from ground loop potential difference.

A second reason for using an isolated amplifier is protection against high voltage. When industrial equipment is running at hundreds of volts, isolated amplifiers can safely withstand high-voltage surges that can damage equipment or harm humans. Here are some common examples of applications for isolated amplifiers, current sensing and voltage sensing. TI offers a variety of isolated amplifiers for shunt-based current sensing with input range of either plus/minus 50 millivolts or plus/minus 250 millivolts.

The input voltage range of the isolated amplifier determines the value of the shunt resistor based on the current to be measured. For voltage sensing applications that use voltage division, TI offers isolated amplifiers with input ranges up to plus/minus 12 volts. For additional information on isolated amplifiers, please visit ti.com/isolation and click on the isolated amplifiers link.

When choosing an isolated amplifier, both amplifier and isolation specifications have to be considered. Let's start with defining input offset voltage. Input offset voltage is a differential input voltage that would have to be applied to force the amplifier's output to 0 volts. Offset voltage should be small as possible in order to minimize the error at the output of the amplifier. Keep in mind that the input offset voltage is depending on changes over temperature which is quantified by input offset drift.

Next is gain error which is the percentage difference between the ideal gain and the measured gain. To describe the changes in gain over temperature, the gain error drift parameter is used. Now, we will go over some common isolation specifications. First, we will consider isolation working voltage, which is the maximum voltage defined in volts RMS that the isolated amplifier has to handle on a continuous basis throughout its operating life.

Next, isolation transient overvoltage is the voltage defined at volts peak that the isolated amplifier can tolerate for 60 seconds. Lastly, we have to define the Common-Mode Transient Immunity, or CMTI, which describes the maximum rate of change and ground potential difference that the isolated amplifier can withstand without errors. That concludes this video. Thank you for watching. Please try the quiz at the link below to check your understanding of this video's content.