ADUM3221 - FAQ
What are optocouplers? What is optical isolation?
Optocouplers are a form of optical isolation that employs light to transmit information across an isolation barrier. Typically, a light emitting diode (LED) transmits information to a light sensitive receiver (e.g., a transistor). Optical isolation's primary benefit is that it is widely used and accepted as a low-cost isolation solution for transmission of slower digital signals; high-speed, digital optocouplers tend to be expensive. Optocoupler-based methods are commonly used in applications in which the DC state of a signal is important. Because LEDs may wear out over time, optical isolation typically requires compensation and guardbanding to guarantee application operability over life.
What is iCoupler technology?
iCoupler technology is a transformer-based approach to isolation that combines the advantages of optocoupler, transformer, and semiconductor technologies. By integrating micro-transformers onto semiconductor dice, isolation is provided without the detrimental characteristics of the electro-optical conversions present in optocouplers. These include excessive power consumption, large timing errors, and constraining data rate limitations. Safety agency-recognized insulation is achieved through the used of a special insulation layer between the transformer coils. The common transformer problem of achieving DC-correctness is avoided by the use of patented "refresh" circuitry that updates the output with the correct input state in the absence of input signal transitions.
What is isolation rating?
An isolator's isolation rating (also called the test voltage) is a measure of the protection provided against short-duration, common-mode voltage differences. Usually specified in terms of a 60 Hz RMS value, it is a rating of how much voltage can be safely applied between the input and output terminals of the device for a duration of one minute. A common isolation rating found on isolation devices is 2.5KVRMS. Other devices have ratings such as 3.75KVRMS or higher. The isolation rating does not describe how much voltage can be safely applied across the part on an long-term continuous basis – this is described by the "working voltage" or "rated mains voltage."
What is capacitive isolation?
Capacitive isolation employs capacitors to couple data signals across an isolation barrier. This approach can offer significant performance advantages relative to optocoupler but typically suffer a high vulnerability to common-mode and ESD transients.
What is the relationship between working voltage and isolation rating?
The relationship between a given working voltage and the required test voltage is complex and is a function of the application, the magnitude of common-mode voltage transients (Installation Category), the "cleanliness" of the environment (Pollution Degree), and the required insulation type (Insulation Level). As a representative example based on IEC 1010-1 (international standard for measurement and control equipment), a measurement and control application involving installation category II, a pollution degree of 2, basic insulation, and a working voltage of 300 VRMS has a required test voltage of 1.35KVRMS. This means that for an isolation component to be suitable for this application, it must support steady-state common-mode voltages differences of at least 300 VRMS and transient common-mode voltages differences of at least 1.35KVRMS. The relationship between working voltage and isolation rating is defined by the specific safety standard for the end-equipment under consideration.
What is isolation? Why is it needed?
Isolation is a means of preventing current from flowing between two communicating points. Typically, isolation is used in two general situations. The first is where there is the potential for current surges that may damage equipment or harm humans. The second is where interconnections involve different ground potentials and disruptive ground loops are to be avoided. In both cases, isolation is used to prevent current flow yet allow for data or power flow between the two points.
What are some of the distinguishing features of the iCoupler products?
Distinguishing features of iCoupler products include integration that optimizes cost and size as well as the ability to support high signal data rates and the ability to operate with extremely low supply currents. iCoupler products provide an excellent cost/size advantage over competing alternatives due to integration of multiple channels as well as other functions including isolated power.
The speed and power consumption benefits can be seen in products such as the ADuM1100 that supports NRZ data rates of up to 100 Mbps and the ADuM344x that supports up to 150 Mbps.
iCoupler products also have the ability to perform up to a maximum temperature of 125° C and to be operated with either 3.3V or 5V supplies. Also of benefit to users is the incorporation of integrated driver circuitry providing compatibility with standard digital interfaces.
What communications protocols are supported by iCoupler products?
iCoupler products are capable of supporting most standard communications protocols but may require additional components to ensure communication integrity. Products such as the ADuM1401 are ideal for SPI where four channels are required for Clock, Data In, Data Out, and Select.The ADuM125x and ADuM225x have been designed for compliance to the I2C, PMBus and SMBus protocols.
The ADM2483, ADM2485, ADM2486 and ADM2490E are complete isolated RS-485 solutions.
What other parameters are important when considering an isolation device?
The ideal isolator consumes no power, imposes no signal errors, and supports any type of input signal. As a result, the important performance metrics are as follows:
Which of the above metrics are important to a particular user is a function of the application in question. Often in a specific application, certain parameters are important and others are not.
Other performance characteristics important to users are an isolator's power-up/down characteristics, its performance in the presence of input noise, and its performance in the presence of a DC input or after a loss of power. In all cases, the output of an isolator should properly reflect the correct input state.
How much isolation can iCoupler products provide?
As of this writing, most iCoupler products carry an isolation rating of either 2.5KVRMS or 5.0KVRMS (the ADuM240x and ADuM225x isolators carry an isolation rating of 5.0KVRMS). The maximum working voltage that iCoupler products support is a function of both the safety approvals a given product has as well as its insulation lifetime as evaluated by testing performed by Analog Devices in addition to that of the safety agencies.
All iCoupler products support working voltages up to 400 VRMS. The ADuM240x and ADuM225x families support higher working voltages.
The maximum working voltage supported by the ADuM240x or ADuM225x depends on the nature of the common-mode waveform that is imposed across the isolation barrier. This is due to the fact that the iCoupler insulation system is stressed differently for the various waveforms shown below. In the case of bipolar AC waveforms, the maximum working voltage is set by the criteria that the insulation have a 50 year minimum lifetime. In the case of other types of voltage waveforms, the maximum working voltage is limited by regulatory safety approvals – despite the insulation's ability to withstand higher voltages with a 50 year minimum lifetime. The table below summarizes the maximum recommended voltages for the ADuM240x and ADuM225x as a function of waveform type:
|AC Voltage, Bipolar Waveform||400 VRMS (565 (VPK)||50-year minimum lifetime|
|AC Voltage, Unipolar Waveform||519 VRMS (848 (VPK||Maximum CSA/VDE approved working voltage|
|DC Voltage||848 VPK||Maximum CSA/VDE approved working voltage|
Table 3: ADuM240x and ADuM225x Maximum Working Voltages
How is isolation specified?
The isolation characteristics of an isolator are specified in a variety of manners, including:
The degree to which an isolator successfully insulates one side of an isolation barrier from high voltages on the other side is commonly described by the isolator's isolation rating and by its maximum working voltage (or rated mains voltage). In addition, the degree to which an isolator continues to correctly transfer a signal across an isolation barrier in the presence of a common-mode transient is described by its transient immunity (or common-mode rejection). Each of these three parameters is described below.
What is the difference between basic and double (or reinforced) insulation levels?
In general, protection against electric shock may be provided by adequate clearance (a physical spacing from the live part) or by one of two insulation levels. These insulation levels are basic insulation and double (or reinforced) insulation. The required insulation level is determined by the voltage levels involved as well as the presence or absence of a connection from accessible parts to earth ground. In general, compared to an isolator providing basic insulation, an isolator providing double or reinforced insulation has greater requirements on its test voltage as well as on its input-to-output spacing. The required insulation level is defined by the specific safety standard for the end-equipment under consideration.
Can I replace an optocoupler with an iCoupler product in an existing design?
In most cases, an iCoupler product may be used as a replacement for an optocoupler in a given application in order to achieve higher performance, lower cost, greater space savings, or improved quality and reliability. In some cases, Analog Devices products are pin-for-pin compatible with industry standard optocouplers. For example, the ADuM1100, Analog Devices' first iCoupler product, is a single-channel digital isolator intended for applications requiring high-speed digital isolation. It is footprint- and pin-compatible with Agilent Technologies' HCPL-0710, -0720, 0721, -0723 optocouplers and provides substantial performance, power consumption, and cost advantages over these optocoupler components.
For all other products, please refer to the selection tables available at www.analog.com/iCoupler to identify the appropriate part. For assistance, please visit our web site or contact Analog Devices or one of our authorized sales representatives or distributors.
What are transient immunity and common-mode rejection?
An isolator's transient immunity specifies how fast of a common-mode transient between input and output a part can be subjected to while maintaining correct signal transmission. Many isolators have no specification on this parameter while others have values ranging from 5 to 25 KV/µs. All iCoupler products have a transient immunity specification of at least 25 KV/µs.
With which regulatory standards do iCoupler products comply?
iCoupler products typically carry at least a 2.5 kVRMS UL rating and a 400 VRMS working voltage approval from CSA and UL.
The approvals and details for each specific product are summarized in the table available at www.analog.com/iCouplerSafety
Do iCoupler products have VDE certification for reinforced insulation?
Yes. iCoupler products have now been approved for reinforced insulation by VDE. Until recently, no VDE standard covered reinforced insulation for magnetic couplers such as the iCoupler family of digital isolators; however, VDE released DIN V VDE V 0884-10 (VDE V 0884-10) in December 2006, and iCouplerproducts successfully completed testing to this standard. For details about specific working voltages and isolation ratings for each iCoupler product, and to view the safety certificates, please visit www.analog.com/iCouplerSafety.
Does Analog Devices provide other products that employ iCoupler technology?
A significant benefit of iCoupler technology is its ability to integrate isolation in a wide range of products. Integrating iCoupler isolation with other products simplifies designs, reduces the bill-of-materials, lowers cost, reduces size and increases reliability.
The ADM2483, ADM2485, ADM2486 and ADM2490E are RS-485 transceivers that employ iCoupler technology. Likewise, the AD7400 and AD7401 are isolated ΣΔ analog-to-digital converters. In the future, Analog Devices will release additional products integrated isolation via iCoupler technology.
How do iCoupler products differ from interface products?
iCoupler isolation products are a subset of traditional interface products. Typically, a given iCoupler product may be suitable for a wide array of applications, whereas interface-specific products are limited in scope (e.g., RS-232 or RS-485 transceivers). In addition, interface-specific products may not require isolation; however, it is common that such interfaces also require some level of isolation. Analog Devices offers a variety of interface products, some of which integrate iCoupler technology for a fully integrated solution (see below).