Category Archives: IC’s

Double-Sided Cooling for MOSFETs

Emission regulations for the automotive industry are increasingly tightening. To meet these demands, the industry is moving rapidly towards the electrification of vehicles. Primarily, they are making use of batteries and electric motors for the purpose. However, they also must use power electronics for controlling the performance of hybrid and electric vehicles.

In this context, European companies are leading the way with their innovative technologies. This is especially so in the development of power components and modules, and specifically in the compound semiconductor materials field.

ICs used for handling electrical power are now increasingly using gallium nitride (GaN) and silicon carbide (SiC). Most of these devices are wide-bandwidth devices, and work at high temperatures and voltages, but with the high efficiency that is typically demanded of them in automotive applications.

Silicon Carbide is particularly appealing to the automotive industry because of its physical properties. While silicon can withstand an electrical field of 0.3 MV/cm before it breaks down, SiC can withstand 2.8 MV/cm. Additionally, SiC offers an internal resistance 100 times lower than that of silicon. These parameters imply that a smaller chip of SiC can handle the same level of current while operating at a higher voltage level. This allows smaller systems if made of SiC.

Apart from functioning more efficiently at elevated temperatures, a full SiC MOSFET module can reduce switching losses by 64%, when operating at a chip temperature of 125 °C. Power control units for controlling traction motors in hybrid electric vehicles must operate from engine compartments, and this places additional thermal loads on them.

Manufacturers are now exploring various solutions for improving the efficiency, durability, and reliability of SiC MOSFETs under the above operating conditions. One of these is to reduce the amount of wire bonding by using double-sided cooling structures. This cools the power semiconductor chips more effectively. Therefore, overmolded modules with double side cooling are rapidly becoming more popular, especially for mid-power and low-cost applications.

As a result of the research at the North Carolina State University, researchers have developed a prototype inverter using SiC MOSFETs that can transfer 99% of the input energy to the motor. This is about 2% higher than silicon-based inverters under regular conditions.

While an electric vehicle could achieve only 4.1 kW/L in the year 2010, new SiC-based inverters can deliver about 12.1 kW/L of power. This is very close to the goal of 13.4 kW/L that the US Department of Energy has set for inverters to be achieved by 2020.

With the new power component using double-sided cooling, it is capable of dissipating more heat effectively in comparison to earlier versions. These double-sided air-cooled inverters can operate up to 35 kW, easily eliminating the need for heavy and bulky liquid cooling systems.

The power modules use FREEDM Power Chip on Bus MOSFET devices to reduce parasitic inductance. The integrated power interconnect structure helps achieve this. With the power chips attached directly to the busbar, their thermal performance improves further. Air, as dielectric fluid, provides the necessary electrical isolation, while the busbar also doubles as an integrated heatsink. Thermal resistance for the power module can reach about 0.5 °C/w.

TI Driver for BLDC Motors

When simple motors were more frequently used, it was relatively easy to design products with them. Controlling such motors was simple, whether it was a brushed DC motor or a single-phase AC motor. There was no need for sophisticated hardware or software for designing a product with a motor.

However, sophisticated BLDC or brush-less DC motors are replacing most of the above motors because of several advantages like quiet operation and high efficiency. But these advantages come at the cost of design knowledge and effort, requiring both hardware and software development. Texas Instruments has developed a new integrated circuit that allows designers to achieve all the benefits easily from these motors.

The biggest benefit offered by BLDC motors over older designs is their improved power efficiency. Most government regulators today demand that electrical products meet strict efficiency standards. In most cases, meeting these requirements is possible only through the use of BLDC motors.

Motors are mechanical devices and therefore, they make noise when operating. Although the quiet operation is not usually a design goal for most products, using a BLDC motor offers a way to achieve low noise operation.

There are further advantages to using BLDC motors. One of them is low voltage operation, and the other is a longer life. Manufacturers of BLDC motors are now offering them in larger sizes for use in bigger products.

As stated earlier, BLDC motors are now replacing brushed DC motors and in some cases, AC motors as well. Some practical examples are robotic vacuum cleaners, pumps, fans, washing machines, humidifiers, and air purifiers. They are useful for multiple automotive devices as well.

Functionally, a BLDC motor works under the same principles that govern the operation of all motors—rotation is from the interaction of two magnetic fields, one fixed and the other movable. Frequently, the BLDC motor will have multiple stator coils embedded in the periphery of the motor assembly. With the stator coil wired into three groups, it performs as a three-phase motor does. The rotor on the BLDC motor consists of several permanent magnets rotating in the circle formed by the stator coils. The user only has to apply a sequence of pulses to the stator coils.

The timing of the pulses must match their interaction with the permanent magnets. The control circuitry that drives the stator coils gets the correct timing from multiple sensors indicating the orientation of the rotor. These sensors are mostly Hall-effect devices that produce signals that the controller requires for moving the magnetic fields on the stator coil.

There are numerous variations of the approach to control the BLDC motor. One of them is a sensor-less method using the back electromotive force the rotating rotor magnets induce into the stator coils. The sensor-less method typically reads the feedback voltages in the motor stator winding and processes them into control signals.

Many motor controllers are pre-programmed and packaged BLDC motor control modules. This is usually satisfactory for common applications. Others, however, require a custom design. The MCF8316A from TI is a single chip BLDC motor controller chip that only requires inputs for speed, direction, and torque. The IC takes care of the rest.

Low-Power Circuit Timing using SPXOs

A wide range of electronic devices relies on circuit timing as a critical function. These include microcontrollers, Bluetooth, Ethernet, Wi-Fi, USB, and other interfaces. In addition, circuit timing is essential for consumer electronics, wearables, the Internet of Things (IoT), industrial control and automation, test and measuring equipment, medical devices, computing devices and peripherals, and more. Although designing crystal-controlled oscillators seems an easy process for providing system timing, there are numerous design requirements and parameters that designers must consider when matching a quartz crystal to an oscillator chip.

Among the several considerations are the negative resistance of the oscillator, its drive level, resonant mode, and the motional impedance of the crystal. When the designer is making the circuit layout, they must consider the parasitic capacitance of the PC board. They must also consider the on-chip integrated capacitance, and include a guard band around the crystal. Finally, the design must not only be compact, with a minimum number of components, and reliable. While the circuit must be capable of operating with a wide range of input voltages, consuming minimal power, it must also have a small root-mean-square jitter.

An optimal solution to the above is to use simply packaged crystal oscillators or SPXOs. Manufacturers optimize SPXOs for low RMS jitter and minimal power consumption. These devices can operate with any supply voltage ranging from 1.6 VDC to 3.6 VDC. With these continuous-voltage oscillators, designers can implement solutions requiring minimal effort while integrating them into digital systems.

In small, battery-powered, wireless devices, power consumption is always a very important consideration. That is why designers prefer to base such devices on the system on a chip or SoC processor that consumes very low power to support battery lives of several years. Moreover, device cost depends on the battery size, as the battery is easily the most expensive component in the device—minimizing the battery size is, therefore, an important factor in small wireless devices. For battery life consideration, one of the important parameters is the standby current, apart from the self-discharge current of the battery. Minimizing the current drawn by the clock oscillator is important, as this is greater than the standby current.

Designing low-power oscillators can be challenging. Designers are tempted to save energy by allowing the circuit to enter a disabled state for minimizing the standby current while starting the oscillator when needed. However, this is not an easy task as starting crystal oscillators quickly is not a simple and reliable task. Reliable start-up conditions require careful design efforts when designers attempt it across all environmental and operating conditions.

Most low-power wireless SoCs favor the Pierce oscillator configuration. The circuit has crystal and tow load capacitors. It uses an inverting amplifier that has an internal feedback resistor. With the amplifier feeding back its output to its input, the right conditions cause a negative resistance to start the oscillations going.

Quartz crystal oscillators can have jitters caused by power supply noise, improper load, improper termination conditions, the presence of integer harmonics of the signal frequency, circuit configurations, and amplifier noise. The designer must use several methods to minimize jitter.

Small LED Driver IC

Although Switch-mode and PWM or Pulse Width Modulation methods make very efficient drivers for LEDs, they are also a good source of electromagnetic noise. Many applications require low noise conditions for proper operations, especially those related to medical. These low-noise applications benefit from linear circuits that introduce far lower noise in the system as compared to the Switch-mode or PWM topologies.

For driving LEDs linearly, Infineon Technologies AG offers a small LED driver IC, the BCR431U. Available in a tiny SMT package SOT23-6, the BCR431U can regulate the operating current to the LED in a standalone operation without requiring the help of an external power transistor.

Operating between a voltage range of 6 to 42 VDC, the BCR431U can drive LED currents up to 37 mA. A high-value resistor connected between the pins Rset and RS of the IC allows setting the desired LED current level.

The major advantage of the BCR431U LED driver is its low-drop feature. At a full load of 37 mA, the IC drops only 200 mVDC across from the supply to the output. At 15 mA load, this voltage drop is only 105 mVDC. This feature has two benefits—one, the power dissipation in the driver IC at full load is only 7.4 mW, and two, the user can drive a string of LEDs in series connection mode by adjusting the input voltage. Over the entire current range, the driver IC maintains precision of ±10% of the set value of the LED current.

The low voltage drop across the BCR431U LED driver improves the system efficiency substantially while allowing extra voltage headroom to compensate for the tolerances of forward voltages of LEDs. Therefore, even if some LEDs in the string have different forward voltages, the driver IC can accommodate them with an increase or decrease in the driving voltage. Likewise, it can accommodate tolerances in supply voltage sources when used in multiple applications.

Internal circuit configurations ensure the BCR431U LED driver IC can keep the LED current under control, even when the temperature changes. If the junction temperature of the driver IC rises, a temperature controlling circuit within the IC reduces the LED current, thereby helping to bring down the junction temperature. Therefore, the BCR431U can protect itself from thermal runaway.

The linear low-drop LED current driver IC BCR431U is eminently suitable for driving long strips of low-power and low-voltage LEDs. Highly flexible in adjusting to 12, 24, or 36 VDC power supplies, the driver IC offers high precision and efficiency when driving LEDs. Internal thermal protection built into the driver IC ensures long-life operations, preventing accidental damages and protections against surge events. Infineon has designed the IC BCR431U to be robust enough to withstand high ESD conditions.

Applications for BCR431U are almost endless including driving LED strips, LED channel letters and displays, architectural LED lights and displays, emergency lights, retail lights for decoration, shop window LED lights, and many more. The driver IC is especially helpful in shops for driving LED lights in shops showcasing items in different colors.

High-Speed Ceramic Digital Isolator

Isolated systems can communicate digitally among themselves without conducting ground loops or presenting hazardous voltages—simply by using digital isolators. A capacitive isolation barrier exists between the isolated systems. The transmitter side modulates its digital data with a high-frequency signal that allows it to transmit across the capacitive isolation. Receivers on the other side detect the signal, demodulate it to extract the digital data, and use it.

Digital isolators offer thick insulation distances of greater than 0.5 mm, with reliable high-voltage insulation. ON Semiconductor has patented an off-chip galvanic capacitor isolation technology and offer a full-duplex, high-speed, bi-directional, dual-channel digital isolator—the NCID9211.

NCID9211 supports isolated communications. Therefore, isolated systems do not need conducting ground loops to communicate with digital signals, and it is possible for them to avoid hazardous voltages. The optimized IC design and the off-chip galvanic capacitor isolation technology that ON Semiconductors has developed ensures high noise immunity and high insulation. The power supply rejection and common-mode rejection ratio specifications of the NCID9211 support this. Compared to coreless transformers and thin-film on-chip capacitors, the thick film substrates offer capacitors with 25 times the dielectric thickness.

The digital isolator offers a unique combination of an insulating barrier and an electrical performance along with safety and reliability that only optocouplers had offered so far. NCID9211 comes in a 16-pin small outline package with a wide body. The device has features with several advantages.

NCID9211 is the only digital isolator in the market today that includes insulation reliability matching that offered by optocouplers while offering the same level of safety. The device has a distance through insulation or DTI or over 0.5 mm and uses off-chip capacitive isolation for achieving maximum high-voltage insulation reaching 2000 Vpeak.

The off-chip capacitive isolation offers better long-term reliability and safety compared to other digital isolation methodologies available in the market. ON Semiconductors guarantees the specifications of the NCID9211 over a supply voltage range of 2.5-5.5 DVC and an extended temperature range of -40 °C to +125 °C. The device does not require overdesign as the device performance remains stable over voltage and temperature.

NCID9211 offers a high-speed communication of NRZ or non-return to zero data at rates of 50 Mbits per second. The maximum propagation delay is only 25 ns, while the maximum distortion of the pulse width is only 10 ns.

ON Semiconductor claims NCID9211 has better performance over optocouplers. Compared to optocouplers, NCID9211 does not exhibit insulation material wear out over time up to 1500 V, there is no LED to degrade over time, and the performance across devices is more consistent. Compared to optocouplers, NCID9211 has a longer lifetime expectancy.

With a minimum common-mode rejection of 100 KV/µs, the NCID9211 has a superior noise immunity and it meets stringent performance requirements of EMI/EMC. However, for meeting reliable high-voltage insulation requirements, there must be a minimum creepage and clearance distance of 8 mm between the input and the output.

With full-duplex and bi-directional communication, the NCID9211 has several applications such as isolated PWM control, SPI and I2C type micro-controller interfaces, voltage level translators, isolated data acquisition systems, and many more.

Using Integrated Power Switches

Power switches are most commonly in demand for their simplicity in turning on and off a voltage rail or for protecting a power path. Engineers find load switches easier to use compared to discrete power MOSFETs. For complete power protection of the system, eFuses offer an integrated approach. The combination of load switches and eFuses offers more than significant PCB space savings. Compared to discrete circuits, the combination of load switches and eFuses, also known as integrated switches, offers substantial improvements in performance, while resolving common power management challenges such as faster current limiting, detecting, and responding to mistakes in field wirings, and improving battery life and power density.

In fact, using the right integrated switch helps to reduce EMI and heat generation, while improving the power efficiencies to 90%. Bad power management leads to several side effects such as the generation of excess heat, electromagnetic interference, inaccurate voltage control, and these can lead not only to poor device performance but even to its outright failure. For the above reasons, designers are using integrated switches in electronic equipment such as desktop computers, LCD TVs, and plasma TVs.

Using integrated power switches offers several advantages over solutions of discrete controller and MOSFET. The loser component count leads to lower cost and higher reliability.

With electronic products shrinking in size, PCB space is almost always at a premium. The integrated power switch with its lower footprint has a better advantage over discrete components. Several manufacturers offer a variety of integrated power switches, and these include Fairchild Semiconductors, Power Integrations, ON Semiconductors, and ST.

Fairchild Semiconductor offers their new Green FPS e-Series of integrated switches as a replacement for conventional, flyback converters using hard switches. The new e-Series are a versatile set of devices for improving efficiency by reducing switching losses in the MOSFET with quasi-resonant operation.

It is also possible to use the e-Series in the continuous conduction mode or CCM in fixed frequency operations. The design offers simplicity and lowers the ripple current. Using an advanced burst mode technique, devices of the e-Series also conform to several governmental agency requirements for standby efficiency.

Fairchild uses a prefix of FSQ in the part number of these devices, and they are available for applications that can deliver up to 90 W. Depending on the requirement, it is possible to avail the series in seven different packages including DIP, TO-220F, LSOP, and others.

The devices use valley switching along with inherent frequency modulation for the quasi-resonant operation. The improves efficiency while reducing the EMI signature of the power supply. Valley switching uses the natural resonance of the primary inductance of the transformer and both circuit capacitance and parasitic capacitance for turning the MOSFET on only when the drain-to-source voltage is at its minimum. This reduces the amplitude of the current spike at turn-on typically found in hard-switched converters.

The increased efficiency from reducing the turn-on current spike also reduces the stress on the MOSFET.  However, with valley switching, the power supply can operate with a variable switching frequency, changing with changes in the line and load conditions, helping to reduce the EMI the power supply generates.

Isolated RS-485 Transceivers

A standard RS-485 transceiver sends and receives digital signals between digital equipment. They use positive and negative signals limited to 5 VDC levels. One can connect them as simple point-to-point configuration or as multi-point connections with two or more devices communicating. RS-485 transceivers allow high-speed communication in electrically noisy environments, as is usual within industrial plants.

Each of the two output lines on an RS-485 transceiver uses square waves to send serial data to another distant transceiver. A capacitive line offers high impedance to high-speed transmission, distorting the rise and fall of the signals. A capacitive line is one where the line carrying the signals is close to the signal ground.

Rather than use capacitive lines, RS-485 transceivers use a balanced line where the two output lines carry voltages of opposite polarity all the time. In balanced lines, the signal rise and fall times are much better, resulting in transmitting high-speed signals over longer distances.

Using +5 VDC and -5 VDC for each of the two output lines alternately an RS-485 transceiver can offer either non-inverting or inverting signals on its output lines. When the output is non-inverting, its polarity is the same as that at the input of the transceiver. For the inverting pin, the polarity is always opposite to that at the input.

In an industrial application, using isolated RS-485 transceivers is the normal practice, as the interconnecting cable often must pass through an environment with high voltages present. The isolation prevents any high-voltage spike inadvertently appearing on the interconnecting cable and passing on to the circuit driving the transceivers.

Analog Devices offers two types of isolate RS-485 transceivers. The ADM2867E is signal and power isolated up to 5.7 kV rms, while the ADM2561E has isolation levels up to 3 kV rms.

Both transceivers pass radiated emission testing, conforming to the requirements of EN55032 Class B standard. The tests use a double-layer PCB with two small 0402 size external ferrite beads on isolated ground and power pins.

Both devices feature integrated but isolated DC-DC converters generating low EMI. The isolation barrier offers immunity to system-level EMC standards. On the A, B, Y, and Z pins of the RS-485, a family of isolator devices offer ±15 kV air and ±12 kV contact ESD protection complying with the IEC6100-4-2 standard. Cable invert pins on the device allow users to reverse cable connections to quickly correct the connection while maintaining fail-safe performance on the receivers.

A double-layered PCB reduces the design time and material costs while providing Class B radiated Emissions. The cable invert feature reduces debug time during system install by allowing users to easily correct installation errors. With a greater than 8 mm creepage and clearance, and the IEC 6100-4-2 ESD and 5.7 kV digital isolation, the RS-485 transceivers from Analog Devices can maintain signal integrity even when the signals are passing through the harshest of environments.

Isolated RS-485 transceivers are useful for industrial automation, communication, building and infrastructure, and in aerospace and defense, mainly because of their cable invert feature, high isolation, low EMI/EMC capabilities, good surge protection, and improved ESD safeguards.

24-Bit Quad-Channel ADC

Analog Devices is offering a 24-bit Quad-Channel ADC, the AD7134, a low noise, precision type, simultaneous sampling Analog to Digital Converter that while offering exceptional functionality and performance, is also easy to use.

The AD7134 operates on the continuous-time sigma-delta or CTSD modulation scheme. This helps to remove the traditional requirement of a sampling switched capacitor circuitry preceding the sigma-delta modulator—simplifying the input driving requirement for the ADC. The device also has inherent antialiasing capability, arising out of the CTSD architecture rejecting signals around the aliasing frequency band of the ADC. Therefore, this ADC does not need the regular complex antialiasing filter.

The four independent converter channels of the AD7134 operate in parallel, and each of them has its own CTSD modulator along with a digital filtering and decimation path. Therefore, the user can sample four separate analog signals, each with a maximum input bandwidth of 391.5 kHz. The four signal measurements can also achieve tight phase matching among themselves. Therefore, the AD7134 can offer a high density of multichannel data acquisition in a small form factor, because of its simplified requirement of analog front-end, and a high level of channel integration.

ADCs normally require a complicated signal chain and an analog front-end circuitry that introduces distortion, mismatch, error, and noise at the ADC output. As the AD7134 simplifies the signal chain requirements, it also improves the system level performance of the device.

Offering excellent AC and DC performance, the bandwidth for each ADC channel of the device ranges from 0 to 391.5 kHz. Therefore, the AD7134 is an ideal choice for acquiring data with universal precision, and capable of supporting a variety of sensor types ranging from shock and vibration to pressure and temperature.

With several configuration options and features, the AD7134 offers the user the flexibility of achieving an optimal balance between power, accuracy, noise, and bandwidth for specific applications.

Analog Devices has integrated an asynchronous sample rate converter or ASRC with their AD7134 for precise control of the decimation ratio. This, in turn, allows the AD7134 to support a wide range of output data rates or ODR frequencies ranging from 0.01 kSPS to 1496 kSPS as the ODR uses interpolation and resampling techniques. Furthermore, as the adjustment resolution between the ODRs is less than 0.01 SPS, the user can vary the sampling speed granularly to achieve coherent sampling.

The user can also use multiple AD7134 devices with synchronous sampling between them using a single system clock, and this is because of the ASRC slave mode operation. The slave mode simplifies the requirement of clock distribution for a data acquisition system of medium bandwidth as each ADC no longer requires routing of low jitter, high-frequency master clock from the digital back end.

The AD1734 can perform on-board averaging between two or four of its input channels. This results in improving the dynamic range while the device maintains its bandwidth. Combining two channels improves the results by about 3 dB, while combining all the four channels offers an improvement of nearly 6 dB.

Why Low Dropout Regulators?

In this era of high-efficiency switching power supplies and voltage regulators, low dropout (LDO) regulators seem almost out of place. Contrary to popular belief, low dropout regulators are small components, simple to use, and cost-effective for obtaining an output of regulated voltage from an input of higher voltage.

For system designers, low dropout regulators offer a simple method of obtaining a voltage from a source that is very close to the output voltage. This is one major reason designers use LDO regulators widely. The second reason is LDO regulators are analog devices, and unlike switching regulators, introduce very low noise into the system.

Small LDO regulator devices such as those from Diodes Incorporated offer a variety of features such as high-power supply rejection ratio, ultra-low quiescent current, wide input voltage handling capability, physically small footprint, and high output current supply capability.

Keeping in line with other SMT components, manufacturers are making LDO regulators in smaller form factors, enabling designers to use PCB space more effectively. Designers can make better use of the newer families of LDO regulators in highly dense PCBs as these components are of very small size, and occupy the minimum space, while they offer the same high-quality performance.

Not all power supply sources offer clean and regulated outputs. LDO regulators help filter out most of the noise from unregulated power sources with their high-power supply rejection ratio specifications. By rejecting the noise from the power source, LDO regulators provide noiseless and spike-free DC power to ensure the system operates reliably.

Many systems do not require continuous power. In remote areas, where it is difficult to deliver power, engineers rely on batteries to power their equipment. LDO regulators with ultra-low quiescent current consumption are a boon, as they consume the minimum amount of power when the system is idle, resulting in a significant increase in the life of the battery.

LDO regulators can handle a wide range of input voltages, in some cases, up to as high as 40 VDC. In multi-voltage systems, which are now common-place, such LDO regulators are very cost-effective, and they make the design more robust and reliable.

Sensors and related electronics work better with clean power supplies. Noise from switching regulators can limit the sensitivity of sensors drastically, resulting in reduced coverage or misleading measurements. LDO regulators supplying clean and efficient power with high current output allow using components for sensitive measurements, without the introduction of ripple and noise. Even with their high current output, LDO regulators work with voltage differentials as low as 350 mVDC.

Automotive applications require high-temperature reliability, and LDO regulators are available that cover a wide temperature range of -40 ºC to +125 ºC. This is a necessary feature in an automobile, as many applications must work concurrently to keep the vehicle operational.

The new family of LDO regulators are ideal for portable and small consumer devices, such as smartwatches, smartphones, wearables, wireless earphones, smart homes, smart offices, and different sensor applications. The industry uses these LDO regulators for other applications such as healthcare devices, smart meters, and other devices powered by batteries.

LTM2893 μModule isolator for ADCs

Analog to digital converters (ADCs) need to float to the common mode of the input signal to absorb the harsh voltage conditions and transients. The best way to do this is to place an isolation barrier between the ADC and the external signal. Even applications that perform under moderate conditions can benefit from the presence of an isolator. The LTM2893 from Linear Technology provides such isolation, improving on system safety, especially when reading from high-resolution successive approximation register type of ADCs.

Ideally, the isolator for an ADC should be near invisible. Its function would be to manage the control and data signals, maximizing the sampling rate, and minimizing the effects of jitter on the performance of signal to noise ratio. The LTM2893 μModule isolator from Linear Technology meets all the above criteria, achieving these for ADCs with SPI interfaces, offers a 1 Msps range, while supporting a 6K Vrms isolation rating.

Options that are more traditional exist, but provide limited functionality, especially when reading data from high-resolution successive approximation register (SAR) ADCs. Most traditional high speed digital isolators work maximum up to 25 MHz, with a few special devices reaching 40 MHz On the other hand, the LTM2893 can easily read data samples at rates up to 100 MHz. Additionally, it is flexible enough to be able to handle multiple ADCs. This effectively solves timing issues and other limitations of the standard digital isolator interfacing that SAR ADCs face.

Test and process equipment need isolation so that their inputs are not damaged if accidentally misconnected or from overvoltage events. Usually, engineers use an isolator as a high voltage level shifter for extending the common mode range thereby reducing the ground noise. The LTM2893 is intelligent enough to ignore transients events of the common mode type up to 50K V/μs, as this provides a low-capacitance isolation barrier along with fully differential data communication.

When dedicated SPI isolators and other general-purpose digital isolators isolate ADCs, they use multiple digital isolators for supporting signals such as busy status or conversion start signals. In addition, they offer a 3- or 4-wire SPI port. They also suffer from signal propagation delays, as the isolated SPI port must wait for the return of the acknowledgement signal before the next data latching can occur. Adding all the propagation and the response delays from the ADC SPI port, a single read may suffer a delay of about 35 ns. Therefore, although the initially rating of a digital isolator may be at 150 Mbps, in reality, the delays reduce the effective frequency to 25 MHz or even less.

Linear Technology has provided the LTM2893 with a dedicated master SPI engine on its isolated side, and a dedicated slave engine and a buffer on the logic side. The master SPI engine of the LTM2893 monitors the status signals from the ADC, fetching the data as soon as its BUSY signal goes low. There is no interaction with the logic side once the conversion has started.

The buffer register on the slave SPI engine on the logic side receives data from the isolated side via the isolated barrier. The two sides therefore, operate independently of each other.