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High Performance ICs for Aerospace Application

By Maurizio Di Paolo Emilio for Mouser Electronics

In every communication system, an analog-to-digital converter (ADC) and its counterpart, the digital-analog converter (DAC) are essential. For the aerospace telecommunications payloads that have a large number of converters, mass and power consumption are significant. Furthermore, the inclusion of ADCs in the payload makes new compromises in design (jitter, quantization noise) necessary.

Space requirements

One of the first obstacles for space electronics is the vibration generated at launch and at higher launch stages. When, for example, there is a separation between satellite and rocket, many vibrations occur in the body structure due to pyrotechnic shock.

Degassing is another concern. The vapor that comes out of plastic devices can deposit hazardous material on optical devices thus degrading their performance. The Earth's orbit causes a cloud of contaminants around the spacecraft with high levels of contamination that can contribute to electrostatic discharge. Satellites are vulnerable to charge flows that cause changes in the electrostatic potential. To prevent these effects, space applications require components without floating metal (e.g. ‘floating gate’).

The primary mechanism of the floating charge on the surface of the satellite is mainly due to photoelectric effects. Discharge voltages in the order of 20kV occur on the satellite in geosynchronous orbits. These discharges create an accumulation of energy that can damage the devices. A design solution consists in covering all external surfaces of the satellite with a conductive material.

The atmosphere in the various orbits is composed of about 95% of atomic oxygen that can react with organic materials located in the outer areas of the spacecraft and thus gradually damage the structure. The erosion of materials by atomic oxygen was noted in the first missions of NASA's space shuttle, where the presence of atomic oxygen caused several problems in the microelectronics of the devices. NASA has remedied this problem by developing a thin film coating that is immune to reaction with atomic oxygen.

Another obstacle is the very high temperature variations that occur in different spatial environments. In particular, the spatial environment can be considered in two application cases: when it is illuminated by the sun and during the eclipse. In the first case, the temperature can reach over 200°C. Because it is closer to the Sun, temperature fluctuations on a satellite in the GEO (Geostationary Earth Orbit) stationary orbit will be much higher than temperature variations on a satellite in LEO (Low Earth Orbit). In the other case, freezing temperatures are reached. On the moon, for example, the temperature on the surface can vary from about -150°C to +150°C. These temperature variations can promote the degradation of electronic components. Therefore, it is necessary to optimize their quality with regards to heat dissipation. In the vacuum of space, there is no thermal convection or conduction in place. Radioactive heat transfer is the primary method of transferring heat into the vacuum, so the satellites are cooled by radiating heat into space (Figure 1).

Figure 1: Typical environment in space around earth. (Source: NASA)

There are significant variations in levels and types of radiation depending on the orbit. Each space program must be evaluated regarding reliability, tolerance to radiation, environmental stress, launch date, and expected life cycle of the mission. The spatial radiation environment can have detrimental effects on the electronics of the spacecraft.

Radiation damage occurs in transistors, both in the field-effect (like the MOSFET) and in the bipolar junction (BJT). In the case of MOSFETs, ionization is caused by a charged particle that affects silicon dioxide (insulating layer); it induces the breaking of the electron-hole pairs by directing them towards the silicon-oxide interface where it creates "trap" states that decrease the number of actual carriers and therefore the threshold voltage of the transistor. In BJT, instead, these particles increase the base current and consequently decrease the overall gain (Figure 2).

Figure 2: A transient current or voltage spike may propagate through logic gates, and produces system failures. (Source: Author)

The effects of radiation can be classified into two broad groups:

• Cumulative effects
• Single Event Effects (SEE)

Cumulative effects are the damages that irretrievably accumulate over the years, making the electronics in space devices unusable. These damages are predictable in the laboratory and allow us to establish a useful average life estimate for each aircraft. We can differentiate them in Total Ionizing Dose (TID) and Displacement Damage (DD).

Single Event Effects (SEE), on the other hand, are unpredictable and can appear at any time from electronic equipment to space. EEAs are grouped into two categories: transient effects (or soft errors) such as Single Event Transient (SET) and Single Event Upset (SEU); catastrophic effects such as Single Event Burnout (SEB), Single Event Gate Rupture (SEGR) and Single Event Latch-up (SEL).

Data conversion for Space

In the growing digital world, the processing and transmission of digital data in spatial environments have become crucial for the correct encoding and decryption of data. The use of space-qualified components must be an integral part of the design process. Evaluation parameters for an ADC can be the following:

• Bit resolution: it is the number of bits that the converter itself can supply in output.
• Conversion time: this is the time required for the converter to output the result of the digital conversion. The time may vary from a few μs for economic ADCs up to some ns for Flash ADCs.
• Input signal range: voltage or current range that the drive can accept.

The EV12AQ600 is a 12-bit Teledyne ADC with Cross Point Switch (CPS) as part of the INTERSTELLAR project of the European Union. This ADC allows the device to operate its four cores simultaneously to achieve a sampling rate of 6 GS/s. Engineers can design systems that use the EV12AQ600 independently or synchronized, in quad-channel 1.5Giga Sample per second (GSPS), dual-channel at 3 GSPS or single-channel at 6 GSPS (Figure 3).

Figure 3: Block diagram of the EV12AQ600 (Source: Teledyne e2v)

STMicroelectronics rad-hard data converters are ideal for a range of satellite applications, including imaging for geology applications. The solutions come in ceramic packages that meet the requirements for radiation immunity and qualification criteria to be included in the QML-V and EPPL lists. The RHF1201 is a 12-bit, 50 MS/s analog-to-digital converter that uses simple 0.25 μm CMOS technology to meet the demands of very low power consumption for the space environment. RHF1201 integrates a track-and-hold microelectronic structure that makes it ideal for applications up to 150 MHz (Figure 4).

Figure 4: Power consumption as a function of the sampling frequency for the IC RHF1201 (Source: STMicroelectronics)

To address the challenges of aerospace applications, Analog Devices offers a series of monolithic hermetic devices with MIL-PRF-38535 support for high QMLV spatial quality. Test methods used as MIL-STD-883, 1000-hour test, and electrical shielding represent an essential part of the compliance of space ICs. In the field of data converters, we can distinguish the AD9731 DAC with 10-bit resolution and AD9246 ADC with 14-bit resolution 80 MSPS / 105 MSPS / 125 MSPS (Figure 5).

Figure 5: Block diagram of the AD9246 (Source: Analog Devices)

Power ICs in satellites

In modern satellites, a centralized power source unit must provide a wide range of voltages bus required by the modules. For this reason, the power conditioning units are very sophisticated and have to be customized with ultra-low fluctuations for each payload. The volume and weight, in addition to efficiency, contribute significantly to the costs of space missions. Any satellite with its payloads requires specific conditioning and distribution for each bus voltage level.

The reliability of dc-dc converters for space must meet different standards in addition to quality requirements. The parameters of temperature, tightness, and compliance with specifications must be verified with appropriate reliability tests.

MIL-PRF-38534 also regulates Radiation-resistant, space-based hybrid DC-DC converters. The DC-DC space converters are available on SMD and are generally used in Class K. The Microsemi SA-series DC-DC converters are designed and qualified for stringent space requirements both regarding total ionizing dose (TID) and single event events (SEE). The SA component family can be synchronized externally with external frequency sources at 220 Hz. To ensure the precise regulation of the load voltage, each output can be individually adjusted using a voltage reference with a precision of less than 1%. The SA50-120 family is ideal for use on the International Space Station (ISS) supporting 120 V_in (Figure 6).

Figure 6: Block diagram of the SA50-120 for space applications (Source: Microsemi)

Intersil offers ISL70003ASEH with an input voltage range of 3.0V to 13.2V and with integrated low R_DS(ON)-MOSFET. The continuous output load current capacity is 9A for T_J <= +125°C and 6A for T_J <= +150°C.

ISL70003ASEH contains a buffer amplifier to generate the VREF voltage; moreover, it uses the control architecture in voltage mode with feed-forward and switching at a frequency of 500 kHz or 300 kHz appropriately selectable. The loop compensation is externally adjustable to allow an optimal balance between stability and dynamic output performance (Figure 7).

Figure 7: Power distribution solution for hard low power FPGAs (Source: Intersil)

Conclusion

Operational requirements make it necessary for the instruments to work under certain conditions in the presence of noise, vibration, and physical trauma, challenges that are not trivial for designers. An example is the low quantization noise. The analog input to the ADC is a continuous signal with an infinite number of possible states, and on the contrary, the digital output is a discrete function with different states determined by the resolution of the device. Many applications require much signal conditioning to reduce noise, increasing the dynamic range of operation. Designers of military and aerospace systems face a set of challenges regarding not only noise and data management, but also thermal issues. Today's advanced aerospace electronics systems for applications such as communications, multi-computing and signal processing must take into account the significant increase in dissipated power and high data management for digital conversion.

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. Working as a software/hardware developer in the data acquisition system, he participated as the designer of the thermal compensation system (TCS) for the optical system used in the Virgo/Ligo Experiment (an experiment for detection of the gravitational wave that achieved the 2017 Nobel Prize in Physics). Actually, he collaborates with University of L'Aquila and INFN to design devices for radiobiological and microscopy applications and new data acquisition and control systems for space applications. Moreover he works in the software/hardware engineering field as editor and technical writer. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.