Anthony Vaughan

Size and power consumption are two of the key design aspects for circuits used in low Earth orbit (LEO) satellites. These systems require components that are:

• Radiation tolerant, to ensure proper operation in orbit. The operation of LEO satellites in an environment without the protection of the earth’s atmosphere subjects electronics to radiation and possible damage. Radiation-tolerant components are less likely to be affected negatively by radiation.
• Small in size, to minimize board space. Size and their corresponding weight in satellites are at a premium because lighter weights reduce overall launch costs. Small electronic components can minimize the size of circuit boards used in satellites.
• Low power, to reduce required battery weights and solar array size. Most satellites are powered with solar arrays and batteries that affect the weight of the system. Using electronic components that minimize power consumption will reduce the overall size of the satellite’s battery and solar array, thus reducing the weight of the system and, again, the cost to launch the satellite into orbit.

Addressing Design Challenges with FDAs

Many analog-to-digital converters (ADCs) for LEO satellite systems have fully differential inputs to help improve dynamic range and take advantage of the fidelity improvements of differential signals. To drive the input of differential ADCs, it is best to choose a fully differential amplifier (FDA) that you can use with either differential or single-ended signals. Several sensors do provide a single-ended signal that can be efficiently converted to a fully differential signal. And while it is possible to convert a single-ended signal to a differential signal by using two separate operational amplifiers, FDAs offer smaller size, lower power, lower noise and an overall improved dynamic range.

Figure 1 shows an FDA configured to convert a single-ended signal to a fully differential signal in order to drive the inputs of a fully differential ADC.

Reducing Noise to Improve Performance

Using a single FDA architecture to drive an ADC will provide 1/√2 lower noise for the same power when compared to using a pair of single-ended operational amplifiers. For example, an operational amplifier with an input voltage noise of 3 nV√Hz will have a total input voltage noise of 3 ´ √2 nV/√Hz in a dual operational amplifier circuit. Some FDAs, such as the LMH5485-SEP amplifier, feature a common-mode output Vocm pin to set the output common-mode voltage of the amplifier to perfectly match the expected ADC input common-mode voltage. Many ADCs include a common-mode voltage pin designed to connect to the Vocm pin. You can also leave the Vocm pin on the amplifier floating if your intended common-mode voltage is at the midpoint of the supplies.

Meeting Mission Radiation Requirements

FDAs used in satellite systems also need to meet the radiation requirements of the mission. The 850-MHz LMH5485-SEP FDA is radiation tolerant, with a total ionizing dose assured up to 30 krad. The device is also single-event latch-up immune to a linear energy transfer of –43 MeV-cm²/mg. The LMH5485-SEP is in a small 3.0-mm-by-3.0-mm very thin shrink small-outline package package, shown in Figure 2, and meets TI’s space-enhanced plastic (SEP) qualification. You can learn more about SEP products from TI in the article “How Space-Enhanced Plastic Devices Address Challenges in Low-Earth Orbit Applications.”

Designers of LEO satellite signal chains face many challenges when selecting components for their systems. The LMH5485-SEP FDA is one component in an expanding portfolio that enables next-generation LEO satellites to achieve greater performance at smaller sizes and lower power. The LMH5485-SEP achieves radiation tolerance and small size while also providing low-noise performance to achieve the maximum dynamic range and minimize total harmonic distortion for differential analog signal chains. Take a look at the Additional Resources section to learn more about other SEP products from TI, and how to optimize signal-chain design with FDAs.

Additional Resources

• Check out these application notes:
  • “Reduce the Risk in Low-Earth Orbit Missions with Space Enhanced Plastic Products.”
  • “Maximizing Signal Chain Distortion Performance Using High Speed Amplifiers.”
• Learn more about TI's portfolio of space-grade devices with the “TI Space Products Guide.”
• Watch the TI Precision Labs video series about FDAs.