APL has a long history of digital, analog, and mixed-signal application-specific integrated circuit (ASIC) development. At APL, ASICs have ranged from 32-bit language directed real-time processors, radar chirp generators, signal processing and communications ASICs, as well as numerous gate arrays and mass memory input controllers. Within APL's Space Department, the development of ASICs has been focused on mixed-signal design targeted at both spacecraft bus and instrument applications. ASICs provide many advantages to spacecraft design; they also come with unique challenges. One of the most significant challenges is the radiation environment in which these devices must operate.
Radiation environments can have negative effects on electronic devices, resulting in sudden changes to internal signals that can cause an electronic component to malfunction or to be damaged. Additionally, prolonged exposure to radiation can change the electrical properties of ASIC transistors. Long-term radiation exposure has been successfully dealt with at APL by using a specialized radiation-hardened manufacturing processes. For example, the temperature remote input/output (TRIO) ASIC has found widespread use in many APL spacecraft. The TRIO's simple interface and multichannel capacity simplify the acquisition of temperatures across a spacecraft. APL has also developed a multi-channel instrument front end and an innovative time-to-digital converter. Both of these ASICs have allowed the development of advanced spacecraft instruments for missions such as MESSENGER and New Horizons.
While these specialized radiation-hardened processes offer the ASIC developer an avenue for designing components to be used in space, they do not come without difficulties. The market for such technologies is small, and the processes are expensive with long lead times, making prototypes expensive. Generally, these specialized processes also have a limited longevity, so expenses relating to adopting a new process are significant. Lack of process longevity also hampers long-term use of a "good" ASIC because of the difficulties in manufacturing new parts. As a process is retired, an existing part must be retargeted to a newer process.
To address these issues, APL has expanded its ASIC work away from rad-hard foundries to commercial foundries. Commercial foundries offer a wider variety of processes, expanding the range of possible ASIC applications. Commercial processes using multi-project wafers allow for faster turnaround time and for the development of ASICs at substantially reduced costs over purchasing entire wafer lots. Although commercial manufacturing processes are not inherently designed to deal with radiation effects, mitigation is possible through Radiation Hardened By Design (RHBD) layout techniques. These techniques leverage the advantages in cost, access, longevity, and variety inherent in commercial foundries to circumvent the limitations of traditional dedicated radiation-hardened ASIC lines.
Using RHBD methodologies such as these, we have developed a number of mixed-signal ASICs. These ASICs are largely targeted to sensor and system monitoring functions. A partial list of APL-designed RHBD ASICs includes:
- Micro digital solar attitude detector (µDSAD): A singl- chip Sun sensor that uses an on-chip focal plane, analog processing, and internal analog to digital to provide solar attitude information via a four-wire interface. Total power dissipation is less than 20 mW.
- Pressure remote input/output (PRIO): A controller chip for implementing power switch functions in spacecraft and instruments. PRIO provides eight channels of voltage, and eight channels of isolated current measurements with circuit breaker functions.
- Quad digital to analog converter (QuadDAC): Four-channel, 8-bit voltage output digital-to-analog converter with inter-integrated circuit interface. QuadDAC is capable of driving a 10-kW load while consuming only 15 mW. It is currently used on MESSENGER and New Horizons spacecraft.
- Receiver processor unit (RPU): A rad-hard mixed-signal microcontroller that combines 68HC11 processor core with Programmable Gain Amplifier, 10-bit analog-to-digital converter, 10-bit digital-to-analog converter, voltage reference, and electrically erasable programmable read-only memory. RPU is designed for an extended temperature range (–125°C to 85°C) to accommodate martian surface conditions.
We are currently expanding our development of RHBD mixed-signal ASICs to include smaller feature sizes as well as higher voltage operation. These newer technologies will allow us to migrate our RHBD methodologies to tackle the problems of precision analog and power systems components.