Radiation hardening of analog and mixed-signal circuits is critical for aerospace applications, where total ionizing dose (TID) and single-event effects (SEE) can degrade performance or cause catastrophic failures. Analog-to-digital converters (ADCs), phase-locked loops (PLLs), and other mixed-signal components must maintain functionality in high-radiation environments while balancing trade-offs in bandwidth, noise, and power consumption.
### Radiation Effects on Analog and Mixed-Signal Circuits
TID accumulates over time and causes threshold voltage shifts, leakage current increases, and transconductance degradation in MOS devices. In bipolar technologies, TID leads to gain degradation in transistors due to increased base current. SEE, including single-event transients (SETs) and single-event upsets (SEUs), introduce abrupt disturbances such as voltage glitches or phase errors in PLLs and ADCs.
Analog circuits are particularly sensitive because small perturbations in bias conditions or signal integrity can cascade into significant performance deviations. For example, a SET in a PLL’s voltage-controlled oscillator (VCO) can induce phase noise or loss of lock, while TID-induced leakage in an ADC’s comparator may increase offset errors.
### Hardening Techniques for TID Mitigation
**Process Selection:**
Radiation-hardened semiconductor processes, such as silicon-on-insulator (SOI) or bipolar-CMOS (BiCMOS), reduce TID susceptibility by minimizing parasitic leakage paths. SOI technologies exhibit lower charge trapping in buried oxides compared to bulk CMOS.
**Layout Strategies:**
Enclosed-gate (edgeless) transistor layouts mitigate leakage by eliminating field oxide edges where charge trapping occurs. Guard rings and deep n-wells isolate sensitive nodes from substrate noise and latch-up triggers.
**Circuit Design Adjustments:**
- **Biasing Techniques:** Using cascode structures or dynamic biasing compensates for threshold voltage shifts.
- **Redundancy:** Dual-interleaved ADCs or redundant PLL paths average out radiation-induced errors.
- **Feedback Control:** Adaptive biasing circuits adjust operating points in real-time to counteract TID drift.
### Hardening Techniques for SEE Mitigation
**SET Filtering:**
Low-pass filters at critical nodes (e.g., VCO control lines in PLLs) attenuate transient glitches. Time-delayed voting circuits in ADCs reject outlier samples caused by SETs.
**Triple Modular Redundancy (TMR):**
Critical analog blocks, such as comparators or reference buffers, can be triplicated with majority voting to mask SEUs. However, TMR increases power and area overhead.
**Charge Cancellation:**
Differential circuit topologies (e.g., fully differential amplifiers) inherently reject common-mode SETs. Charge-canceling dummy transistors absorb injected charge from ion strikes.
### Trade-offs in Bandwidth, Noise, and Power
**Bandwidth Limitations:**
SET filtering and redundancy reduce bandwidth. For example, a low-pass filter with a 10 MHz cutoff may suppress SETs but limit an ADC’s effective sampling rate. Radiation-hardened ADCs for aerospace often trade off sample rates (e.g., < 100 MS/s) for robustness.
**Noise Considerations:**
Redundant circuits and guard rings introduce additional parasitic capacitance, increasing thermal noise. In a 12-bit ADC, TID-hardened designs may exhibit 1–2 dB higher noise floors than commercial counterparts.
**Power Overhead:**
TMR and adaptive biasing increase static power. A radiation-hardened PLL might consume 20–30% more power than a non-hardened equivalent due to redundant VCOs and bias generators.
### Aerospace-Specific Design Challenges
- **Extended Mission Durations:** Satellites and deep-space probes require decades of reliable operation, necessitating ultra-low leakage and aging-resistant designs.
- **Extreme Temperature Swings:** Radiation-hardened circuits must also tolerate -55°C to 125°C thermal cycles without performance drift.
- **Weight and Size Constraints:** Redundant systems must fit within strict payload limits, favoring monolithic integration over discrete redundancy.
### Case Study: Radiation-Hardened ADCs
Aerospace-grade ADCs often employ:
- **Pipeline architectures** with error-correcting interstage redundancy.
- **Sub-radix-2 DACs** to tolerate comparator offset shifts from TID.
- **On-chip self-calibration** to periodically correct radiation-induced drifts.
For example, a 14-bit 50 MS/s ADC hardened for TID > 300 krad(Si) may achieve 11.5 effective number of bits (ENOB) post-irradiation, compared to 12.5 ENOB pre-irradiation, with a power increase from 150 mW to 190 mW.
### Future Directions
Emerging techniques include:
- **Machine learning-assisted calibration** to dynamically compensate for radiation drift.
- **3D-integrated designs** where redundant layers are stacked vertically to save area.
- **Novel materials** like SiC or GaN for inherent radiation tolerance in extreme environments.
In summary, radiation hardening for analog and mixed-signal circuits requires a multi-layered approach spanning process technology, circuit design, and system architecture. While trade-offs in bandwidth, noise, and power are unavoidable, careful optimization ensures reliable operation in aerospace applications without compromising critical performance metrics.